summary refs log tree commit diff
path: root/manual/resource.texi
blob: 4a814c9e4a331fa6daa508c87a95823618a18376 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
@node Resource Usage And Limitation, Non-Local Exits, Date and Time, Top
@c %MENU% Functions for examining resource usage and getting and setting limits
@chapter Resource Usage And Limitation
This chapter describes functions for examining how much of various kinds of
resources (CPU time, memory, etc.) a process has used and getting and setting
limits on future usage.

@menu
* Resource Usage::		Measuring various resources used.
* Limits on Resources::		Specifying limits on resource usage.
* Priority::			Reading or setting process run priority.
* Memory Resources::            Querying memory available resources.
* Processor Resources::         Learn about the processors available.
@end menu


@node Resource Usage
@section Resource Usage

@pindex sys/resource.h
The function @code{getrusage} and the data type @code{struct rusage}
are used to examine the resource usage of a process.  They are declared
in @file{sys/resource.h}.

@comment sys/resource.h
@comment BSD
@deftypefun int getrusage (int @var{processes}, struct rusage *@var{rusage})
This function reports resource usage totals for processes specified by
@var{processes}, storing the information in @code{*@var{rusage}}.

In most systems, @var{processes} has only two valid values:

@table @code
@comment sys/resource.h
@comment BSD
@item RUSAGE_SELF
Just the current process.

@comment sys/resource.h
@comment BSD
@item RUSAGE_CHILDREN
All child processes (direct and indirect) that have already terminated.
@end table

In the GNU system, you can also inquire about a particular child process
by specifying its process ID.

The return value of @code{getrusage} is zero for success, and @code{-1}
for failure.

@table @code
@item EINVAL
The argument @var{processes} is not valid.
@end table
@end deftypefun

One way of getting resource usage for a particular child process is with
the function @code{wait4}, which returns totals for a child when it
terminates.  @xref{BSD Wait Functions}.

@comment sys/resource.h
@comment BSD
@deftp {Data Type} {struct rusage}
This data type stores various resource usage statistics.  It has the
following members, and possibly others:

@table @code
@item struct timeval ru_utime
Time spent executing user instructions.

@item struct timeval ru_stime
Time spent in operating system code on behalf of @var{processes}.

@item long int ru_maxrss
The maximum resident set size used, in kilobytes.  That is, the maximum
number of kilobytes of physical memory that @var{processes} used
simultaneously.

@item long int ru_ixrss
An integral value expressed in kilobytes times ticks of execution, which
indicates the amount of memory used by text that was shared with other
processes.

@item long int ru_idrss
An integral value expressed the same way, which is the amount of
unshared memory used for data.

@item long int ru_isrss
An integral value expressed the same way, which is the amount of
unshared memory used for stack space.

@item long int ru_minflt
The number of page faults which were serviced without requiring any I/O.

@item long int ru_majflt
The number of page faults which were serviced by doing I/O.

@item long int ru_nswap
The number of times @var{processes} was swapped entirely out of main memory.

@item long int ru_inblock
The number of times the file system had to read from the disk on behalf
of @var{processes}.

@item long int ru_oublock
The number of times the file system had to write to the disk on behalf
of @var{processes}.

@item long int ru_msgsnd
Number of IPC messages sent.

@item long int ru_msgrcv
Number of IPC messages received.

@item long int ru_nsignals
Number of signals received.

@item long int ru_nvcsw
The number of times @var{processes} voluntarily invoked a context switch
(usually to wait for some service).

@item long int ru_nivcsw
The number of times an involuntary context switch took place (because
a time slice expired, or another process of higher priority was
scheduled).
@end table
@end deftp

@code{vtimes} is a historical function that does some of what
@code{getrusage} does.  @code{getrusage} is a better choice.

@code{vtimes} and its @code{vtimes} data structure are declared in
@file{sys/vtimes.h}.
@pindex sys/vtimes.h
@comment vtimes.h

@deftypefun int vtimes (struct vtimes @var{current}, struct vtimes @var{child})

@code{vtimes} reports resource usage totals for a process.

If @var{current} is non-null, @code{vtimes} stores resource usage totals for
the invoking process alone in the structure to which it points.  If
@var{child} is non-null, @code{vtimes} stores resource usage totals for all
past children (which have terminated) of the invoking process in the structure
to which it points.

@deftp {Data Type} {struct vtimes}
This data type contains information about the resource usage of a process.
Each member corresponds to a member of the @code{struct rusage} data type
described above.

@table @code
@item vm_utime
User CPU time.  Analogous to @code{ru_utime} in @code{struct rusage}
@item vm_stime
System CPU time.  Analogous to @code{ru_stime} in @code{struct rusage}
@item vm_idsrss
Data and stack memory.  The sum of the values that would be reported as
@code{ru_idrss} and @code{ru_isrss} in @code{struct rusage}
@item vm_ixrss
Shared memory.  Analogous to @code{ru_ixrss} in @code{struct rusage}
@item vm_maxrss
Maximent resident set size.  Analogous to @code{ru_maxrss} in
@code{struct rusage}
@item vm_majflt
Major page faults.  Analogous to @code{ru_majflt} in @code{struct rusage}
@item vm_minflt
Minor page faults.  Analogous to @code{ru_minflt} in @code{struct rusage}
@item vm_nswap
Swap count.  Analogous to @code{ru_nswap} in @code{struct rusage}
@item vm_inblk
Disk reads.  Analogous to @code{ru_inblk} in @code{struct rusage}
@item vm_oublk
Disk writes.  Analogous to @code{ru_oublk} in @code{struct rusage}
@end table
@end deftp


The return value is zero if the function succeeds; @code{-1} otherwise.



@end deftypefun
An additional historical function for examining resource usage,
@code{vtimes}, is supported but not documented here.  It is declared in
@file{sys/vtimes.h}.

@node Limits on Resources
@section Limiting Resource Usage
@cindex resource limits
@cindex limits on resource usage
@cindex usage limits

You can specify limits for the resource usage of a process.  When the
process tries to exceed a limit, it may get a signal, or the system call
by which it tried to do so may fail, depending on the resource.  Each
process initially inherits its limit values from its parent, but it can
subsequently change them.

There are two per-process limits associated with a resource:
@cindex limit

@table @dfn
@item current limit
The current limit is the value the system will not allow usage to
exceed.  It is also called the ``soft limit'' because the process being
limited can generally raise the current limit at will.
@cindex current limit
@cindex soft limit

@item maximum limit
The maximum limit is the maximum value to which a process is allowed to
set its current limit.  It is also called the ``hard limit'' because
there is no way for a process to get around it.  A process may lower
its own maximum limit, but only the superuser may increase a maximum
limit.
@cindex maximum limit
@cindex hard limit
@end table

@pindex sys/resource.h
The symbols for use with @code{getrlimit}, @code{setrlimit},
@code{getrlimit64}, and @code{setrlimit64} are defined in
@file{sys/resource.h}.

@comment sys/resource.h
@comment BSD
@deftypefun int getrlimit (int @var{resource}, struct rlimit *@var{rlp})
Read the current and maximum limits for the resource @var{resource}
and store them in @code{*@var{rlp}}.

The return value is @code{0} on success and @code{-1} on failure.  The
only possible @code{errno} error condition is @code{EFAULT}.

When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a
32-bit system this function is in fact @code{getrlimit64}.  Thus, the
LFS interface transparently replaces the old interface.
@end deftypefun

@comment sys/resource.h
@comment Unix98
@deftypefun int getrlimit64 (int @var{resource}, struct rlimit64 *@var{rlp})
This function is similar to @code{getrlimit} but its second parameter is
a pointer to a variable of type @code{struct rlimit64}, which allows it
to read values which wouldn't fit in the member of a @code{struct
rlimit}.

If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a
32-bit machine, this function is available under the name
@code{getrlimit} and so transparently replaces the old interface.
@end deftypefun

@comment sys/resource.h
@comment BSD
@deftypefun int setrlimit (int @var{resource}, const struct rlimit *@var{rlp})
Store the current and maximum limits for the resource @var{resource}
in @code{*@var{rlp}}.

The return value is @code{0} on success and @code{-1} on failure.  The
following @code{errno} error condition is possible:

@table @code
@item EPERM
@itemize @bullet
@item
The process tried to raise a current limit beyond the maximum limit.

@item
The process tried to raise a maximum limit, but is not superuser.
@end itemize
@end table

When the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a
32-bit system this function is in fact @code{setrlimit64}.  Thus, the
LFS interface transparently replaces the old interface.
@end deftypefun

@comment sys/resource.h
@comment Unix98
@deftypefun int setrlimit64 (int @var{resource}, const struct rlimit64 *@var{rlp})
This function is similar to @code{setrlimit} but its second parameter is
a pointer to a variable of type @code{struct rlimit64} which allows it
to set values which wouldn't fit in the member of a @code{struct
rlimit}.

If the sources are compiled with @code{_FILE_OFFSET_BITS == 64} on a
32-bit machine this function is available under the name
@code{setrlimit} and so transparently replaces the old interface.
@end deftypefun

@comment sys/resource.h
@comment BSD
@deftp {Data Type} {struct rlimit}
This structure is used with @code{getrlimit} to receive limit values,
and with @code{setrlimit} to specify limit values for a particular process
and resource.  It has two fields:

@table @code
@item rlim_t rlim_cur
The current limit

@item rlim_t rlim_max
The maximum limit.
@end table

For @code{getrlimit}, the structure is an output; it receives the current
values.  For @code{setrlimit}, it specifies the new values.
@end deftp

For the LFS functions a similar type is defined in @file{sys/resource.h}.

@comment sys/resource.h
@comment Unix98
@deftp {Data Type} {struct rlimit64}
This structure is analogous to the @code{rlimit} structure above, but
its components have wider ranges.  It has two fields:

@table @code
@item rlim64_t rlim_cur
This is analogous to @code{rlimit.rlim_cur}, but with a different type.

@item rlim64_t rlim_max
This is analogous to @code{rlimit.rlim_max}, but with a different type.
@end table

@end deftp

Here is a list of resources for which you can specify a limit.  Memory
and file sizes are measured in bytes.

@table @code
@comment sys/resource.h
@comment BSD
@item RLIMIT_CPU
@vindex RLIMIT_CPU
The maximum amount of CPU time the process can use.  If it runs for
longer than this, it gets a signal: @code{SIGXCPU}.  The value is
measured in seconds.  @xref{Operation Error Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_FSIZE
@vindex RLIMIT_FSIZE
The maximum size of file the process can create.  Trying to write a
larger file causes a signal: @code{SIGXFSZ}.  @xref{Operation Error
Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_DATA
@vindex RLIMIT_DATA
The maximum size of data memory for the process.  If the process tries
to allocate data memory beyond this amount, the allocation function
fails.

@comment sys/resource.h
@comment BSD
@item RLIMIT_STACK
@vindex RLIMIT_STACK
The maximum stack size for the process.  If the process tries to extend
its stack past this size, it gets a @code{SIGSEGV} signal.
@xref{Program Error Signals}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_CORE
@vindex RLIMIT_CORE
The maximum size core file that this process can create.  If the process
terminates and would dump a core file larger than this, then no core
file is created.  So setting this limit to zero prevents core files from
ever being created.

@comment sys/resource.h
@comment BSD
@item RLIMIT_RSS
@vindex RLIMIT_RSS
The maximum amount of physical memory that this process should get.
This parameter is a guide for the system's scheduler and memory
allocator; the system may give the process more memory when there is a
surplus.

@comment sys/resource.h
@comment BSD
@item RLIMIT_MEMLOCK
The maximum amount of memory that can be locked into physical memory (so
it will never be paged out).

@comment sys/resource.h
@comment BSD
@item RLIMIT_NPROC
The maximum number of processes that can be created with the same user ID.
If you have reached the limit for your user ID, @code{fork} will fail
with @code{EAGAIN}.  @xref{Creating a Process}.

@comment sys/resource.h
@comment BSD
@item RLIMIT_NOFILE
@vindex RLIMIT_NOFILE
@itemx RLIMIT_OFILE
@vindex RLIMIT_OFILE
The maximum number of files that the process can open.  If it tries to
open more files than this, its open attempt fails with @code{errno}
@code{EMFILE}.  @xref{Error Codes}.  Not all systems support this limit;
GNU does, and 4.4 BSD does.

@comment sys/resource.h
@comment Unix98
@item RLIMIT_AS
@vindex RLIMIT_AS
The maximum size of total memory that this process should get.  If the
process tries to allocate more memory beyond this amount with, for
example, @code{brk}, @code{malloc}, @code{mmap} or @code{sbrk}, the
allocation function fails.

@comment sys/resource.h
@comment BSD
@item RLIM_NLIMITS
@vindex RLIM_NLIMITS
The number of different resource limits.  Any valid @var{resource}
operand must be less than @code{RLIM_NLIMITS}.
@end table

@comment sys/resource.h
@comment BSD
@deftypevr Constant int RLIM_INFINITY
This constant stands for a value of ``infinity'' when supplied as
the limit value in @code{setrlimit}.
@end deftypevr


The following are historical functions to do some of what the functions
above do.  The functions above are better choices.

@code{ulimit} and the command symbols are declared in @file{ulimit.h}.
@pindex ulimit.h

@comment ulimit.h
@comment BSD
@deftypefun int ulimit (int @var{cmd}, ...)

@code{ulimit} gets the current limit or sets the current and maximum
limit for a particular resource for the calling process according to the
command @var{cmd}.a

If you are getting a limit, the command argument is the only argument.
If you are setting a limit, there is a second argument:
@code{long int} @var{limit} which is the value to which you are setting
the limit.

The @var{cmd} values and the operations they specify are:
@table @code

@item GETFSIZE
Get the current limit on the size of a file, in units of 512 bytes.

@item SETFSIZE
Set the current and maximum limit on the size of a file to @var{limit} *
512 bytes.

@end table

There are also some other @var{cmd} values that may do things on some
systems, but they are not supported.

Only the superuser may increase a maximum limit.

When you successfully get a limit, the return value of @code{ulimit} is
that limit, which is never negative.  When you successfully set a limit,
the return value is zero.  When the function fails, the return value is
@code{-1} and @code{errno} is set according to the reason:

@table @code
@item EPERM
A process tried to increase a maximum limit, but is not superuser.
@end table


@end deftypefun

@code{vlimit} and its resource symbols are declared in @file{sys/vlimit.h}.
@pindex sys/vlimit.h

@comment sys/vlimit.h
@comment BSD
@deftypefun int vlimit (int @var{resource}, int @var{limit})

@code{vlimit} sets the current limit for a resource for a process.

@var{resource} identifies the resource:

@table @code
@item LIM_CPU
Maximum CPU time.  Same as @code{RLIMIT_CPU} for @code{setrlimit}.
@item LIM_FSIZE
Maximum file size.  Same as @code{RLIMIT_FSIZE} for @code{setrlimit}.
@item LIM_DATA
Maximum data memory.  Same as @code{RLIMIT_DATA} for @code{setrlimit}.
@item LIM_STACK
Maximum stack size.  Same as @code{RLIMIT_STACK} for @code{setrlimit}.
@item LIM_CORE
Maximum core file size.  Same as @code{RLIMIT_COR} for @code{setrlimit}.
@item LIM_MAXRSS
Maximum physical memory.  Same as @code{RLIMIT_RSS} for @code{setrlimit}.
@end table

The return value is zero for success, and @code{-1} with @code{errno} set
accordingly for failure:

@table @code
@item EPERM
The process tried to set its current limit beyond its maximum limit.
@end table

@end deftypefun

@node Priority
@section Process CPU Priority And Scheduling
@cindex process priority
@cindex cpu priority
@cindex priority of a process

When multiple processes simultaneously require CPU time, the system's
scheduling policy and process CPU priorities determine which processes
get it.  This section describes how that determination is made and
GNU C library functions to control it.

It is common to refer to CPU scheduling simply as scheduling and a
process' CPU priority simply as the process' priority, with the CPU
resource being implied.  Bear in mind, though, that CPU time is not the
only resource a process uses or that processes contend for.  In some
cases, it is not even particularly important.  Giving a process a high
``priority'' may have very little effect on how fast a process runs with
respect to other processes.  The priorities discussed in this section
apply only to CPU time.

CPU scheduling is a complex issue and different systems do it in wildly
different ways.  New ideas continually develop and find their way into
the intricacies of the various systems' scheduling algorithms.  This
section discusses the general concepts, some specifics of systems
that commonly use the GNU C library, and some standards.

For simplicity, we talk about CPU contention as if there is only one CPU
in the system.  But all the same principles apply when a processor has
multiple CPUs, and knowing that the number of processes that can run at
any one time is equal to the number of CPUs, you can easily extrapolate
the information.

The functions described in this section are all defined by the POSIX.1
and POSIX.1b standards (the @code{sched@dots{}} functions are POSIX.1b).
However, POSIX does not define any semantics for the values that these
functions get and set.  In this chapter, the semantics are based on the
Linux kernel's implementation of the POSIX standard.  As you will see,
the Linux implementation is quite the inverse of what the authors of the
POSIX syntax had in mind.

@menu
* Absolute Priority::               The first tier of priority.  Posix
* Realtime Scheduling::             Scheduling among the process nobility
* Basic Scheduling Functions::      Get/set scheduling policy, priority
* Traditional Scheduling::          Scheduling among the vulgar masses
* CPU Affinity::                    Limiting execution to certain CPUs
@end menu



@node Absolute Priority
@subsection Absolute Priority
@cindex absolute priority
@cindex priority, absolute

Every process has an absolute priority, and it is represented by a number.
The higher the number, the higher the absolute priority.

@cindex realtime CPU scheduling
On systems of the past, and most systems today, all processes have
absolute priority 0 and this section is irrelevant.  In that case,
@xref{Traditional Scheduling}.  Absolute priorities were invented to
accommodate realtime systems, in which it is vital that certain processes
be able to respond to external events happening in real time, which
means they cannot wait around while some other process that @emph{wants
to}, but doesn't @emph{need to} run occupies the CPU.

@cindex ready to run
@cindex preemptive scheduling
When two processes are in contention to use the CPU at any instant, the
one with the higher absolute priority always gets it.  This is true even if the
process with the lower priority is already using the CPU (i.e., the
scheduling is preemptive).  Of course, we're only talking about
processes that are running or ``ready to run,'' which means they are
ready to execute instructions right now.  When a process blocks to wait
for something like I/O, its absolute priority is irrelevant.

@cindex runnable process
@strong{NB:}  The term ``runnable'' is a synonym for ``ready to run.''

When two processes are running or ready to run and both have the same
absolute priority, it's more interesting.  In that case, who gets the
CPU is determined by the scheduling policy.  If the processes have
absolute priority 0, the traditional scheduling policy described in
@ref{Traditional Scheduling} applies.  Otherwise, the policies described
in @ref{Realtime Scheduling} apply.

You normally give an absolute priority above 0 only to a process that
can be trusted not to hog the CPU.  Such processes are designed to block
(or terminate) after relatively short CPU runs.

A process begins life with the same absolute priority as its parent
process.  Functions described in @ref{Basic Scheduling Functions} can
change it.

Only a privileged process can change a process' absolute priority to
something other than @code{0}.  Only a privileged process or the
target process' owner can change its absolute priority at all.

POSIX requires absolute priority values used with the realtime
scheduling policies to be consecutive with a range of at least 32.  On
Linux, they are 1 through 99.  The functions
@code{sched_get_priority_max} and @code{sched_set_priority_min} portably
tell you what the range is on a particular system.


@subsubsection Using Absolute Priority

One thing you must keep in mind when designing real time applications is
that having higher absolute priority than any other process doesn't
guarantee the process can run continuously.  Two things that can wreck a
good CPU run are interrupts and page faults.

Interrupt handlers live in that limbo between processes.  The CPU is
executing instructions, but they aren't part of any process.  An
interrupt will stop even the highest priority process.  So you must
allow for slight delays and make sure that no device in the system has
an interrupt handler that could cause too long a delay between
instructions for your process.

Similarly, a page fault causes what looks like a straightforward
sequence of instructions to take a long time.  The fact that other
processes get to run while the page faults in is of no consequence,
because as soon as the I/O is complete, the high priority process will
kick them out and run again, but the wait for the I/O itself could be a
problem.  To neutralize this threat, use @code{mlock} or
@code{mlockall}.

There are a few ramifications of the absoluteness of this priority on a
single-CPU system that you need to keep in mind when you choose to set a
priority and also when you're working on a program that runs with high
absolute priority.  Consider a process that has higher absolute priority
than any other process in the system and due to a bug in its program, it
gets into an infinite loop.  It will never cede the CPU.  You can't run
a command to kill it because your command would need to get the CPU in
order to run.  The errant program is in complete control.  It controls
the vertical, it controls the horizontal.

There are two ways to avoid this: 1) keep a shell running somewhere with
a higher absolute priority.  2) keep a controlling terminal attached to
the high priority process group.  All the priority in the world won't
stop an interrupt handler from running and delivering a signal to the
process if you hit Control-C.

Some systems use absolute priority as a means of allocating a fixed
percentage of CPU time to a process.  To do this, a super high priority
privileged process constantly monitors the process' CPU usage and raises
its absolute priority when the process isn't getting its entitled share
and lowers it when the process is exceeding it.

@strong{NB:}  The absolute priority is sometimes called the ``static
priority.''  We don't use that term in this manual because it misses the
most important feature of the absolute priority:  its absoluteness.


@node Realtime Scheduling
@subsection Realtime Scheduling
@cindex realtime scheduling

Whenever two processes with the same absolute priority are ready to run,
the kernel has a decision to make, because only one can run at a time.
If the processes have absolute priority 0, the kernel makes this decision
as described in @ref{Traditional Scheduling}.  Otherwise, the decision
is as described in this section.

If two processes are ready to run but have different absolute priorities,
the decision is much simpler, and is described in @ref{Absolute
Priority}.

Each process has a scheduling policy.  For processes with absolute
priority other than zero, there are two available:

@enumerate
@item
First Come First Served
@item
Round Robin
@end enumerate

The most sensible case is where all the processes with a certain
absolute priority have the same scheduling policy.  We'll discuss that
first.

In Round Robin, processes share the CPU, each one running for a small
quantum of time (``time slice'') and then yielding to another in a
circular fashion.  Of course, only processes that are ready to run and
have the same absolute priority are in this circle.

In First Come First Served, the process that has been waiting the
longest to run gets the CPU, and it keeps it until it voluntarily
relinquishes the CPU, runs out of things to do (blocks), or gets
preempted by a higher priority process.

First Come First Served, along with maximal absolute priority and
careful control of interrupts and page faults, is the one to use when a
process absolutely, positively has to run at full CPU speed or not at
all.

Judicious use of @code{sched_yield} function invocations by processes
with First Come First Served scheduling policy forms a good compromise
between Round Robin and First Come First Served.

To understand how scheduling works when processes of different scheduling
policies occupy the same absolute priority, you have to know the nitty
gritty details of how processes enter and exit the ready to run list:

In both cases, the ready to run list is organized as a true queue, where
a process gets pushed onto the tail when it becomes ready to run and is
popped off the head when the scheduler decides to run it.  Note that
ready to run and running are two mutually exclusive states.  When the
scheduler runs a process, that process is no longer ready to run and no
longer in the ready to run list.  When the process stops running, it
may go back to being ready to run again.

The only difference between a process that is assigned the Round Robin
scheduling policy and a process that is assigned First Come First Serve
is that in the former case, the process is automatically booted off the
CPU after a certain amount of time.  When that happens, the process goes
back to being ready to run, which means it enters the queue at the tail.
The time quantum we're talking about is small.  Really small.  This is
not your father's timesharing.  For example, with the Linux kernel, the
round robin time slice is a thousand times shorter than its typical
time slice for traditional scheduling.

A process begins life with the same scheduling policy as its parent process.
Functions described in @ref{Basic Scheduling Functions} can change it.

Only a privileged process can set the scheduling policy of a process
that has absolute priority higher than 0.

@node Basic Scheduling Functions
@subsection Basic Scheduling Functions

This section describes functions in the GNU C library for setting the
absolute priority and scheduling policy of a process.

@strong{Portability Note:}  On systems that have the functions in this
section, the macro _POSIX_PRIORITY_SCHEDULING is defined in
@file{<unistd.h>}.

For the case that the scheduling policy is traditional scheduling, more
functions to fine tune the scheduling are in @ref{Traditional Scheduling}.

Don't try to make too much out of the naming and structure of these
functions.  They don't match the concepts described in this manual
because the functions are as defined by POSIX.1b, but the implementation
on systems that use the GNU C library is the inverse of what the POSIX
structure contemplates.  The POSIX scheme assumes that the primary
scheduling parameter is the scheduling policy and that the priority
value, if any, is a parameter of the scheduling policy.  In the
implementation, though, the priority value is king and the scheduling
policy, if anything, only fine tunes the effect of that priority.

The symbols in this section are declared by including file @file{sched.h}.

@comment sched.h
@comment POSIX
@deftp {Data Type} {struct sched_param}
This structure describes an absolute priority.
@table @code
@item int sched_priority
absolute priority value
@end table
@end deftp

@comment sched.h
@comment POSIX
@deftypefun int sched_setscheduler (pid_t @var{pid}, int @var{policy}, const struct sched_param *@var{param})

This function sets both the absolute priority and the scheduling policy
for a process.

It assigns the absolute priority value given by @var{param} and the
scheduling policy @var{policy} to the process with Process ID @var{pid},
or the calling process if @var{pid} is zero.  If @var{policy} is
negative, @code{sched_setscheduler} keeps the existing scheduling policy.

The following macros represent the valid values for @var{policy}:

@table @code
@item SCHED_OTHER
Traditional Scheduling
@item SCHED_FIFO
First In First Out
@item SCHED_RR
Round Robin
@end table

@c The Linux kernel code (in sched.c) actually reschedules the process,
@c but it puts it at the head of the run queue, so I'm not sure just what
@c the effect is, but it must be subtle.

On success, the return value is @code{0}.  Otherwise, it is @code{-1}
and @code{ERRNO} is set accordingly.  The @code{errno} values specific
to this function are:

@table @code
@item EPERM
@itemize @bullet
@item
The calling process does not have @code{CAP_SYS_NICE} permission and
@var{policy} is not @code{SCHED_OTHER} (or it's negative and the
existing policy is not @code{SCHED_OTHER}.

@item
The calling process does not have @code{CAP_SYS_NICE} permission and its
owner is not the target process' owner.  I.e., the effective uid of the
calling process is neither the effective nor the real uid of process
@var{pid}.
@c We need a cross reference to the capabilities section, when written.
@end itemize

@item ESRCH
There is no process with pid @var{pid} and @var{pid} is not zero.

@item EINVAL
@itemize @bullet
@item
@var{policy} does not identify an existing scheduling policy.

@item
The absolute priority value identified by *@var{param} is outside the
valid range for the scheduling policy @var{policy} (or the existing
scheduling policy if @var{policy} is negative) or @var{param} is
null.  @code{sched_get_priority_max} and @code{sched_get_priority_min}
tell you what the valid range is.

@item
@var{pid} is negative.
@end itemize
@end table

@end deftypefun


@comment sched.h
@comment POSIX
@deftypefun int sched_getscheduler (pid_t @var{pid})

This function returns the scheduling policy assigned to the process with
Process ID (pid) @var{pid}, or the calling process if @var{pid} is zero.

The return value is the scheduling policy.  See
@code{sched_setscheduler} for the possible values.

If the function fails, the return value is instead @code{-1} and
@code{errno} is set accordingly.

The @code{errno} values specific to this function are:

@table @code

@item ESRCH
There is no process with pid @var{pid} and it is not zero.

@item EINVAL
@var{pid} is negative.

@end table

Note that this function is not an exact mate to @code{sched_setscheduler}
because while that function sets the scheduling policy and the absolute
priority, this function gets only the scheduling policy.  To get the
absolute priority, use @code{sched_getparam}.

@end deftypefun


@comment sched.h
@comment POSIX
@deftypefun int sched_setparam (pid_t @var{pid}, const struct sched_param *@var{param})

This function sets a process' absolute priority.

It is functionally identical to @code{sched_setscheduler} with
@var{policy} = @code{-1}.

@c in fact, that's how it's implemented in Linux.

@end deftypefun

@comment sched.h
@comment POSIX
@deftypefun int sched_getparam (pid_t @var{pid}, const struct sched_param *@var{param})

This function returns a process' absolute priority.

@var{pid} is the Process ID (pid) of the process whose absolute priority
you want to know.

@var{param} is a pointer to a structure in which the function stores the
absolute priority of the process.

On success, the return value is @code{0}.  Otherwise, it is @code{-1}
and @code{ERRNO} is set accordingly.  The @code{errno} values specific
to this function are:

@table @code

@item ESRCH
There is no process with pid @var{pid} and it is not zero.

@item EINVAL
@var{pid} is negative.

@end table

@end deftypefun


@comment sched.h
@comment POSIX
@deftypefun int sched_get_priority_min (int *@var{policy});

This function returns the lowest absolute priority value that is
allowable for a process with scheduling policy @var{policy}.

On Linux, it is 0 for SCHED_OTHER and 1 for everything else.

On success, the return value is @code{0}.  Otherwise, it is @code{-1}
and @code{ERRNO} is set accordingly.  The @code{errno} values specific
to this function are:

@table @code
@item EINVAL
@var{policy} does not identify an existing scheduling policy.
@end table

@end deftypefun

@comment sched.h
@comment POSIX
@deftypefun int sched_get_priority_max (int *@var{policy});

This function returns the highest absolute priority value that is
allowable for a process that with scheduling policy @var{policy}.

On Linux, it is 0 for SCHED_OTHER and 99 for everything else.

On success, the return value is @code{0}.  Otherwise, it is @code{-1}
and @code{ERRNO} is set accordingly.  The @code{errno} values specific
to this function are:

@table @code
@item EINVAL
@var{policy} does not identify an existing scheduling policy.
@end table

@end deftypefun

@comment sched.h
@comment POSIX
@deftypefun int sched_rr_get_interval (pid_t @var{pid}, struct timespec *@var{interval})

This function returns the length of the quantum (time slice) used with
the Round Robin scheduling policy, if it is used, for the process with
Process ID @var{pid}.

It returns the length of time as @var{interval}.
@c We need a cross-reference to where timespec is explained.  But that
@c section doesn't exist yet, and the time chapter needs to be slightly
@c reorganized so there is a place to put it (which will be right next
@c to timeval, which is presently misplaced).  2000.05.07.

With a Linux kernel, the round robin time slice is always 150
microseconds, and @var{pid} need not even be a real pid.

The return value is @code{0} on success and in the pathological case
that it fails, the return value is @code{-1} and @code{errno} is set
accordingly.  There is nothing specific that can go wrong with this
function, so there are no specific @code{errno} values.

@end deftypefun

@comment sched.h
@comment POSIX
@deftypefun int sched_yield (void)

This function voluntarily gives up the process' claim on the CPU.

Technically, @code{sched_yield} causes the calling process to be made
immediately ready to run (as opposed to running, which is what it was
before).  This means that if it has absolute priority higher than 0, it
gets pushed onto the tail of the queue of processes that share its
absolute priority and are ready to run, and it will run again when its
turn next arrives.  If its absolute priority is 0, it is more
complicated, but still has the effect of yielding the CPU to other
processes.

If there are no other processes that share the calling process' absolute
priority, this function doesn't have any effect.

To the extent that the containing program is oblivious to what other
processes in the system are doing and how fast it executes, this
function appears as a no-op.

The return value is @code{0} on success and in the pathological case
that it fails, the return value is @code{-1} and @code{errno} is set
accordingly.  There is nothing specific that can go wrong with this
function, so there are no specific @code{errno} values.

@end deftypefun

@node Traditional Scheduling
@subsection Traditional Scheduling
@cindex scheduling, traditional

This section is about the scheduling among processes whose absolute
priority is 0.  When the system hands out the scraps of CPU time that
are left over after the processes with higher absolute priority have
taken all they want, the scheduling described herein determines who
among the great unwashed processes gets them.

@menu
* Traditional Scheduling Intro::
* Traditional Scheduling Functions::
@end menu

@node Traditional Scheduling Intro
@subsubsection Introduction To Traditional Scheduling

Long before there was absolute priority (See @ref{Absolute Priority}),
Unix systems were scheduling the CPU using this system.  When Posix came
in like the Romans and imposed absolute priorities to accommodate the
needs of realtime processing, it left the indigenous Absolute Priority
Zero processes to govern themselves by their own familiar scheduling
policy.

Indeed, absolute priorities higher than zero are not available on many
systems today and are not typically used when they are, being intended
mainly for computers that do realtime processing.  So this section
describes the only scheduling many programmers need to be concerned
about.

But just to be clear about the scope of this scheduling: Any time a
process with a absolute priority of 0 and a process with an absolute
priority higher than 0 are ready to run at the same time, the one with
absolute priority 0 does not run.  If it's already running when the
higher priority ready-to-run process comes into existence, it stops
immediately.

In addition to its absolute priority of zero, every process has another
priority, which we will refer to as "dynamic priority" because it changes
over time.  The dynamic priority is meaningless for processes with
an absolute priority higher than zero.

The dynamic priority sometimes determines who gets the next turn on the
CPU.  Sometimes it determines how long turns last.  Sometimes it
determines whether a process can kick another off the CPU.

In Linux, the value is a combination of these things, but mostly it is
just determines the length of the time slice.  The higher a process'
dynamic priority, the longer a shot it gets on the CPU when it gets one.
If it doesn't use up its time slice before giving up the CPU to do
something like wait for I/O, it is favored for getting the CPU back when
it's ready for it, to finish out its time slice.  Other than that,
selection of processes for new time slices is basically round robin.
But the scheduler does throw a bone to the low priority processes: A
process' dynamic priority rises every time it is snubbed in the
scheduling process.  In Linux, even the fat kid gets to play.

The fluctuation of a process' dynamic priority is regulated by another
value: The ``nice'' value.  The nice value is an integer, usually in the
range -20 to 20, and represents an upper limit on a process' dynamic
priority.  The higher the nice number, the lower that limit.

On a typical Linux system, for example, a process with a nice value of
20 can get only 10 milliseconds on the CPU at a time, whereas a process
with a nice value of -20 can achieve a high enough priority to get 400
milliseconds.

The idea of the nice value is deferential courtesy.  In the beginning,
in the Unix garden of Eden, all processes shared equally in the bounty
of the computer system.  But not all processes really need the same
share of CPU time, so the nice value gave a courteous process the
ability to refuse its equal share of CPU time that others might prosper.
Hence, the higher a process' nice value, the nicer the process is.
(Then a snake came along and offered some process a negative nice value
and the system became the crass resource allocation system we know
today).

Dynamic priorities tend upward and downward with an objective of
smoothing out allocation of CPU time and giving quick response time to
infrequent requests.  But they never exceed their nice limits, so on a
heavily loaded CPU, the nice value effectively determines how fast a
process runs.

In keeping with the socialistic heritage of Unix process priority, a
process begins life with the same nice value as its parent process and
can raise it at will.  A process can also raise the nice value of any
other process owned by the same user (or effective user).  But only a
privileged process can lower its nice value.  A privileged process can
also raise or lower another process' nice value.

GNU C Library functions for getting and setting nice values are described in
@xref{Traditional Scheduling Functions}.

@node Traditional Scheduling Functions
@subsubsection Functions For Traditional Scheduling

@pindex sys/resource.h
This section describes how you can read and set the nice value of a
process.  All these symbols are declared in @file{sys/resource.h}.

The function and macro names are defined by POSIX, and refer to
"priority," but the functions actually have to do with nice values, as
the terms are used both in the manual and POSIX.

The range of valid nice values depends on the kernel, but typically it
runs from @code{-20} to @code{20}.  A lower nice value corresponds to
higher priority for the process.  These constants describe the range of
priority values:

@vtable @code
@comment sys/resource.h
@comment BSD
@item PRIO_MIN
The lowest valid nice value.

@comment sys/resource.h
@comment BSD
@item PRIO_MAX
The highest valid nice value.
@end vtable

@comment sys/resource.h
@comment BSD,POSIX
@deftypefun int getpriority (int @var{class}, int @var{id})
Return the nice value of a set of processes; @var{class} and @var{id}
specify which ones (see below).  If the processes specified do not all
have the same nice value, this returns the lowest value that any of them
has.

On success, the return value is @code{0}.  Otherwise, it is @code{-1}
and @code{ERRNO} is set accordingly.  The @code{errno} values specific
to this function are:

@table @code
@item ESRCH
The combination of @var{class} and @var{id} does not match any existing
process.

@item EINVAL
The value of @var{class} is not valid.
@end table

If the return value is @code{-1}, it could indicate failure, or it could
be the nice value.  The only way to make certain is to set @code{errno =
0} before calling @code{getpriority}, then use @code{errno != 0}
afterward as the criterion for failure.
@end deftypefun

@comment sys/resource.h
@comment BSD,POSIX
@deftypefun int setpriority (int @var{class}, int @var{id}, int @var{niceval})
Set the nice value of a set of processes to @var{niceval}; @var{class}
and @var{id} specify which ones (see below).

The return value is @code{0} on success, and @code{-1} on
failure.  The following @code{errno} error condition are possible for
this function:

@table @code
@item ESRCH
The combination of @var{class} and @var{id} does not match any existing
process.

@item EINVAL
The value of @var{class} is not valid.

@item EPERM
The call would set the nice value of a process which is owned by a different
user than the calling process (i.e., the target process' real or effective
uid does not match the calling process' effective uid) and the calling
process does not have @code{CAP_SYS_NICE} permission.

@item EACCES
The call would lower the process' nice value and the process does not have
@code{CAP_SYS_NICE} permission.
@end table

@end deftypefun

The arguments @var{class} and @var{id} together specify a set of
processes in which you are interested.  These are the possible values of
@var{class}:

@vtable @code
@comment sys/resource.h
@comment BSD
@item PRIO_PROCESS
One particular process.  The argument @var{id} is a process ID (pid).

@comment sys/resource.h
@comment BSD
@item PRIO_PGRP
All the processes in a particular process group.  The argument @var{id} is
a process group ID (pgid).

@comment sys/resource.h
@comment BSD
@item PRIO_USER
All the processes owned by a particular user (i.e., whose real uid
indicates the user).  The argument @var{id} is a user ID (uid).
@end vtable

If the argument @var{id} is 0, it stands for the calling process, its
process group, or its owner (real uid), according to @var{class}.

@comment unistd.h
@comment BSD
@deftypefun int nice (int @var{increment})
Increment the nice value of the calling process by @var{increment}.
The return value is the new nice value on success, and @code{-1} on
failure.  In the case of failure, @code{errno} will be set to the
same values as for @code{setpriority}.


Here is an equivalent definition of @code{nice}:

@smallexample
int
nice (int increment)
@{
  int result, old = getpriority (PRIO_PROCESS, 0);
  result = setpriority (PRIO_PROCESS, 0, old + increment);
  if (result != -1)
      return old + increment;
  else
      return -1;
@}
@end smallexample
@end deftypefun


@node CPU Affinity
@subsection Limiting execution to certain CPUs

On a multi-processor system the operating system usually distributes
the different processes which are runnable on all available CPUs in a
way which allows the system to work most efficiently.  Which processes
and threads run can be to some extend be control with the scheduling
functionality described in the last sections.  But which CPU finally
executes which process or thread is not covered.

There are a number of reasons why a program might want to have control
over this aspect of the system as well:

@itemize @bullet
@item
One thread or process is responsible for absolutely critical work
which under no circumstances must be interrupted or hindered from
making process by other process or threads using CPU resources.  In
this case the special process would be confined to a CPU which no
other process or thread is allowed to use.

@item
The access to certain resources (RAM, I/O ports) has different costs
from different CPUs.  This is the case in NUMA (Non-Uniform Memory
Architecture) machines.  Preferably memory should be accessed locally
but this requirement is usually not visible to the scheduler.
Therefore forcing a process or thread to the CPUs which have local
access to the mostly used memory helps to significantly boost the
performance.

@item
In controlled runtimes resource allocation and book-keeping work (for
instance garbage collection) is performance local to processors.  This
can help to reduce locking costs if the resources do not have to be
protected from concurrent accesses from different processors.
@end itemize

The POSIX standard up to this date is of not much help to solve this
problem.  The Linux kernel provides a set of interfaces to allow
specifying @emph{affinity sets} for a process.  The scheduler will
schedule the thread or process on on CPUs specified by the affinity
masks.  The interfaces which the GNU C library define follow to some
extend the Linux kernel interface.

@comment sched.h
@comment GNU
@deftp {Data Type} cpu_set_t
This data set is a bitset where each bit represents a CPU.  How the
system's CPUs are mapped to bits in the bitset is system dependent.
The data type has a fixed size; in the unlikely case that the number
of bits are not sufficient to describe the CPUs of the system a
different interface has to be used.

This type is a GNU extension and is defined in @file{sched.h}.
@end deftp

To manipulate the bitset, to set and reset bits, a number of macros is
defined.  Some of the macros take a CPU number as a parameter.  Here
it is important to never exceed the size of the bitset.  The following
macro specifies the number of bits in the @code{cpu_set_t} bitset.

@comment sched.h
@comment GNU
@deftypevr Macro int CPU_SETSIZE
The value of this macro is the maximum number of CPUs which can be
handled with a @code{cpu_set_t} object.
@end deftypevr

The type @code{cpu_set_t} should be considered opaque; all
manipulation should happen via the next four macros.

@comment sched.h
@comment GNU
@deftypefn Macro void CPU_ZERO (cpu_set_t *@var{set})
This macro initializes the CPU set @var{set} to be the empty set.

This macro is a GNU extension and is defined in @file{sched.h}.
@end deftypefn

@comment sched.h
@comment GNU
@deftypefn Macro void CPU_SET (int @var{cpu}, cpu_set_t *@var{set})
This macro adds @var{cpu} to the CPU set @var{set}.

The @var{cpu} parameter must not have side effects since it is
evaluated more than once.

This macro is a GNU extension and is defined in @file{sched.h}.
@end deftypefn

@comment sched.h
@comment GNU
@deftypefn Macro void CPU_CLR (int @var{cpu}, cpu_set_t *@var{set})
This macro removes @var{cpu} from the CPU set @var{set}.

The @var{cpu} parameter must not have side effects since it is
evaluated more than once.

This macro is a GNU extension and is defined in @file{sched.h}.
@end deftypefn

@comment sched.h
@comment GNU
@deftypefn Macro int CPU_ISSET (int @var{cpu}, const cpu_set_t *@var{set})
This macro returns a nonzero value (true) if @var{cpu} is a member
of the CPU set @var{set}, and zero (false) otherwise.

The @var{cpu} parameter must not have side effects since it is
evaluated more than once.

This macro is a GNU extension and is defined in @file{sched.h}.
@end deftypefn


CPU bitsets can be constructed from scratch or the currently installed
affinity mask can be retrieved from the system.

@comment sched.h
@comment GNU
@deftypefun int sched_getaffinity (pid_t @var{pid}, size_t @var{cpusetsize}, cpu_set_t *@var{cpuset})

This functions stores the CPU affinity mask for the process or thread
with the ID @var{pid} in the @var{cpusetsize} bytes long bitmap
pointed to by @var{cpuset}.  If successful, the function always
initializes all bits in the @code{cpu_set_t} object and returns zero.

If @var{pid} does not correspond to a process or thread on the system
the or the function fails for some other reason, it returns @code{-1}
and @code{errno} is set to represent the error condition.

@table @code
@item ESRCH
No process or thread with the given ID found.

@item EFAULT
The pointer @var{cpuset} is does not point to a valid object.
@end table

This function is a GNU extension and is declared in @file{sched.h}.
@end deftypefun

Note that it is not portably possible to use this information to
retrieve the information for different POSIX threads.  A separate
interface must be provided for that.

@comment sched.h
@comment GNU
@deftypefun int sched_setaffinity (pid_t @var{pid}, size_t @var{cpusetsize}, const cpu_set_t *@var{cpuset})

This function installs the @var{cpusetsize} bytes long affinity mask
pointed to by @var{cpuset} for the process or thread with the ID @var{pid}.
If successful the function returns zero and the scheduler will in future
take the affinity information into account.

If the function fails it will return @code{-1} and @code{errno} is set
to the error code:

@table @code
@item ESRCH
No process or thread with the given ID found.

@item EFAULT
The pointer @var{cpuset} is does not point to a valid object.

@item EINVAL
The bitset is not valid.  This might mean that the affinity set might
not leave a processor for the process or thread to run on.
@end table

This function is a GNU extension and is declared in @file{sched.h}.
@end deftypefun


@node Memory Resources
@section Querying memory available resources

The amount of memory available in the system and the way it is organized
determines oftentimes the way programs can and have to work.  For
functions like @code{mmap} it is necessary to know about the size of
individual memory pages and knowing how much memory is available enables
a program to select appropriate sizes for, say, caches.  Before we get
into these details a few words about memory subsystems in traditional
Unix systems will be given.

@menu
* Memory Subsystem::           Overview about traditional Unix memory handling.
* Query Memory Parameters::    How to get information about the memory
                                subsystem?
@end menu

@node Memory Subsystem
@subsection Overview about traditional Unix memory handling

@cindex address space
@cindex physical memory
@cindex physical address
Unix systems normally provide processes virtual address spaces.  This
means that the addresses of the memory regions do not have to correspond
directly to the addresses of the actual physical memory which stores the
data.  An extra level of indirection is introduced which translates
virtual addresses into physical addresses.  This is normally done by the
hardware of the processor.

@cindex shared memory
Using a virtual address space has several advantage.  The most important
is process isolation.  The different processes running on the system
cannot interfere directly with each other.  No process can write into
the address space of another process (except when shared memory is used
but then it is wanted and controlled).

Another advantage of virtual memory is that the address space the
processes see can actually be larger than the physical memory available.
The physical memory can be extended by storage on an external media
where the content of currently unused memory regions is stored.  The
address translation can then intercept accesses to these memory regions
and make memory content available again by loading the data back into
memory.  This concept makes it necessary that programs which have to use
lots of memory know the difference between available virtual address
space and available physical memory.  If the working set of virtual
memory of all the processes is larger than the available physical memory
the system will slow down dramatically due to constant swapping of
memory content from the memory to the storage media and back.  This is
called ``thrashing''.
@cindex thrashing

@cindex memory page
@cindex page, memory
A final aspect of virtual memory which is important and follows from
what is said in the last paragraph is the granularity of the virtual
address space handling.  When we said that the virtual address handling
stores memory content externally it cannot do this on a byte-by-byte
basis.  The administrative overhead does not allow this (leaving alone
the processor hardware).  Instead several thousand bytes are handled
together and form a @dfn{page}.  The size of each page is always a power
of two byte.  The smallest page size in use today is 4096, with 8192,
16384, and 65536 being other popular sizes.

@node Query Memory Parameters
@subsection How to get information about the memory subsystem?

The page size of the virtual memory the process sees is essential to
know in several situations.  Some programming interface (e.g.,
@code{mmap}, @pxref{Memory-mapped I/O}) require the user to provide
information adjusted to the page size.  In the case of @code{mmap} is it
necessary to provide a length argument which is a multiple of the page
size.  Another place where the knowledge about the page size is useful
is in memory allocation.  If one allocates pieces of memory in larger
chunks which are then subdivided by the application code it is useful to
adjust the size of the larger blocks to the page size.  If the total
memory requirement for the block is close (but not larger) to a multiple
of the page size the kernel's memory handling can work more effectively
since it only has to allocate memory pages which are fully used.  (To do
this optimization it is necessary to know a bit about the memory
allocator which will require a bit of memory itself for each block and
this overhead must not push the total size over the page size multiple.

The page size traditionally was a compile time constant.  But recent
development of processors changed this.  Processors now support
different page sizes and they can possibly even vary among different
processes on the same system.  Therefore the system should be queried at
runtime about the current page size and no assumptions (except about it
being a power of two) should be made.

@vindex _SC_PAGESIZE
The correct interface to query about the page size is @code{sysconf}
(@pxref{Sysconf Definition}) with the parameter @code{_SC_PAGESIZE}.
There is a much older interface available, too.

@comment unistd.h
@comment BSD
@deftypefun int getpagesize (void)
The @code{getpagesize} function returns the page size of the process.
This value is fixed for the runtime of the process but can vary in
different runs of the application.

The function is declared in @file{unistd.h}.
@end deftypefun

Widely available on @w{System V} derived systems is a method to get
information about the physical memory the system has.  The call

@vindex _SC_PHYS_PAGES
@cindex sysconf
@smallexample
  sysconf (_SC_PHYS_PAGES)
@end smallexample

@noindent
returns the total number of pages of physical the system has.
This does not mean all this memory is available.  This information can
be found using

@vindex _SC_AVPHYS_PAGES
@cindex sysconf
@smallexample
  sysconf (_SC_AVPHYS_PAGES)
@end smallexample

These two values help to optimize applications.  The value returned for
@code{_SC_AVPHYS_PAGES} is the amount of memory the application can use
without hindering any other process (given that no other process
increases its memory usage).  The value returned for
@code{_SC_PHYS_PAGES} is more or less a hard limit for the working set.
If all applications together constantly use more than that amount of
memory the system is in trouble.

The GNU C library provides in addition to these already described way to
get this information two functions.  They are declared in the file
@file{sys/sysinfo.h}.  Programmers should prefer to use the
@code{sysconf} method described above.

@comment sys/sysinfo.h
@comment GNU
@deftypefun {long int} get_phys_pages (void)
The @code{get_phys_pages} function returns the total number of pages of
physical the system has.  To get the amount of memory this number has to
be multiplied by the page size.

This function is a GNU extension.
@end deftypefun

@comment sys/sysinfo.h
@comment GNU
@deftypefun {long int} get_avphys_pages (void)
The @code{get_phys_pages} function returns the number of available pages of
physical the system has.  To get the amount of memory this number has to
be multiplied by the page size.

This function is a GNU extension.
@end deftypefun

@node Processor Resources
@section Learn about the processors available

The use of threads or processes with shared memory allows an application
to take advantage of all the processing power a system can provide.  If
the task can be parallelized the optimal way to write an application is
to have at any time as many processes running as there are processors.
To determine the number of processors available to the system one can
run

@vindex _SC_NPROCESSORS_CONF
@cindex sysconf
@smallexample
  sysconf (_SC_NPROCESSORS_CONF)
@end smallexample

@noindent
which returns the number of processors the operating system configured.
But it might be possible for the operating system to disable individual
processors and so the call

@vindex _SC_NPROCESSORS_ONLN
@cindex sysconf
@smallexample
  sysconf (_SC_NPROCESSORS_ONLN)
@end smallexample

@noindent
returns the number of processors which are currently inline (i.e.,
available).

For these two pieces of information the GNU C library also provides
functions to get the information directly.  The functions are declared
in @file{sys/sysinfo.h}.

@comment sys/sysinfo.h
@comment GNU
@deftypefun int get_nprocs_conf (void)
The @code{get_nprocs_conf} function returns the number of processors the
operating system configured.

This function is a GNU extension.
@end deftypefun

@comment sys/sysinfo.h
@comment GNU
@deftypefun int get_nprocs (void)
The @code{get_nprocs} function returns the number of available processors.

This function is a GNU extension.
@end deftypefun

@cindex load average
Before starting more threads it should be checked whether the processors
are not already overused.  Unix systems calculate something called the
@dfn{load average}.  This is a number indicating how many processes were
running.  This number is average over different periods of times
(normally 1, 5, and 15 minutes).

@comment stdlib.h
@comment BSD
@deftypefun int getloadavg (double @var{loadavg}[], int @var{nelem})
This function gets the 1, 5 and 15 minute load averages of the
system. The values are placed in @var{loadavg}.  @code{getloadavg} will
place at most @var{nelem} elements into the array but never more than
three elements.  The return value is the number of elements written to
@var{loadavg}, or -1 on error.

This function is declared in @file{stdlib.h}.
@end deftypefun