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When mutiple threads waiting for lock at the same time, once lock owner
releases the lock, waiters will see lock available and all try to lock,
which may cause an expensive CAS storm.
Binary exponential backoff with random jitter is introduced. As try-lock
attempt increases, there is more likely that a larger number threads
compete for adaptive mutex lock, so increase wait time in exponential.
A random jitter is also added to avoid synchronous try-lock from other
threads.
v2: Remove read-check before try-lock for performance.
v3:
1. Restore read-check since it works well in some platform.
2. Make backoff arch dependent, and enable it for x86_64.
3. Limit max backoff to reduce latency in large critical section.
v4: Fix strict-prototypes error in sysdeps/nptl/pthread_mutex_backoff.h
v5: Commit log updated for regression in large critical section.
Result of pthread-mutex-locks bench
Test Platform: Xeon 8280L (2 socket, 112 CPUs in total)
First Row: thread number
First Col: critical section length
Values: backoff vs upstream, time based, low is better
non-critical-length: 1
1 2 4 8 16 32 64 112 140
0 0.99 0.58 0.52 0.49 0.43 0.44 0.46 0.52 0.54
1 0.98 0.43 0.56 0.50 0.44 0.45 0.50 0.56 0.57
2 0.99 0.41 0.57 0.51 0.45 0.47 0.48 0.60 0.61
4 0.99 0.45 0.59 0.53 0.48 0.49 0.52 0.64 0.65
8 1.00 0.66 0.71 0.63 0.56 0.59 0.66 0.72 0.71
16 0.97 0.78 0.91 0.73 0.67 0.70 0.79 0.80 0.80
32 0.95 1.17 0.98 0.87 0.82 0.86 0.89 0.90 0.90
64 0.96 0.95 1.01 1.01 0.98 1.00 1.03 0.99 0.99
128 0.99 1.01 1.01 1.17 1.08 1.12 1.02 0.97 1.02
non-critical-length: 32
1 2 4 8 16 32 64 112 140
0 1.03 0.97 0.75 0.65 0.58 0.58 0.56 0.70 0.70
1 0.94 0.95 0.76 0.65 0.58 0.58 0.61 0.71 0.72
2 0.97 0.96 0.77 0.66 0.58 0.59 0.62 0.74 0.74
4 0.99 0.96 0.78 0.66 0.60 0.61 0.66 0.76 0.77
8 0.99 0.99 0.84 0.70 0.64 0.66 0.71 0.80 0.80
16 0.98 0.97 0.95 0.76 0.70 0.73 0.81 0.85 0.84
32 1.04 1.12 1.04 0.89 0.82 0.86 0.93 0.91 0.91
64 0.99 1.15 1.07 1.00 0.99 1.01 1.05 0.99 0.99
128 1.00 1.21 1.20 1.22 1.25 1.31 1.12 1.10 0.99
non-critical-length: 128
1 2 4 8 16 32 64 112 140
0 1.02 1.00 0.99 0.67 0.61 0.61 0.61 0.74 0.73
1 0.95 0.99 1.00 0.68 0.61 0.60 0.60 0.74 0.74
2 1.00 1.04 1.00 0.68 0.59 0.61 0.65 0.76 0.76
4 1.00 0.96 0.98 0.70 0.63 0.63 0.67 0.78 0.77
8 1.01 1.02 0.89 0.73 0.65 0.67 0.71 0.81 0.80
16 0.99 0.96 0.96 0.79 0.71 0.73 0.80 0.84 0.84
32 0.99 0.95 1.05 0.89 0.84 0.85 0.94 0.92 0.91
64 1.00 0.99 1.16 1.04 1.00 1.02 1.06 0.99 0.99
128 1.00 1.06 0.98 1.14 1.39 1.26 1.08 1.02 0.98
There is regression in large critical section. But adaptive mutex is
aimed for "quick" locks. Small critical section is more common when
users choose to use adaptive pthread_mutex.
Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
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