Reader-writer lock #3786
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Reader-writer lock #3786
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Not strictly necessary to check in but anyone who wants to run the benchmark will appreciate it.
uuid1 just seems more liable to cause a collision for minimal benefit.
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Description of change
This implements the reader-write lock requested in #2372. It is a pretty sizeable standalone feature that may take a while to review.
The interface and implementation I think are explained decently well in docstrings and comments. Briefly: the lock state is stored in three keys:
<prefix>:write: Standard mutex lease<prefix>:write_semaphore: Semaphore that tracks waiting writers, implemented as an ordered set.<prefix>:read: Semaphore that tracks readers.I tried to keep the interface (and implementation) similar to the existing
Lockclass while taking inspiration from the Rust and C++ standard libraries for the new bits. I got rid of thread-local state since each read/write guard now generates its own token.Other than supporting multiple readers, the only really "new" feature is the
max_writersparameter, which can be used to cap the number of waiting writers. Typically I expect most users would only choose 0/None (unlimited) or 1 (guarantee single writer).Benchmark
I included a benchmark that simulates a group of workers sharing the lock for reading and writing. Perhaps it doesn't measure numbers that are super meaningful to optimize, but it shows that the lock demonstrates the expected behavior, and you can run parameter sweeps to see how it handles different levels of contention.
I added dev dependencies on pandas (for printing stats) and matplotlib (for reproducing graphs). matplotlib is not needed to run the benchmark, only to visualize the time series output after a run.
Sample run output
duration = 5, seed = 0, num_workers = 2, wr_ratio = 0.01, io_duration = 0.025, max_writers = 0 iops: 79.60 min mean p95 max read_acquire_time 0.09ms 0.53ms 0.20ms 76.15ms read_release_time 0.07ms 0.11ms 0.15ms 0.24ms write_acquire_time 25.46ms 33.87ms 48.12ms 50.63ms write_release_time 0.14ms 0.15ms 0.16ms 0.16ms success timeout aborted read_acquire_status 395 0 0 write_acquire_status 3 0 0duration = 5, seed = 0, num_workers = 2, wr_ratio = 0.01, io_duration = 0.025, max_writers = 1 iops: 74.00 min mean p95 max read_acquire_time 0.09ms 0.62ms 0.19ms 75.74ms read_release_time 0.07ms 0.10ms 0.13ms 0.30ms write_acquire_time 25.35ms 25.38ms 25.40ms 25.41ms write_release_time 0.10ms 0.11ms 0.13ms 0.13ms success timeout aborted read_acquire_status 367 0 0 write_acquire_status 3 0 0
duration = 5, seed = 0, num_workers = 2, wr_ratio = 0.1, io_duration = 0.025, max_writers = 0 iops: 59.60 min mean p95 max read_acquire_time 0.10ms 5.82ms 50.63ms 151.60ms read_release_time 0.07ms 0.11ms 0.14ms 0.19ms write_acquire_time 0.15ms 27.89ms 50.73ms 50.84ms write_release_time 0.11ms 0.15ms 0.18ms 0.20ms success timeout aborted read_acquire_status 267 0 0 write_acquire_status 31 0 0
duration = 5, seed = 0, num_workers = 2, wr_ratio = 0.1, io_duration = 0.025, max_writers = 1 iops: 58.80 min mean p95 max read_acquire_time 0.09ms 5.52ms 50.63ms 126.46ms read_release_time 0.07ms 0.11ms 0.14ms 0.22ms write_acquire_time 0.13ms 27.85ms 63.30ms 75.91ms write_release_time 0.08ms 0.13ms 0.19ms 0.34ms success timeout aborted read_acquire_status 263 0 0 write_acquire_status 31 0 0
duration = 5, seed = 0, num_workers = 4, wr_ratio = 0.01, io_duration = 0.025, max_writers = 0 iops: 137.40 min mean p95 max read_acquire_time 0.09ms 2.49ms 0.26ms 151.62ms read_release_time 0.07ms 0.11ms 0.15ms 0.27ms write_acquire_time 25.41ms 60.28ms 108.79ms 126.30ms write_release_time 0.13ms 0.15ms 0.17ms 0.17ms success timeout aborted read_acquire_status 679 0 0 write_acquire_status 8 0 0
duration = 5, seed = 0, num_workers = 4, wr_ratio = 0.01, io_duration = 0.025, max_writers = 1 iops: 148.60 min mean p95 max read_acquire_time 0.09ms 1.58ms 0.23ms 75.89ms read_release_time 0.06ms 0.11ms 0.14ms 0.30ms write_acquire_time 25.32ms 38.05ms 50.77ms 50.78ms write_release_time 0.09ms 0.12ms 0.13ms 0.13ms success timeout aborted read_acquire_status 737 0 0 write_acquire_status 6 0 0
duration = 5, seed = 0, num_workers = 4, wr_ratio = 0.1, io_duration = 0.025, max_writers = 0 iops: 73.20 min mean p95 max read_acquire_time 0.09ms 23.85ms 101.31ms 278.17ms read_release_time 0.07ms 0.11ms 0.15ms 0.28ms write_acquire_time 0.14ms 50.70ms 101.35ms 152.18ms write_release_time 0.09ms 0.14ms 0.19ms 0.20ms success timeout aborted read_acquire_status 323 0 0 write_acquire_status 43 0 0
duration = 5, seed = 0, num_workers = 4, wr_ratio = 0.1, io_duration = 0.025, max_writers = 1 iops: 86.40 min mean p95 max read_acquire_time 0.09ms 18.82ms 126.28ms 202.43ms read_release_time 0.07ms 0.12ms 0.15ms 0.28ms write_acquire_time 0.12ms 41.44ms 126.43ms 126.59ms write_release_time 0.09ms 0.14ms 0.19ms 0.20ms success timeout aborted read_acquire_status 391 0 0 write_acquire_status 41 0 9
duration = 5, seed = 0, num_workers = 8, wr_ratio = 0.01, io_duration = 0.025, max_writers = 0 iops: 229.60 min mean p95 max read_acquire_time 0.09ms 7.51ms 50.78ms 176.95ms read_release_time 0.06ms 0.12ms 0.18ms 0.49ms write_acquire_time 25.45ms 85.16ms 126.53ms 126.55ms write_release_time 0.11ms 0.16ms 0.20ms 0.21ms success timeout aborted read_acquire_status 1137 0 0 write_acquire_status 11 0 0
duration = 5, seed = 0, num_workers = 8, wr_ratio = 0.01, io_duration = 0.025, max_writers = 1 iops: 263.20 min mean p95 max read_acquire_time 0.09ms 4.76ms 50.56ms 151.84ms read_release_time 0.07ms 0.11ms 0.17ms 0.50ms write_acquire_time 50.53ms 71.33ms 113.95ms 126.63ms write_release_time 0.10ms 0.13ms 0.16ms 0.16ms success timeout aborted read_acquire_status 1305 0 0 write_acquire_status 11 0 1
duration = 5, seed = 0, num_workers = 8, wr_ratio = 0.1, io_duration = 0.025, max_writers = 0 iops: 107.40 min mean p95 max read_acquire_time 0.10ms 45.93ms 201.72ms 302.87ms read_release_time 0.07ms 0.12ms 0.16ms 0.32ms write_acquire_time 0.12ms 64.21ms 151.78ms 227.21ms write_release_time 0.09ms 0.14ms 0.18ms 0.21ms success timeout aborted read_acquire_status 479 0 0 write_acquire_status 58 0 0
duration = 5, seed = 0, num_workers = 8, wr_ratio = 0.1, io_duration = 0.025, max_writers = 1 iops: 112.00 min mean p95 max read_acquire_time 0.10ms 44.48ms 176.69ms 303.13ms read_release_time 0.07ms 0.12ms 0.16ms 0.48ms write_acquire_time 0.14ms 59.12ms 125.25ms 126.61ms write_release_time 0.10ms 0.15ms 0.18ms 0.23ms success timeout aborted read_acquire_status 518 0 0 write_acquire_status 42 0 14