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This is an automatically generated e-mail. To reply, visit:
https://reviewboard.asterisk.org/r/3405/
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(Updated March 27, 2014, 2:57 p.m.)
Review request for Asterisk Developers.
Changes
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EDIT: I forgot to mention I'm working on a GAS fallback.
Bugs: ASTERISK-23553
https://issues.asterisk.org/jira/browse/ASTERISK-23553
Repository: Asterisk
Description (updated)
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Still testing but I'd like some initial feedback.
In some circumstances the atomic fetch/add/test calls are not quite flexible
enough but a full fledged mutex or rwlock is too expensive.
Spin locks are a good solution. They should be used only for protecting very
short blocks of critical code such as simple compares combined with integer
math. Operations that may block, hold a lock, or cause the thread to give up
it's timeslice should NEVER be attempted in a spin lock.
So,
Add the following APIs to lock.h
ast_spinlock_init
ast_spinlock_lock
ast_spinlock_trylock
ast_spinlock_unlock
Depending on the capabilities determined by configure, the following
implementations will be chosen in order of preference... OSX Atomics (for OSX
only), GCC Atomics, Pthread Spinlock and as a final fallback.. Pthread Mutex.
Performance...
Simple test 25,000,000 iterations of (lock, test, calculate, test, unlock) per
thread.
All times are milliseconds. I have no way to test OSX Atomics.
gcc_atomics Threads: 1 Real: 204, User: 20, Sys: 0, Tot: 20
pthread_spinlock Threads: 1 Real: 194, User: 19, Sys: 0, Tot: 19
pthread_mutex Threads: 1 Real: 464, User: 46, Sys: 0, Tot: 46
pthread_adaptive Threads: 1 Real: 464, User: 46, Sys: 0, Tot: 46
gcc_atomics Threads: 2 Real: 1016, User: 191, Sys: 0, Tot: 191
pthread_spinlock Threads: 2 Real: 1142, User: 209, Sys: 0, Tot: 209
pthread_mutex Threads: 2 Real: 2902, User: 362, Sys: 201, Tot: 563
pthread_adaptive Threads: 2 Real: 4557, User: 896, Sys: 9, Tot: 905
gcc_atomics Threads: 3 Real: 2273, User: 556, Sys: 0, Tot: 556
pthread_spinlock Threads: 3 Real: 2318, User: 589, Sys: 0, Tot: 589
pthread_mutex Threads: 3 Real: 6257, User: 834, Sys: 947, Tot: 1781
pthread_adaptive Threads: 3 Real: 12121, User: 3368, Sys: 177, Tot: 3545
gcc_atomics Threads: 4 Real: 3439, User: 1025, Sys: 0, Tot: 1025
pthread_spinlock Threads: 4 Real: 3370, User: 1267, Sys: 0, Tot: 1267
pthread_mutex Threads: 4 Real: 10111, User: 851, Sys: 2901, Tot: 3752
pthread_adaptive Threads: 4 Real: 18888, User: 6317, Sys: 845, Tot: 7162
pthread adaptive mutexes are supposed to give you the best of both spin and
mutex lock but testing shows that in this scenario, it's always worse than
mutex. Although it does have less kernel time than plain mutexes, I removed
the implementation from this patch.
pthread_mutex is universally supported but shows the effect of context
switching when there's lock contention. It's the last resort and maybe should
be removed in favor of a #error stating that no spinlock implementation could
be found.
pthread_spinlock is gaining support but is not in all pthread implementations
(OSX for one). No kernel time at all.
gcc_atomics is also gaining support and seems to be more widely supported than
pthread_spinlock. No kernel time at all.
With infrequent lock contention, both gcc_atomics and pthread_spinlock are
comparable to ast_atomic_fetchadd_int in performance.
Although I don't have any empirical data to back me up (yet), I believe with
osx_atomics, gcc_atomics and pthread_spinlock all major platforms are supported.
EDIT: I forgot to mention I'm working on a GAS fallback.
Diffs
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branches/12/include/asterisk/lock.h 411364
branches/12/include/asterisk/autoconfig.h.in 411364
branches/12/configure.ac 411364
branches/12/configure UNKNOWN
Diff: https://reviewboard.asterisk.org/r/3405/diff/
Testing
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Thanks,
George Joseph
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