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List: asterisk-dev
Subject: Re: [asterisk-dev] [Code Review] 3405: Add ast_spinlock capability to lock.h
From: "George Joseph" <reviewboard () asterisk ! org>
Date: 2014-03-28 16:56:18
Message-ID: 20140328165618.21712.99284 () sonic ! digium ! api
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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 28, 2014, 10:56 a.m.)
Review request for Asterisk Developers.
Changes
-------
Added link to test harness and test results.
Bugs: ASTERISK-23553
https://issues.asterisk.org/jira/browse/ASTERISK-23553
Repository: Asterisk
Description (updated)
-------
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...
Test harness: https://github.com/gtjoseph/spintest
Results are in spintest.csv
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
-----
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
-------
Thanks,
George Joseph
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This is an automatically generated e-mail. To reply, visit:
<a href="https://reviewboard.asterisk.org/r/3405/">https://reviewboard.asterisk.org/r/3405/</a>
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<div>Review request for Asterisk Developers.</div>
<div>By George Joseph.</div>
<p style="color: grey;"><i>Updated March 28, 2014, 10:56 a.m.</i></p>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Changes</h1>
<table width="100%" bgcolor="#ffffff" cellspacing="0" cellpadding="10" style="border: \
1px solid #b8b5a0"> <tr>
<td>
<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;">Added link to test harness and test results.</pre> </td>
</tr>
</table>
<div style="margin-top: 1.5em;">
<b style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Bugs: </b>
<a href="https://issues.asterisk.org/jira/browse/ASTERISK-23553">ASTERISK-23553</a>
</div>
<div style="margin-top: 1.5em;">
<b style="color: #575012; font-size: 10pt;">Repository: </b>
Asterisk
</div>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description \
(updated)</h1> <table width="100%" bgcolor="#ffffff" cellspacing="0" \
cellpadding="10" style="border: 1px solid #b8b5a0"> <tr>
<td>
<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: \
-moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: \
break-word;">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...
Test harness: https://github.com/gtjoseph/spintest
Results are in spintest.csv
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.
</pre>
</td>
</tr>
</table>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
<ul style="margin-left: 3em; padding-left: 0;">
<li>branches/12/include/asterisk/lock.h <span style="color: \
grey">(411364)</span></li>
<li>branches/12/include/asterisk/autoconfig.h.in <span style="color: \
grey">(411364)</span></li>
<li>branches/12/configure.ac <span style="color: grey">(411364)</span></li>
<li>branches/12/configure <span style="color: grey">(UNKNOWN)</span></li>
</ul>
<p><a href="https://reviewboard.asterisk.org/r/3405/diff/" style="margin-left: \
3em;">View Diff</a></p>
</td>
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</table>
</div>
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