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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 April 19, 2014, 10:57 a.m.)
Review request for Asterisk Developers, Corey Farrell, Joshua Colp, and
rmudgett.
Changes
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Fixed a problem including compiler.h
Bugs: ASTERISK-23553
https://issues.asterisk.org/jira/browse/ASTERISK-23553
Repository: Asterisk
Description
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Now ready for prime time...
This patch adds support for spinlocks in Asterisk.
There are cases in Asterisk where it might be desirable to lock a short
critical code section but not incur the context switch and yield penalty of a
mutex or rwlock. The primary spinlock implementations execute exclusively in
userspace and therefore don't incur those penalties. Spinlocks are NOT meant
to be a general replacement for mutexes. They should be used only for
protecting short blocks of critical code such as simple compares and
assignments. Operations that may block, hold a lock, or cause the thread to
give up it's timeslice should NEVER be attempted in a spinlock.
The first use case for spinlocks is in astobj2 - internal_ao2_ref. Currently
the manipulation of the reference counter is done with an
ast_atomic_fetchadd_int which works fine. When weak reference containers are
introduced however, there's an additional comparison and assignment that'll
need to be done while the lock is held. A mutex would be way too expensive
here, hence the spinlock. Given that lock contention in this situation would
be infrequent, the overhead of the spinlock is only a few more machine
instructions than the current ast_atomic_fetchadd_int call.
7 implementations were tested on various i366, x86_64, ARM and Sparc platforms
running Linux, FreeBSD, OSX and Solaris. The test suite and results can be
found here... https://github.com/gtjoseph/spintest
Summary...
Tested various spinlock implementations.
+ GCC Atomics gcc_atomics
+ x86 assembly gas_x86
+ ARM assembly gas_arm
+ Sparc assembly gas_sparc
+ OSX Atomics osx_atomics
+ pthreads spinlock pthread_spinlock
+ pthreads mutex pthread_mutex
+ pthreads adaptive mutex pthread_mutex_adaptive_np
Not all implementations were available on all platforms. For instance, the
gas_x86 implementation won't be available on an arm or sparc platform.
Each implementation was tested with the same locking scenario which is a short
block of compares and assignments representing the most anticipated Asterisk
use cases. The test case was run in a tight loop of 25,000,000 iterations
executed in parallel by 1..5 simultaneous threads.
The test suite was run on the following platforms:
+ Darwin-OSX-x86_64-2core
+ FreeBSD-BSD-amd64-4core
+ FreeBSD-BSD-i386-1core
+ Linux-CentOS-i686-1core
+ Linux-Debian-i686-4core
+ Linux-Debian-x86_64-4core
+ Linux-Fedora-armv7l-1core
+ Linux-Fedora-i686-1core
+ Linux-Fedora-x86_64-12core
+ Linux-Fedora-x86_64-1core
+ Linux-Fedora-x86_64-2core
+ Linux-Fedora-x86_64-4core
+ Linux-Fedora-x86_64-8core
+ Linux-Gentoo-sparc64-32core
+ SunOS-Solaris-i86pc-4core
For each platform tested, the real, user and system times were captured in csv
form and the final real and system times graphed.
Observations:
+ The GCC Atomics implementation was available on all platforms and generally
had the best performance.
+ The native assembly implementations generally performed on par with the GCC
Atomics implementation.
+ The pthread_spinlock implementation was not available on Darwin but performed
well on the other platforms.
+ The OSX Atomics implementation performed well on Darwin.
+ The pthread_mutex_adaptive implementation was not available on all platforms
and it's performance was noticeably worse on the supported platforms.
+ The pthread_mutex implementation was available on all platforms but clearly
showed the effects of context switching (high sys time) as lock contention grew.
Conclusions:
+ GCC Atomics should be the first implementation choice.
+ The native assembly implementations should be the second choices.
+ The pthread_spinlock implementation should be the third choice on non-Darwin
platforms.
+ OSX Atomics should be the third choice on Darwin.
+ The pthread_mutex_adaptive implementation should be eliminated.
+ The pthread_mutex implementation should be the implementation of last resort.
+ If none of the implementations are available, a compilation #error will be
raised.
Diffs (updated)
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branches/12/include/asterisk/spinlock.h PRE-CREATION
branches/12/include/asterisk/autoconfig.h.in 412427
branches/12/configure.ac 412427
branches/12/configure UNKNOWN
Diff: https://reviewboard.asterisk.org/r/3405/diff/
Testing
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See above.
Thanks,
George Joseph
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