Patchwork [5/7] Add manual for lock elision

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Submitter Andi Kleen
Date Dec. 19, 2014, 7:37 a.m.
Message ID <1418974667-32587-6-git-send-email-andi@firstfloor.org>
Download mbox | patch
Permalink /patch/4357/
State New
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Andi Kleen - Dec. 19, 2014, 7:37 a.m.
From: Andi Kleen <ak@linux.intel.com>

This adds the original manual (from the original code submission)
for elision, with a description of all the new tuning options.

2014-12-17  Andi Kleen  <ak@linux.intel.com>

	* manual/Makefile: Add elision.texi.
	* manual/threads.texi: Link to elision.
	* manual/elision.texi: New file.
	* manual/intro.texi: Link to elision.
	* manual/lang.texi: dito.
---
 manual/Makefile     |   3 +-
 manual/elision.texi | 264 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 manual/intro.texi   |   3 +
 manual/lang.texi    |   2 +-
 4 files changed, 270 insertions(+), 2 deletions(-)
 create mode 100644 manual/elision.texi

Patch

diff --git a/manual/Makefile b/manual/Makefile
index 1f481f2..dccebfe 100644
--- a/manual/Makefile
+++ b/manual/Makefile
@@ -38,7 +38,8 @@  chapters = $(addsuffix .texi, \
 		       message search pattern io stdio llio filesys	\
 		       pipe socket terminal syslog math arith time	\
 		       resource setjmp signal startup process ipc job	\
-		       nss users sysinfo conf crypt debug threads probes)
+		       nss users sysinfo conf crypt debug threads probes \
+		       elision)
 add-chapters = $(wildcard $(foreach d, $(add-ons), ../$d/$d.texi))
 appendices = lang.texi header.texi install.texi maint.texi platform.texi \
 	     contrib.texi
diff --git a/manual/elision.texi b/manual/elision.texi
new file mode 100644
index 0000000..8f0b3e5
--- /dev/null
+++ b/manual/elision.texi
@@ -0,0 +1,264 @@ 
+@node Lock elision, Language Features, POSIX Threads, Top
+@c %MENU% Lock elision
+@chapter Lock elision
+
+@c create the bizarre situation that lock elision is documented, but pthreads isn't
+
+This chapter describes the elided lock implementation for POSIX thread locks.
+
+@menu
+* Lock elision introduction::	What is lock elision?
+* Semantic differences of elided locks::
+* Tuning lock elision::
+* Setting elision for individual @code{pthread_mutex_t}::
+* Setting @code{pthread_mutex_t} elision using environment variables::
+* Setting @code{pthread_rwlock_t} elision using environment variables::
+@end menu 
+
+@node Lock elision introduction
+@section Lock elision introduction
+
+Lock elision is a technique to improve lock scaling. It runs
+lock regions in parallel using hardware support for a transactional execution
+mode. The lock region is executed speculatively, and as long
+as there is no conflict or other reason for transaction abort the lock
+will executed in parallel. If an transaction abort occurs, any 
+side effect of the speculative execution is undone, the lock is taken
+for real and the lock region re-executed. This improves scalability
+of the program because locks do not need to wait for each other.
+
+The standard @code{pthread_mutex_t} mutexes and @code{pthread_rwlock_t} rwlocks
+can be transparently elided by @theglibc{}.
+
+Lock elision may lower performance if transaction aborts occur too frequently.
+In this case it is recommended to use a PMU profiler to find the causes for
+the aborts first and try to eliminate them. If that is not possible
+elision can be disabled for a specific lock or for the whole program.
+Alternatively elision can be disabled completely, and only enabled for 
+specific locks that are known to be elision friendly.
+
+The defaults locks are adaptive. The library decides whether elision
+is profitable based on the abort rates, and automatically disables
+elision for a lock when it aborts too often. After some time elision
+is re-tried, in case the workload changed.
+
+Lock elision is currently supported for default (timed) mutexes, and
+rwlocks.  Other lock types (including @code{PTHREAD_MUTEX_NORMAL}) do not elide.
+Condition variables also do not elide. This may change in future versions.
+
+@node Semantic differences of elided locks
+@section Semantic differences of elided locks
+
+Elided locks have some semantic (visible) differences to classic locks. These differences 
+are only visible when the lock is successfully elided. Since elision may always
+fail a program cannot rely on any of these semantics.
+
+@itemize
+@item
+timedlocks may not time out.
+
+@smallexample
+pthread_mutex_lock (&lock);
+if (pthread_mutex_timedlock (&lock, &timeout) == 0) 
+     /* With elision we always come here */
+else
+     /* With no elision we always come here because timeout happens. */
+@end smallexample
+
+Similar semantic changes apply to @code{pthread_rwlock_trywrlock} and
+@code{pthread_rwlock_timedwrlock}.
+
+A program like
+
+@smallexample
+/* lock is not a recursive lock type */
+pthread_mutex_lock (&lock);
+/* Relock same lock in same thread */
+pthread_mutex_lock (&lock);
+@end smallexample
+
+will immediately hang on the second lock (dead lock) without elision. With
+elision the deadlock will only happen on an abort, which can happen
+early or could happen later, but will likely not happen every time.
+
+This behavior is allowed in POSIX for @code{PTHREAD_MUTEX_DEFAULT}, but not for
+@code{PTHREAD_MUTEX_NORMAL}. When @code{PTHREAD_MUTEX_NORMAL} is
+set for a mutex using @code{pthread_mutexattr_settype} elision is implicitly
+disabled.  Note that @code{PTHREAD_MUTEX_INITIALIZER} sets a
+@code{PTHREAD_MUTEX_DEFAULT} type, thus allows elision.
+
+Depending on the ABI version @theglibc{} may not distinguish between
+@code{PTHREAD_MUTEX_NORMAL} and @code{PTHREAD_MUTEX_DEFAULT}, as they may
+have the same numerical value. If that is the case any call to
+@code{pthread_mutexattr_settype} with either type will disable elision.
+
+@item
+@code{pthread_mutex_destroy} does not return an error when the lock is locked
+and will clear the lock state.
+
+@item
+@code{pthread_mutex_t} and @code{pthread_rwlock_t} appear free from other threads.
+
+This can be visible through trylock or timedlock.
+In most cases checking this is a existing latent race in the program, but there may 
+be cases when it is not.
+
+@item
+@code{EAGAIN} and @code{EDEADLK} in rwlocks will not happen under elision.
+
+@item
+@code{pthread_mutex_unlock} does not return an error when unlocking a free lock.
+
+@item
+Elision changes timing because locks now run in parallel.
+Timing differences may expose latent race bugs in the program. Programs using time based synchronization
+(as opposed to using data dependencies) may change behavior.
+
+@end itemize
+
+@node Tuning lock elision
+@section Tuning lock elision
+
+Critical regions may need some tuning to get the benefit of lock elision.
+This is based on the abort rates, which can be determined by a PMU profiler
+(e.g. perf on @gnulinuxsystems{}). When the abort rate is too high lock
+scaling will not improve. Generally lock elision feedback should be done
+only based on profile feedback.
+
+Most of these optimizations will improve performance even without lock elision
+because they will minimize cache line bouncing between threads or make
+lock regions smaller.
+
+Common causes of transactional aborts:
+
+@itemize
+@item 
+Not elidable operations like system calls, IO, CPU exceptions.
+
+Try to move out of the critical section when common. Note that these often happen at program startup only.
+@item
+Global statistic counts
+
+Global statistic variables tend to cause conflicts. Either disable, or make per thread or as a last resort sample
+(not update every operation)
+@item
+False sharing of variables or data structures causing conflicts with other threads
+
+Add padding as needed.
+@item 
+Other conflicts on the same cache lines with other threads
+
+Minimize conflicts with other threads. This may require changes to the data structures.
+@item
+Capacity overflow
+
+The memory transaction used for lock elision has a limited capacity. Make the critical region smaller
+or move operations that do not need to be protected by the lock outside.
+
+@item
+Rewriting already set flags
+
+Setting flags or variables in shared objects that are already set may cause conflicts. Add a check
+to only write when the value changed.
+
+@item
+Using @code{pthread_mutex_trylock} or @code{pthread_rwlock_trywrlock}
+nested in another elided lock.
+
+@end itemize
+
+@node Setting elision for individual @code{pthread_mutex_t}
+@section Setting elision for individual @code{pthread_mutex_t}
+
+Elision can be only disabled for each @code{pthread_mutex_t} in the program
+by setting its type using @code{pthread_mutex_settype_np} to @code{PTHREAD_MUTEX_NORMAL}.
+
+/* Force no lock elision for a mutex */
+pthread_mutexattr_t attr;
+pthread_mutexattr_init (&attr);
+pthread_mutexattr_settype_np (&attr, PTHREAD_MUTEX_NORMAL);
+pthread_mutex_init (&object->mylock, &attr);
+@end smallexample
+
+@node Setting @code{pthread_mutex_t} elision using environment variables
+@section Setting @code{pthread_mutex_t} elision using environment variables
+The elision of @code{pthread_mutex_t} mutexes can be configured at runtime with the @code{GLIBC_PTHREAD_MUTEX}
+environment variable.  This will force a specific lock type for all
+mutexes in the program that did not explicitely disable elision
+by setting a non default type.
+
+@smallexample
+# run myprogram with no elision
+GLIBC_PTHREAD_MUTEX=none myprogram
+@end smallexample
+
+The default depends on the @theglibc{} build configuration and whether the hardware
+supports lock elision.
+
+@itemize
+@item
+@code{GLIBC_PTHREAD_MUTEX=elision}
+Use elided mutexes, unless explicitly disabled in the program.
+
+@item
+@code{GLIBC_PTHREAD_MUTEX=none}
+Don't use elide mutexes, unless explicitly enable in the program.
+@end itemize
+
+Additional tunables can be configured through the environment variable,
+like this:
+@code{GLIBC_PTHREAD_MUTEX=adaptive:retry_lock_busy=10,retry_lock_internal_abort=20}
+Note these parameters do not constitute an ABI and may change or disappear
+at any time as the lock elision algorithm evolves.
+
+Currently supported parameters are:
+
+@itemize
+@item
+skip_lock_busy
+How often to not attempt a transaction when the lock is seen as busy.
+Expressed in number of lock attempts.
+
+@item
+skip_lock_internal_abort
+How often to not attempt a transaction after an internal abort is seen.
+Expressed in number of lock attempts.
+
+@item
+retry_try_xbegin
+How often to retry the transaction on external aborts.
+Expressed in number of transaction starts.
+
+@item
+skip_trylock_internal_abort
+How often to skip doing a transaction on internal aborts during trylock.
+This setting is also used for adaptive locks.
+Expressed in number of transaction starts.
+
+@end itemize
+
+@node Setting @code{pthread_rwlock_t} elision using environment variables
+@section Setting @code{pthread_rwlock_t} elision using environment variables
+The elision of @code{pthread_rwlock_t} rwlocks can be configured at
+runtime with the @code{GLIBC_PTHREAD_RWLOCK} environment variable.
+
+@smallexample
+# run myprogram with no elision
+GLIBC_PTHREAD_RWLOCK=none myprogram
+@end smallexample
+
+The default depends on the @theglibc{} build configuration and whether the hardware
+supports lock elision.
+
+@itemize
+@item
+@code{GLIBC_PTHREAD_RWLOCK=elision}
+Use elided rwlockes, unless explicitly disabled in the program.
+
+@item
+@code{GLIBC_PTHREAD_RWLOCK=none}
+Don't use elided rwlocks, unless explicitly enabled in the program.
+@end itemize
+
+The same tunable parameters as documented for @code{GLIBC_PTHREAD_MUTEX} can be
+also used for @code{GLIBC_PTHREAD_RWLOCK}.
diff --git a/manual/intro.texi b/manual/intro.texi
index d4045f2..c54d050 100644
--- a/manual/intro.texi
+++ b/manual/intro.texi
@@ -1470,6 +1470,9 @@  information about the hardware and software configuration your program
 is executing under.
 
 @item
+@ref{Lock elision} describes elided locks in POSIX threads.
+
+@item
 @ref{System Configuration}, tells you how you can get information about
 various operating system limits.  Most of these parameters are provided for
 compatibility with POSIX.
diff --git a/manual/lang.texi b/manual/lang.texi
index 28b21cb..f88f4fd 100644
--- a/manual/lang.texi
+++ b/manual/lang.texi
@@ -1,6 +1,6 @@ 
 @c This node must have no pointers.
 @node Language Features
-@c @node Language Features, Library Summary, , Top
+@c @node Language Features, Library Summary, Lock elision, Top
 @c %MENU% C language features provided by the library
 @appendix C Language Facilities in the Library