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pthread_setconcurrency.3: New page documenting pthread_setconcurrency(3) and pthread_getconcurrency(3)
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Copyright (c) 2009 Michael Kerrisk, <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.TH PTHREAD_SETCONCURRENCY 3 2009-04-10 "Linux" "Linux Programmer's Manual"
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.SH NAME
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pthread_setconcurrency, pthread_getconcurrency \- set/get
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the concurrency level
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.SH SYNOPSIS
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.nf
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.B #include <pthread.h>
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.BI "int pthread_setconcurrency(int " new_level );
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.BI "int pthread_getconcurrency(void);
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.sp
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Compile and link with \fI\-pthread\fP.
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.SH DESCRIPTION
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The
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.BR pthread_setconcurrency ()
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function informs the implementation of the application's
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desired concurrency level, specified in
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.IR new_level .
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The implementation only takes this as a hint:
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POSIX.1 does not specify the level of concurrency that
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should be provided as a result of calling
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.BR pthread_setconcurrency ().
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Specifying
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.I new_level
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as 0 instructs the implementation to manage the concurrency level
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as it deems appropriate.
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.BR pthread_getconcurrency ()
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returns the current value of the concurrency level for this process.
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.SH RETURN VALUE
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On success,
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.BR pthread_setconcurrency ()
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returns 0;
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on error, it returns a non-zero error number.
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.BR pthread_getconcurrency ()
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always succeeds, returning the concurrency level set by a previous call to
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.BR pthread_setconcurrency (),
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or 0, if
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.BR pthread_setconcurrency ()
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has not previously been called.
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.SH ERRORS
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.BR pthread_setconcurrency ()
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can fail with the following error:
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.TP
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.B EINVAL
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.I new_level
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is negative.
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.PP
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POSIX.1-2001 also documents an
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.BR EAGAIN
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error ("the value specified by
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.I new_level
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would cause a system resource to be exceeded").
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.SH VERSIONS
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These functions are available in glibc since version 2.1.
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.SH CONFORMING TO
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POSIX.1-2001.
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.SH NOTES
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The default concurrency level is 0.
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Concurrency levels are only meaningful for M:N threading implementations,
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where at any moment a subset of a process's set of user-level threads
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may be bound to a smaller number of kernel-scheduling entities.
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Setting the concurrency level allows the application to
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give the system a hint as to the number of kernel-scheduling entities
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that should be provided for efficient execution of the application.
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Both LinuxThreads and NPTL are 1:1 threading implementations,
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so setting the concurrency level has no meaning.
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In other words,
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on Linux these functions merely exist for compatibility with other systems,
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and they have no effect on the execution of a program.
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.SH SEE ALSO
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.BR pthread_attr_setscope (3),
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.BR pthreads (7)
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