Section (2) getcpu
Name
getcpu — determine CPU and NUMA node on which the calling thread is running
Synopsis
#include <linux/getcpu.h>
int
getcpu( |
unsigned *cpu, |
unsigned *node, | |
struct getcpu_cache *tcache) ; |
DESCRIPTION
The getcpu
() system call
identifies the processor and node on which the calling thread
or process is currently running and writes them into the
integers pointed to by the cpu
and node
arguments. The processor
is a unique small integer identifying a CPU. The node is a
unique small identifier identifying a NUMA node. When either
cpu
or node
is NULL nothing is written
to the respective pointer.
The third argument to this system call is nowadays unused, and should be specified as NULL unless portability to Linux 2.6.23 or earlier is required (see NOTES).
The information placed in cpu
is guaranteed to be current
only at the time of the call: unless the CPU affinity has
been fixed using sched_setaffinity(2), the
kernel might change the CPU at any time. (Normally this does
not happen because the scheduler tries to minimize movements
between CPUs to keep caches hot, but it is possible.) The
caller must allow for the possibility that the information
returned in cpu
and
node
is no longer
current by the time the call returns.
VERSIONS
getcpu
() was added in kernel
2.6.19 for x86-64 and i386. Library support was added in
glibc 2.29 (Earlier glibc versions did not provide a wrapper
for this system call, necessitating the use of syscall(2).)
NOTES
Linux makes a best effort to make this call as fast as
possible. (On some architectures, this is done via an
implementation in the vdso(7).) The intention of
getcpu
() is to allow programs
to make optimizations with per-CPU data or for NUMA
optimization.
The tcache
argument is unused since Linux 2.6.24. In earlier kernels, if
this argument was non-NULL, then it specified a pointer to a
caller-allocated buffer in thread-local storage that was used
to provide a caching mechanism for getcpu
(). Use of the cache could speed
getcpu
() calls, at the cost
that there was a very small chance that the returned
information would be out of date. The caching mechanism was
considered to cause problems when migrating threads between
CPUs, and so the argument is now ignored.
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
This man page is Copyright (C) 2006 Andi Kleen <akmuc.de>. %%%LICENSE_START(VERBATIM_ONE_PARA) Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. %%%LICENSE_END 2008, mtk, various edits |