1 .\" This man page is Copyright (C) 2006 Andi Kleen <ak@muc.de>.
2 .\" Permission is granted to distribute possibly modified copies
3 .\" of this page provided the header is included verbatim,
4 .\" and in case of nontrivial modification author and date
5 .\" of the modification is added to the header.
6 .\" 2008, mtk, various edits
7 .TH GETCPU 2 2008-06-03 "Linux" "Linux Programmer's Manual"
9 getcpu \- determine CPU and NUMA node on which the calling thread is running
12 .B #include <linux/getcpu.h>
14 .BI "int getcpu(unsigned *" cpu ", unsigned *" node \
15 ", struct getcpu_cache *" tcache );
20 system call identifies the processor and node on which the calling
21 thread or process is currently running and writes them into the
22 integers pointed to by the
27 The processor is a unique small integer identifying a CPU.
28 The node is a unique small identifier identifying a NUMA node.
33 is NULL nothing is written to the respective pointer.
35 The third argument to this system call is nowadays unused.
37 The information placed in
39 is only guaranteed to be current at the time of the call:
40 unless the CPU affinity has been fixed using
41 .BR sched_setaffinity (2),
42 the kernel might change the CPU at any time.
43 (Normally this does not happen
44 because the scheduler tries to minimize movements between CPUs to
45 keep caches hot, but it is possible.)
46 The caller must be prepared to handle the situation when
50 are no longer the current CPU and node.
53 was added in kernel 2.6.19 for x86_64 and i386.
58 Linux makes a best effort to make this call as fast possible.
61 is to allow programs to make optimizations with per-CPU data
62 or for NUMA optimization.
64 Glibc does not provide a wrapper for this system call; call it using
72 argument is unused since Linux 2.6.24.
73 .\" commit 4307d1e5ada595c87f9a4d16db16ba5edb70dcb1
74 .\" Author: Ingo Molnar <mingo@elte.hu>
75 .\" Date: Wed Nov 7 18:37:48 2007 +0100
76 .\" x86: ignore the sys_getcpu() tcache parameter
78 if this argument was non-NULL,
79 then it specified a pointer to a caller-allocated buffer in thread-local
80 storage that was used to provide a caching mechanism for
82 Use of the cache could speed
84 calls, at the cost that there was a very small chance that
85 the returned information would be out of date.
86 The caching mechanism was considered to cause problems when
87 migrating threads between CPUs, and so the argument is now ignored.
89 .\" ===== Before kernel 2.6.24: =====
92 .\" .IR "struct getcpu_cache"
93 .\" that is used as a cache by
95 .\" The caller should put the cache into a thread-local variable
96 .\" if the process is multithreaded,
97 .\" because the cache cannot be shared between different threads.
100 .\" If it is not NULL
102 .\" will use it to speed up operation.
103 .\" The information inside the cache is private to the system call
104 .\" and should not be accessed by the user program.
105 .\" The information placed in the cache can change between kernel releases.
107 .\" When no cache is specified
110 .\" but always retrieve the current CPU and node information.
114 .\" However, the cached information is only updated once per jiffy (see
116 .\" This means that the information could theoretically be out of date,
117 .\" although in practice the scheduler's attempt to maintain
118 .\" soft CPU affinity means that the information is unlikely to change
119 .\" over the course of the caching interval.
122 .BR sched_setaffinity (2),
123 .BR set_mempolicy (2),
124 .BR sched_getcpu (3),