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80997bc3 | 1 | .\" This man page is Copyright (C) 2006 Andi Kleen <ak@muc.de>. |
2297bf0e | 2 | .\" |
00acdba1 | 3 | .\" %%%LICENSE_START(VERBATIM_ONE_PARA) |
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4 | .\" Permission is granted to distribute possibly modified copies |
5 | .\" of this page provided the header is included verbatim, | |
6 | .\" and in case of nontrivial modification author and date | |
7 | .\" of the modification is added to the header. | |
8ff7380d | 8 | .\" %%%LICENSE_END |
4366109b | 9 | .\" |
80997bc3 | 10 | .\" 2008, mtk, various edits |
4784c377 | 11 | .\" |
4c1c5274 | 12 | .TH getcpu 2 (date) "Linux man-pages (unreleased)" |
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13 | .SH NAME |
14 | getcpu \- determine CPU and NUMA node on which the calling thread is running | |
db24e10d AC |
15 | .SH LIBRARY |
16 | Standard C library | |
8fc3b2cf | 17 | .RI ( libc ", " \-lc ) |
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18 | .SH SYNOPSIS |
19 | .nf | |
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20 | .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" |
21 | .B #include <sched.h> | |
68e4db0a | 22 | .PP |
c9eec92c | 23 | .BI "int getcpu(unsigned int *" cpu ", unsigned int *" node ); |
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24 | .fi |
25 | .SH DESCRIPTION | |
26 | The | |
27 | .BR getcpu () | |
28 | system call identifies the processor and node on which the calling | |
29 | thread or process is currently running and writes them into the | |
30 | integers pointed to by the | |
31 | .I cpu | |
32 | and | |
33 | .I node | |
34 | arguments. | |
35 | The processor is a unique small integer identifying a CPU. | |
36 | The node is a unique small identifier identifying a NUMA node. | |
37 | When either | |
38 | .I cpu | |
39 | or | |
40 | .I node | |
41 | is NULL nothing is written to the respective pointer. | |
efeece04 | 42 | .PP |
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43 | The information placed in |
44 | .I cpu | |
33a0ccb2 | 45 | is guaranteed to be current only at the time of the call: |
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46 | unless the CPU affinity has been fixed using |
47 | .BR sched_setaffinity (2), | |
48 | the kernel might change the CPU at any time. | |
49 | (Normally this does not happen | |
50 | because the scheduler tries to minimize movements between CPUs to | |
51 | keep caches hot, but it is possible.) | |
0ab8aeec | 52 | The caller must allow for the possibility that the information returned in |
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53 | .I cpu |
54 | and | |
55 | .I node | |
ded312a3 | 56 | is no longer current by the time the call returns. |
47297adb | 57 | .SH RETURN VALUE |
178df36f MF |
58 | On success, 0 is returned. |
59 | On error, \-1 is returned, and | |
60 | .I errno | |
f6a4078b | 61 | is set to indicate the error. |
178df36f MF |
62 | .SH ERRORS |
63 | .TP | |
64 | .B EFAULT | |
65 | Arguments point outside the calling process's address space. | |
80997bc3 | 66 | .SH VERSIONS |
90fe01c5 | 67 | .BR getcpu () |
9ea5bc66 | 68 | was added in kernel 2.6.19 for x86-64 and i386. |
d8be53ac MK |
69 | Library support was added in glibc 2.29 |
70 | (Earlier glibc versions did not provide a wrapper for this system call, | |
71 | necessitating the use of | |
72 | .BR syscall (2).) | |
3113c7f3 | 73 | .SH STANDARDS |
90fe01c5 | 74 | .BR getcpu () |
76c637e1 | 75 | is Linux-specific. |
80997bc3 | 76 | .SH NOTES |
f55912a9 | 77 | Linux makes a best effort to make this call as fast as possible. |
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78 | (On some architectures, this is done via an implementation in the |
79 | .BR vdso (7).) | |
80997bc3 | 80 | The intention of |
90fe01c5 | 81 | .BR getcpu () |
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82 | is to allow programs to make optimizations with per-CPU data |
83 | or for NUMA optimization. | |
c9eec92c MK |
84 | .\" |
85 | .SS C library/kernel differences | |
86 | The kernel system call has a third argument: | |
87 | .PP | |
88 | .in +4n | |
89 | .nf | |
90 | .BI "int getcpu(unsigned int *" cpu ", unsigned int *" node , | |
91 | .BI " struct getcpu_cache *" tcache ); | |
92 | .fi | |
93 | .in | |
efeece04 | 94 | .PP |
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95 | The |
96 | .I tcache | |
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97 | argument is unused since Linux 2.6.24, |
98 | and (when invoking the system call directly) | |
99 | should be specified as NULL, | |
100 | unless portability to Linux 2.6.23 or earlier is required. | |
101 | .PP | |
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102 | .\" commit 4307d1e5ada595c87f9a4d16db16ba5edb70dcb1 |
103 | .\" Author: Ingo Molnar <mingo@elte.hu> | |
104 | .\" Date: Wed Nov 7 18:37:48 2007 +0100 | |
105 | .\" x86: ignore the sys_getcpu() tcache parameter | |
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106 | In Linux 2.6.23 and earlier, if the |
107 | .I tcache | |
108 | argument was non-NULL, | |
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109 | then it specified a pointer to a caller-allocated buffer in thread-local |
110 | storage that was used to provide a caching mechanism for | |
111 | .BR getcpu (). | |
112 | Use of the cache could speed | |
113 | .BR getcpu () | |
114 | calls, at the cost that there was a very small chance that | |
115 | the returned information would be out of date. | |
116 | The caching mechanism was considered to cause problems when | |
117 | migrating threads between CPUs, and so the argument is now ignored. | |
118 | .\" | |
119 | .\" ===== Before kernel 2.6.24: ===== | |
120 | .\" .I tcache | |
121 | .\" is a pointer to a | |
122 | .\" .IR "struct getcpu_cache" | |
123 | .\" that is used as a cache by | |
124 | .\" .BR getcpu (). | |
125 | .\" The caller should put the cache into a thread-local variable | |
126 | .\" if the process is multithreaded, | |
127 | .\" because the cache cannot be shared between different threads. | |
128 | .\" .I tcache | |
129 | .\" can be NULL. | |
130 | .\" If it is not NULL | |
131 | .\" .BR getcpu () | |
132 | .\" will use it to speed up operation. | |
133 | .\" The information inside the cache is private to the system call | |
134 | .\" and should not be accessed by the user program. | |
135 | .\" The information placed in the cache can change between kernel releases. | |
c17a5c8b | 136 | .\" |
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137 | .\" When no cache is specified |
138 | .\" .BR getcpu () | |
139 | .\" will be slower, | |
140 | .\" but always retrieve the current CPU and node information. | |
141 | .\" With a cache | |
142 | .\" .BR getcpu () | |
143 | .\" is faster. | |
33a0ccb2 | 144 | .\" However, the cached information is updated only once per jiffy (see |
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145 | .\" .BR time (7)). |
146 | .\" This means that the information could theoretically be out of date, | |
147 | .\" although in practice the scheduler's attempt to maintain | |
148 | .\" soft CPU affinity means that the information is unlikely to change | |
149 | .\" over the course of the caching interval. | |
150 | .SH SEE ALSO | |
80997bc3 | 151 | .BR mbind (2), |
f0c34053 | 152 | .BR sched_setaffinity (2), |
80997bc3 | 153 | .BR set_mempolicy (2), |