Hi Michael,
this one is not 'straightforward' for me as I needed to look into the
glibc and kernel source...
I am not very knowledgeable in that area, so my review comments are in
consequence...
Cheers,
Loïc
--
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.TH PTHREAD_SETAFFINITY_NP 3 2008-11-04 "Linux" "Linux Programmer's Manual"
.SH NAME
pthread_setaffinity_np, pthread_getaffinity_np \- set/get
CPU affinity of a thread
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <pthread.h>
.BI "int pthread_setaffinity_np(pthread_t " thread ", size_t " cpusetsize ,
.BI " const cpu_set_t *" cpuset );
.BI "int pthread_getaffinity_np(pthread_t " thread ", size_t " cpusetsize ,
.BI " cpu_set_t *" cpuset );
.sp
Compile and link with \fI\-pthread\fP.
.SH DESCRIPTION
The
.BR pthread_setaffinity_np ()
sets the CPU affinity mask of the thread
.I thread
to the CPU set pointed to by
.IR cpuset .
If the call is successful,
and the thread is not currently running on one of the CPUs in
.IR cpuset ,
then it is migrated to one of those CPUs.
The
.BR pthread_getaffinity_np ()
function returns the CPU affinity mask of the thread
.I thread
in the buffer pointed to by
.IR cpuset .
The argument
.I cpusetsize
is the length (in bytes) of the buffer pointed to by
.IR cpuset .
Normally this argument would be specified as
.IR sizeof(cpu_set_t) .
The constant
.B CPU_SETSIZE
specifies a value one greater than the
maximum CPU number that can be stored in a CPU set.
I came independently to the same conclusion than you. AFAICS,glibc
defines the cpu_set_t to be a 1024 bits long structure. But the kernel
defines the corresponding structure to be a bit field of appropriate
length to store NR_CPUS.
Interesting enough, the exact size of the kernel structure is determined
by the glibc on the 1st call to pthread_setaffinity_np().
We'll run into troubles if run this on a server with more than 1K core ;-)
For more details on CPU affinity masks,
as well as a description of a set of macros
that can be used to manipulate and inspect CPU sets, see
.BR sched_setaffinity (2)
for details.
.SH RETURN VALUE
On success, these functions return 0;
on error, they return a non-zero error number.
.SH ERRORS
.TP
.B EFAULT
A supplied memory address was invalid.
.TP
.B EINVAL
.RB ( pthread_setaffinity_np ())
The affinity bit mask
.I mask
contains no processors that are physically on the system.
If I got it right, it seems that Linux supports (or is preparing)
hotplug of CPUs cards... So when you say that "mask contains no
processors that are physically on the system", it should be understood:
at the point of time where pthread_setaffinity_np() has been issued.
.TP
.BR EINVAL
.RB ( pthread_setaffinity_np ())
.I cpuset
specified a CPU that was outside the range
permitted by the kernel data type
.\" cpumask_t
used to represent CPU sets.
.\" The raw sched_getaffinity() system call returns the size (in bytes)
.\" of the cpumask_t type.
This range is determined by the kernel configuration option
.BR CONFIG_NR_CPUS .
True. But what does it mean for an application programmer? That I am
requesting a processor which is outside of the set supported by the
kernel (structure). Strictly speaking, this processor could be
physically present.
So, If I sum-up the two EINVAL cases above, I get something like:
The affinity bit mask <code>mask</code> contains no processors that are
physically on the system, or contains a processor outside of the set
supported by the kernel.
.TP
.B EINVAL
.RB ( pthread_getaffinity_np ()
.I cpusetsize
is smaller than the size of the affinity mask used by the kernel.
.TP
.B ESRCH
There is no thread matching
.IR thread
(e.g., perhaps that thread has already terminated and been joined).
.SH VERSIONS
These functions are provided by glibc since version 2.3.4.
.SH CONFORMING TO
These functions are non-standard GNU extensions.
.SH NOTES
These functions are interpreted on top of the
.BR sched_setaffinity (2)
and
.BR sched_getaffinity (2)
system calls.
In glibc 2.3.3 only,
versions of these functions were provided that did not have a
.I cpusetsize
argument.
Instead the CPU set size given to the underlying system calls was always
.IR sizeof(cpu_set_t) .
That's funny. Didn't you mention above that "the cpusetsize should be
sizeof(cpu_set_t)"... So for now, it's not that much different ;-)
A new thread created by
.BR pthread_create ()
inherits a copy of its creator's CPU affinity mask.
.SH EXAMPLE
In the following program, the main thread uses
.BR pthread_setaffinity_np ()
to set its CPU affinity mask to include CPUs 0 to 7
(which may not all be available on the system),
and then calls
.BR pthread_getaffinity_np ()
to check the resulting CPU affinity mask of the thread.
.nf
#define _GNU_SOURCE
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#define errExitEN(en, msg) { errno = en; perror(msg); \\
exit(EXIT_FAILURE); }
int
main(int argc, char *argv[])
{
int s, j;
cpu_set_t cpuset;
pthread_t thread;
thread = pthread_self();
/* Set affinity mask to include CPUs 0 to 7 */
CPU_ZERO(&cpuset);
for (j = 0; j < 8; j++)
CPU_SET(j, &cpuset);
s = pthread_setaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
if (s != 0)
errExitEN(s, "sched_setaffinity");
/* Check the actual affinity mask assigned to the thread */
s = pthread_getaffinity_np(thread, sizeof(cpu_set_t), &cpuset);
if (s != 0)
errExitEN(s, "sched_getaffinity");
printf("Set returned by pthread_getaffinity_np() contained:\\n");
for (j = 0; j < CPU_SETSIZE; j++)
if (CPU_ISSET(j, &cpuset))
printf(" CPU %d\\n", j);
exit(EXIT_SUCCESS);
}
.fi
This might be obvious to all of you, but it wasn't to me... The CPU set
used for the affinity is the intersection of the set passed to
pthread_setaffinity_np() and the set of processors supported by the
kernel...
That's why your example shall work, even if the system where it runs has
less than 8 processors.
.SH SEE ALSO
.BR sched_getaffinity (2),
.BR sched_setaffinity (2),
.BR sched_getscheduler (2),
.BR pthread_getaffinity_np (3),
.BR pthread_setaffinity_np (3),
.BR cpuset (7),
.BR pthreads (7)
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