Re: For review: pthread_setaffinity_np.3

["Bert Wesarg" <bert.wesarg-gM/Ye1E23mwN+BqQ9rBEUg@public.gmane.org>]

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On Sat, Nov 8, 2008 at 23:31, Loic Domaigne <tech@domaigne.com> wrote:
> 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
> --
>
>> .\" Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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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 ;-)
Thats why glibc provides a new set of function to handle cpu_set_t
with more than 1K bits.
Look for the definitions of macros with a _S suffix in <sched.h>.

Bert
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