Date: Fri, 4 Feb 2011 13:29:25 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Sri Ram Vemulpali <sri.ram.gmu06@gmail.com>
cc: Kernel-newbies <kernelnewbies@nl.linux.org>,
        linux-kernel-mail <linux-kernel@vger.kernel.org>
Subject: Re: Thread Affinity structure
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On Fri, 4 Feb 2011, Sri Ram Vemulpali wrote:

>     I have doubt regarding how the thread affinity (to processor) is
> defined in a process (task). If there is only single thread in a
> process, then when process calls itself on binding to a core, will let
> run on that core forever. But, what if there are multiple threads, in
> a process (task). If the main thread calls set affinity to a core, is
> it going to get inherited to all other threads, or just the calling
> thread in a task. What happens when forked a process with threads that
> has affinity to a core. Please can anyone point me to the literature
> on this. Thanks in advance.

See "man sched_setaffinity"

"
The affinity mask is actually a per-thread attribute that can be
adjusted independently for each of the threads in a thread group.  The
value returned  from a call to gettid(2) can be passed in the argument
pid.  Specifying pid as 0 will set the attribute for the calling thread,
and passing the value returned from a call to getpid(2) will set the
attribute for the main thread of the thread group.  (If you  are  using
the  POSIX threads API, then use pthread_setaffinity_np(3) instead of
sched_setaffinity().)
"


The setting of the affinity occurs for the running thread if pid == 0.
Affinity masks are inherited across forks.


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