runtime: use sched_getaffinity for getproccount on GNU/Linux

[ianlancetaylor]

On GNU/Linux it might be slightly more efficient to use the sched_getaffinity system
call to get the number of virtual processors.  It would be one system call instead of
three or more, and it would not touch the filesystem.  The function, getproccount in
runtime/thread_linux.c, is called at the start of every program.

But note that if the system is actually using CPU affinity, this will cause
runtime.NumCPU to return the number of virtual processors that the process is permitted
to run on, rather than the number of virtual processors on the system.

[davecheney]

Comment 1:

I think this is an excellent idea.
> But note that if the system is actually using CPU affinity, this will cause
runtime.NumCPU to return the number of virtual processors that the process is permitted
to run on, rather than the number of virtual processors on the system.
Having worked in environments where affinity/cpu isolation is used, this is the
behaviour I would want. The opposite, where most of the cpus are isolated from the
general scheduler, yet the process believes it has access to all the CPUs leads to
surprising results.

[rsc]

Comment 2:

SGTM

[minux]

Comment 3:

@iant, have you started woking on this yet?

[ianlancetaylor]

Comment 4:

No, I haven't done anything about this.  I created the issue so that I didn't forget.

[minux]

Comment 5:

This issue was closed by revision 4f308ed.

Status changed to Fixed.