Re: getting processor numbers

[Davide Libenzi]

On Tue, 3 Apr 2007, Ulrich Drepper wrote:

> More and more code depends on knowing the number of processors in the
> system to efficiently scale the code.  E.g., in OpenMP it is used by
> default to determine how many threads to create.  Creating more threads
> than there are processors/cores doesn't make sense.
> 
> glibc for a long time provides functionality to retrieve the number
> through sysconf() and this is what fortunately most programs use.  The
> problem is that we are currently using /proc/cpuinfo since this is all
> there was available at that time.  Creating /proc/cpuinfo takes the
> kernel quite a long time, unfortunately (I think Jakub said it is mainly
> the interrupt information).
> 
> The alternative today is to use /sys/devices/system/cpu and count the
> number of cpu* directories in it.  This is somewhat faster.  But there
> would be another possibility: simply stat /sys/devices/system/cpu and
> use st_nlink - 2.
> 
> This last step unfortunately it made impossible by recent changes:
> 
>   http://article.gmane.org/gmane.linux.kernel/413178
> 
> I would like to propose changing that patch, move the sched_*
> pseudo-files in some other directly and permanently ban putting any new
> file into /sys/devices/system/cpu.
> 
> To get some numbers, you can try
> 
>   http://people.redhat.com/drepper/nproc-timing.c
> 
> The numbers I see on x86-64:
> 
> cpuinfo 10145810 cycles for 100 accesses
> readdir /sys 3113870 cycles for 100 accesses
> stat /sys 741070 cycles for 100 accesses

It sucks when seen from a micro-bench POV, but does it really matter 
overall? The vast majority of software usually calls 
sysconf(_SC_NPROCESSORS_*) with very little frequency (mostly once at 
initialization time) anyway. That's what 50us / call?



- Davide


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