* Pádraig Brady <p...@draigbrady.com> wrote:
> On 10/08/2013 10:02 AM, Ingo Molnar wrote:
> > +ifeq ($(JOBS),)
> > + JOBS := $(shell grep -c ^processor /proc/cpuinfo 2>/dev/null)
>
> nproc is probably ubiquitous enough to use now
> (available since coreutils 8.1 (end of 2009))
>
> As well as being more concise, it will take
> account of offline CPUs etc.
/proc/cpuinfo takes account of offline CPUs as well:
# grep -c ^processor /proc/cpuinfo 2>/dev/null
16
# echo 0 > /sys/devices/system/cpu/cpu11/online
# grep -c ^processor /proc/cpuinfo 2>/dev/null
15
# echo 1 > /sys/devices/system/cpu/cpu11/online
# grep -c ^processor /proc/cpuinfo 2>/dev/null
16
But nproc is indeed a better choice:
1)
It is scheduler syscall based and will thus will work in limited
environments as well, for example when /proc is not mounted.
2)
It will also properly detect affinity-limited environments:
# taskset 1 nproc
1
3)
It is also faster than grepping /proc/cpuinfo:
# perf stat --null --repeat 100 nproc >/dev/null
Performance counter stats for 'nproc' (100 runs):
0.000652928 seconds time elapsed
( +- 0.53% )
versus:
# perf stat --null --repeat 100 grep -c ^processor /proc/cpuinfo >/dev/null
Performance counter stats for 'grep -c ^processor /proc/cpuinfo' (100 runs):
0.001037034 seconds time elapsed
( +- 0.32% )
so with 0.652 msecs versus 1.037 msecs it's about 60% faster than grep.
Thanks,
Ingo
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