[gmx-developers] checking CPU set

[Szilárd Páll]

On Tue, Oct 30, 2012 at 5:56 PM, Sander Pronk <pronk at kth.se> wrote:

>
> On Oct 30, 2012, at 14:28 , Szilárd Páll <szilard.pall at cbr.su.se> wrote:
>
> On Tue, Oct 30, 2012 at 8:49 AM, Sander Pronk <pronk at kth.se> wrote:
>
>> What's the practical difference between the two cases? In both cases, the
>> thread is allowed to run on all CPUs in the system, right?
>>
>
> The practical difference is that if we run on an 8-core CPU:
> mdrun -ntmpi 1 -ntomp 4
> we lock the threads to the first four cores, but if we run
> taskset 0xFF mdrun -ntmpi 1 -ntomp 4
> without distinguishing between the two cases I've just mentioned, we would
> override the affinity set by taskset. Of course, the above does not make
> much sense, but I would rather not have mdrun exhibit ambiguous behavior.
>
>
>
> In that case we can could go for a rule that says that we'll only *refine*
> a user-set affinity (use fewer CPUs for a given thread than actually
> requested), never *expand* it (use CPUs we were told not to run on).
>

Sounds good.

BTW in my experience, hwloc-ps doesn't always give consistent results
> (especially with icc).
>

I have never seen inconsistencies, but have not checked very much either.
What kind of inconsistencies did you notice?


>
>
>
>>
>> On Oct 30, 2012, at 00:32 , Szilárd Páll <szilard.pall at cbr.su.se> wrote:
>>
>> > Hi,
>> >
>> > In order to be nice and not have mdrun override externally set CPU
>> affinities I want to check whether the affinity is default or it has been
>> changed (by taskset, OpenMP library, etc.). Initially I want to implement
>> this for GNU platforms with sched.h.
>> >
>> > While I initially though if no affinity is set the cpu set should be
>> empty, in fact it in this case CPU_ISSET will give the same result as if
>> all CPUs were in the set -- which actually makes sense. However, this left
>> me quite clueless about how to distinguish between the case when all CPUs
>> are present in the cpu set and when e.g. all CPU are set by taskset, e.g.
>> > $ taskset 0xF mdrun # quite useless on a quad-core machine
>> >
>> > Does anybody have an idea how to distinguish these two cases. It would
>> be possible, because e.g. hwloc-ps can do it (but they do it based on their
>> own mask-representation which makes is a bit hard to figure out what I need
>> from their code).
>> >
>> > Thanks,
>> > --
>> > Szilárd
>> >
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