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Subject: Re: [OMPI devel] known limitation or bug in hwloc?
From: nadia.derbey_at_[hidden]
Date: 2011-08-29 10:35:17


devel-bounces_at_[hidden] wrote on 08/29/2011 04:20:30 PM:

> De : Ralph Castain <rhc_at_[hidden]>
> A : Open MPI Developers <devel_at_[hidden]>
> Date : 08/29/2011 04:26 PM
> Objet : Re: [OMPI devel] known limitation or bug in hwloc?
> Envoyé par : devel-bounces_at_[hidden]
>
> Actually, I'll eat those words. I was looking at the wrong place.
>
> Yes, that is a bug in hwloc. It needs to loop over CPU_MAX for those
> cases where the bit mask extends over multiple words.

But I'm afraid the fix won't be trivial at all: hwloc in itself is
coherent: it loops overs NUM_BITS, but it uses masks that are NUM_BITS
wide (hwloc_bitmap_t set)...

Regards,
Nadia
>
>
> On Aug 29, 2011, at 7:16 AM, Ralph Castain wrote:
>
> > Actually, if you look closely at the definition of those two
> values, you'll see that it really doesn't matter which one we loop
> over. The NUM_BITS value defines the actual total number of bits in
> the mask. The CPU_MAX is the total number of cpus we can support,
> which was set to a value such that the two are equal (i.e., it's a
> power of two that happens to be an integer multiple of 64).
> >
> > I believe the original intent was to allow CPU_MAX to be
> independent of address-alignment questions, so NUM_BITS could
> technically be greater than CPU_MAX. Even if this happens, though,
> all that would do is cause the loop to run across more bits than
required.
> >
> > So it doesn't introduce a limitation at all. In hindsight, we
> could simplify things by eliminating one of those values and just
> putting a requirement on the number that it be a multiple of 64 so
> it aligns with a memory address.
> >
> >
> > On Aug 29, 2011, at 7:05 AM, Kenneth Lloyd wrote:
> >
> >> Nadia,
> >>
> >> Interesting. I haven't tried pushing this to levels above 8 on a
particular
> >> machine. Do you think that the cpuset / paffinity / hwloc only
applies at
> >> the machine level, at which time you need to employ a graph with
carto?
> >>
> >> Regards,
> >>
> >> Ken
> >>
> >> -----Original Message-----
> >> From: devel-bounces_at_[hidden] [mailto:devel-bounces_at_[hidden]]
On
> >> Behalf Of nadia.derbey
> >> Sent: Monday, August 29, 2011 5:45 AM
> >> To: Open MPI Developers
> >> Subject: [OMPI devel] known limitation or bug in hwloc?
> >>
> >> Hi list,
> >>
> >> I'm hitting a limitation with paffinity/hwloc with cpu numbers >= 64.
> >>
> >> In opal/mca/paffinity/hwloc/paffinity_hwloc_module.c, module_set() is
> >> the routine that sets the calling process affinity to the mask given
as
> >> parameter. Note that "mask" is a opal_paffinity_base_cpu_set_t (so we
> >> allow the cpus to be potentially numbered up to
> >> OPAL_PAFFINITY_BITMASK_CPU_MAX - 1).
> >>
> >> The problem with module_set() is that is loops over
> >> OPAL_PAFFINITY_BITMASK_T_NUM_BITS bits to check if these bits are set
in
> >> the mask:
> >>
> >> for (i = 0; ((unsigned int) i) < OPAL_PAFFINITY_BITMASK_T_NUM_BITS;
++i)
> >> {
> >> if (OPAL_PAFFINITY_CPU_ISSET(i, mask)) {
> >> hwloc_bitmap_set(set, i);
> >> }
> >> }
> >>
> >> Given "mask"'s type, I think module_set() should instead loop over
> >> OPAL_PAFFINITY_BITMASK_CPU_MAX bits.
> >>
> >> Note that module_set() uses a type for its internal mask that is
> >> coherent with OPAL_PAFFINITY_BITMASK_T_NUM_BITS (hwloc_bitmap_t).
> >>
> >> So I'm wondering whether this is a known limitation I've never heard
of
> >> or an actual bug?
> >>
> >> Regards,
> >> Nadia
> >>
> >>
> >> _______________________________________________
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> >> devel_at_[hidden]
> >> http://www.open-mpi.org/mailman/listinfo.cgi/devel
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> >
>
>
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