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Subject: Re: [hwloc-devel] Strange results on itanium 2
From: Jirka Hladky (jhladky_at_[hidden])
Date: 2010-10-29 18:01:26


On Friday, October 29, 2010 10:59:25 pm Brice Goglin wrote:
> Le 29/10/2010 21:57, Jirka Hladky a écrit :
> > Hi Samuel,
> >
> > I have attached the output of
> > tests/linux/gather-topology.sh `uname --kernel-release`_`uname --
> > nodename`_gather-topology
> >
> > I'm sorry for the long delay - systems has been used by somebody else, I
> > had to wait for it to be free.
> >
> > System is running kernel 2.6.18-227.el5 (RHEL 5.6). ia64 is not supported
> > on RHEL 6.0 so I cannot really test it on the new kernel.
> >
> > It would be really interesting if you can recognize if it's a kernel bug
> > or hwloc problem.
>
> /sys/devices/system/cpu/cpu*/cache/index4/shared_cpu_map contains an
> empty map. This index4 is a L3 cache. But this map means that this cache
> is near none of the cores... The instruction L2 has the same problem
> (index3 instead of 4). This is a kernel bug.
>
> But, we already have a dedicated work-around in hwloc
> (src/topology-linux.c):
> if (hwloc_bitmap_weight(cacheset) < 1)
> /* mask is wrong (happens on ia64), assumes it's not shared */
> hwloc_bitmap_only(cacheset, i);
>
> This work-around worked fine on old itaniums since they had one L3, one
> L2 and one L1 per core. Your machine has hyperthreading, so our
> work-around creates one L3 per thread, while L1 and L2 (properly
> reported by the kernel) are core-specific. Maybe hwloc should just
> ignore caches with invalid shared_cpu_map.
>
> Brice
>
> _______________________________________________
> hwloc-devel mailing list
> hwloc-devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/hwloc-devel

Hi Brice,

thanks for looking into it! I'm going to open a BZ for it and put you on the
Cc.

BTW, is there some documentation on /sys/devices/system/cpu/* tree?

Thanks!
Jirka