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Subject: Re: [hwloc-users] Thread core affinity
From: Gabriele Fatigati (g.fatigati_at_[hidden])
Date: 2011-08-04 10:34:56


Ok,

so physical OS index 0 and 1 are not true are physically near on the die.

Considering that, how I can use cache locality and cache sharing by cores if
I don't know where my threads will physically bound?

If L#0 and L#1 where I bind my threads are physically far, may give me bad
performance.

2011/8/4 Samuel Thibault <samuel.thibault_at_[hidden]>

> Gabriele Fatigati, le Thu 04 Aug 2011 16:14:35 +0200, a écrit :
> > Socket:
> > ______________
> > | ____ ____ |
> > | |core | |core | |
> > | ____ ____ |
> > | |core | |core | |
> > | ____ ____ |
> > | |core | |core | |
> > | ______________|
> >
> > lstopo how create the numerations?
>
> It does not really matter since there is no topology consideration here
> (no shared cache or such). In that case hwloc will simply follow the
> order as provided by the OS. If there were shared caches, they would
> come into play when sorting the topology.
>
> > It consider physical OS index to list and create cores topology? If
> > yes, maybe Core L#0 and Core L#1 in a single socket are physically
> > near.
>
> Mmm, maybe the confusion comes from the expression "physically near".
> What we call physical index have nothing to do with physical proximity.
> It's just what the physical chip says, which often has nothing to do
> with physical proximity.
>
> There is nothing much fancy in the topology creation really: we simply
> include objects one into the other according to logical inclusion, and
> finally sort by OS (i.e. physical) index after it's all topology-sorted.
>
> Samuel
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>

-- 
Ing. Gabriele Fatigati
HPC specialist
SuperComputing Applications and Innovation Department
Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy
www.cineca.it                    Tel:   +39 051 6171722
g.fatigati [AT] cineca.it