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Re: [Xen-devel] Re: NUMA and SMP

On Tue, 2007-03-20 at 21:10 +0800, tgh wrote:
> I am puzzled ,what is the page migration?
> Thank you in advance

NUMA is clear? NUMA distributes main memory across multiple memory

This used to be a feature reserved to high-end multiprocessor
architectures, but in servers it is becoming sort of a commodity these
days, in part due AMD multiprocessor systems being NUMA systems these
days. AMD64 processors carry an integrated memory controller. So, if you
buy an SMP machine with AMD processors today, you'd find each slice of
the total memory being connected to a different processor inside.

Note that this doesn't break the 'symmetric' in 'SMP': it still remains
a global, flat physical address space. The processors have interconnects
by which memory can be read from remote processors as well, and will do
so transparently to system and application software.

[The alternative is rather the 'classic' model: Multiple processors
interconnected making SMP, but  single memory interface in a single
northbridge (Intel would call it the "MCH") connecting to the front-side
bus, connecting all processors them to main memory. Obviously, that
single memory interface will easily become a bottleneck, if all
processors try to access memory simultaneously.]

NUMA *may* help here: accessing local memory is very fast. Acessing
remote memory is still pretty fast, but not as fast as it could be:
hence 'NUMA' - non-uniform memory access.

So, in order to take advantage of such a memory topology, memory data
would ideally be always at the CPU where the processing happens. But
processes (or domains, regarding xen) may migrate between different
processors. Whether this happens depends on scheduling decisions.
There's a cost involved in migration itself, so schedulers will do it
ideally only if it really-makes-sense(TM).

In order to keep a NUMA-system happy, pages once allocated could be
moved as well, to where the current CPU is. This is page migration.
As you may imagine, even more costly, and unfortunately completely
useless if cpu migration needs to happen on a regular basis. Therefore
it's difficult to get it right. Getting it right depends on how much the
scheduler and memory management knows about where the memory asked for
will be needed -- in advance. This is the hardest part: Most software
won't tell, because the programming models employed today do not even
recognize the fact that it may matter. Even if they would, in many
cases, it would be even difficult to predict at all.


Daniel Stodden
LRR     -      Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B

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