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

[tgh <tianguanhua@xxxxxxxxxx>]

Thank you for your reply


Daniel Stodden åé:
> 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
> interfaces.
>
> 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.
>   
that is ,in the smp with adm64,it is a numa in the hardware 
architecture,while a smp in the system software,is it right?
Thank you in advance

> [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.
>
> regards,
> daniel
>
>   

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