Rich Freeman on 24 Aug 2016 12:39:14 -0700

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On Wed, Aug 24, 2016 at 3:27 PM, Keith C. Perry
<> wrote:
> That's not quite right.  A NUMA system is essentially what SMP used to be.  Its multiple processors (which have multiple cores) so for instance if you type...

SMP means that memory access from any core to any address range is the
same.  NUMA means that regions of memory are closer to some cores than
others.  They never were the same, and still are not.  I just don't
think anybody tends to use SMP, since it typically involves slowing
down everything to the least common denominator (which is worse than
treating a NUMA system as if it were SMP).

> numactl --hardware

Won't this just tell you how the kernel is handling the system, and
not how the actual hardware is wired up?

Clearly it is showing in my case that it is handling the system as if
it were SMP.  That doesn't mean that the actual hardware works that
way.  The kernel strictly speaking doesn't have to know that it is
NUMA because the hardware takes care of everything.  If the kernel
sends a CPU off to fetch some other CPU's memory it will just politely
ask the other CPU to interrupt what its doing and lend it a hand, and
wait a while for the answer.  It is slower, but it still works.

I would need to read up a lot more about how AMD HT works to sort it out...

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