Re: [rfc 08/45] cpu alloc: x86 support

H. Peter Anvin
2007-11-20T02:19:37Z

Christoph Lameter wrote:
> On Mon, 19 Nov 2007, H. Peter Anvin wrote:
>
>> You're making the assumption here that NUMA = large number of CPUs. This
>> assumption is flat-out wrong.
>
> Well maybe. Usually one gets to NUMA because the hardware gets too big to
> be handleed the UMA way.
>
>> On x86-64, most two-socket systems are still NUMA, and I would expect that
>> most distro kernels probably compile in NUMA. However,
>> burning megabytes of memory on a two-socket dual-core system when we're
>> talking about tens of kilobytes used would be more than a wee bit insane.
>
> Yeah yea but the latencies are minimal making the NUMA logic too expensive
> for most loads ... If you put a NUMA kernel onto those then performance
> drops (I think someone measures 15-30%?)
>

How do you handle this memory, in the first place? Do you allocate the
whole 2 MB for the particular CPU, or do you reclaim the upper part of
the large page? (I haven't dug far enough into the source to tell.)

-hpa