Re: [PATCH] slob: poor man's NUMA, take 2.

Nick Piggin
2007-06-13T03:39:15Z

Paul Mundt wrote:
> On Wed, Jun 13, 2007 at 01:24:01PM +1000, Nick Piggin wrote:
>
>>Paul Mundt wrote:
>>
>>>Here's an updated copy of the patch adding simple NUMA support to SLOB,
>>>against the current -mm version of SLOB this time.
>>>
>>>I've tried to address all of the comments on the initial version so far,
>>>but there's obviously still room for improvement.
>>>
>>>This approach is not terribly scalable in that we still end up using a
>>>global freelist (and a global spinlock!) across all nodes, making the
>>>partial free page lookup rather expensive. The next step after this will
>>>be moving towards split freelists with finer grained locking.
>>
>>I just think that this is not really a good intermediate step because
>>you only get NUMA awareness from the first allocation out of a page. I
>>guess that's an easy no-brainer for bigblock allocations, but for SLUB
>>proper, it seems not so good.
>>
>>For a lot of workloads you will have a steady state where allocation and
>>freeing rates match pretty well and there won't be much movement of pages
>>in and out of the allocator. In this case it will be back to random
>>allocations, won't it?
>>
>
> That's why I tossed in the node id matching in slob_alloc() for the
> partial free page lookup. At the moment the logic obviously won't scale,
> since we end up scanning the entire freelist looking for a page that
> matches the node specifier. If we don't find one, we could rescan and
> just grab a block from another node, but at the moment it just continues
> on and tries to fetch a new page for the specified node.

Oh, I didn't notice that. OK, sorry that would work.

... but that goes against Matt's direction of wanting to improve basic
things like SMP scalability before NUMA awareness. I think once we had
per-CPU lists in place for SMP scalability, NUMA come much more naturally
and easily.

--
SUSE Labs, Novell Inc.