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

[Paul Mundt <lethal@linux-sh.org>]

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.

If the freelists are split per node, that makes it a bit more manageable,
but that's more of a scalability issue than a correctness one. Random
alloc/free workloads will stick to their node with the current patch, so
I'm not sure where you see the random node placement as an issue?

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