Re: [PATCH] slub: Avoid direct compaction if possible

From: Christoph Lameter
Date: Fri Jun 14 2013 - 10:32:19 EST

On Fri, 14 Jun 2013, Roman Gushchin wrote:

> Slub tries to allocate contiguous pages even if memory is fragmented and
> there are no free contiguous pages. In this case it calls direct compaction
> to allocate contiguous page. Compaction requires the taking of some heavily
> contended locks (e.g. zone locks). So, running compaction (direct and using
> kswapd) simultaneously on several processors can cause serious performance
> issues.

The main thing that this patch does is to add a nocompact flag to the page
allocator. That needs to be a separate patch. Also fix the description.
Slub does not invoke compaction. The page allocator initiates compaction
under certain conditions.

> It's possible to avoid such problems (or at least to make them less probable)
> by avoiding direct compaction. If it's not possible to allocate a contiguous
> page without compaction, slub will fall back to order 0 page(s). In this case
> kswapd will be woken to perform asynchronous compaction. So, slub can return
> to default order allocations as soon as memory will be de-fragmented.

Sounds like a good idea. Do you have some numbers to show the effect of
this patch?
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