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

[Roman Gushchin]

On 14.06.2013 18:32, Christoph Lameter wrote:
> 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.

Ok, I'll do.

> > 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?

No.
It seems that any numbers here depend on memory fragmentation,
so it's not easy to make a reproducible measurement. If you have
any ideas here, you are welcome.

But there is an actual problem, that this patch solves.
Sometimes I saw the following issue on some machines:
all CPUs are performing compaction, system time is about 80%,
system is completely unreliable. It occurs only on machines
with specific workload (distributed data storage system, so,
intensive disk i/o is performed). A system can fall into
this state fast and unexpectedly or by progressive degradation.

This patch solves this problem.

Thank you for your comments and suggestions!

Regards,
Roman

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