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

From: Roman Gushchin
Date: Fri Jun 14 2013 - 12:54:17 EST

On 14.06.2013 20:08, Christoph Lameter wrote:
On Fri, 14 Jun 2013, Roman Gushchin wrote:

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.

Well that is not a slab allocator specific issue but related to compaction
concurrency. Likely cache line contention is causing a severe slowday. But
that issue could be triggered by any subsystem that does lots of memory
allocations. I would suggest that we try to address the problem in the
compaction logic rather than modifying allocators.

I agree, that it's good to address the original issue. But I'm not sure,
that it's a compaction issue. If someone wants to participate here,
I can provide more information. The main problem here is that it's
__very__ hard to reproduce the issue.

But, I think, all that shouldn't stop us from modifying the allocator.
Falling back to minimal order is in any case better than running
direct compaction. Just because it's faster. Am I wrong?

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