linux-kernel - Re: SLUB performance regression vs SLAB

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Date:	Thu, 4 Oct 2007 19:53:23 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Matthew Wilcox <matthew@....cx>,
	David Miller <davem@...emloft.net>, willy@...ux.intel.com,
	nickpiggin@...oo.com.au, hch@....de, mel@...net.ie,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dgc@....com, jens.axboe@...cle.com, suresh.b.siddha@...el.com
Subject: Re: SLUB performance regression vs SLAB

On Thu, 4 Oct 2007 19:43:58 -0700 (PDT)
Christoph Lameter <clameter@....com> wrote:

> So there could still be page struct contention left if multiple
> processors frequently and simultaneously free to the same slab and
> that slab is not the per cpu slab of a cpu. That could be addressed
> by optimizing the object free handling further to not touch the page
> struct even if we miss the per cpu slab.
> 
> That get_partial* is far up indicates contention on the list lock
> that should be addressable by either increasing the slab size or by
> changing the object free handling to batch in some form.
> 
> This is an SMP system right? 2 cores with 4 cpus each? The main loop
> is always hitting on the same slabs? Which slabs would this be? Am I
> right in thinking that one process allocates objects and then lets
> multiple other processors do work and then the allocated object is
> freed from a cpu that did not allocate the object? If neighboring
> objects in one slab are allocated on one cpu and then are almost
> simultaneously freed from a set of different cpus then this may be
> explain the situation. -

one of the characteristics of the application in use is the following:
all cores submit IO (which means they allocate various scsi and block
structures on all cpus).. but only 1 will free it (the one the IRQ is
bound to). SO it's allocate-on-one-free-on-another at a high rate.

That is assuming this is the IO slab; that's a bit of an assumption
obviously (it's one of the slab things that are hot, but it's a complex
workload, there could be others)
-
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