linux-kernel - Re: [PATCH 0/9] re-shrink 'struct page' when SLUB is on.

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Date:	Fri, 3 Jan 2014 14:18:16 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	penberg@...nel.org, cl@...ux-foundation.org
Subject: Re: [PATCH 0/9] re-shrink 'struct page' when SLUB is on.

On Fri, 03 Jan 2014 10:01:47 -0800 Dave Hansen <dave@...1.net> wrote:

> This is a minor update from the last version.  The most notable
> thing is that I was able to demonstrate that maintaining the
> cmpxchg16 optimization has _some_ value.
> 
> Otherwise, the code changes are just a few minor cleanups.
> 
> ---
> 
> SLUB depends on a 16-byte cmpxchg for an optimization which
> allows it to not disable interrupts in its fast path.  This
> optimization has some small but measurable benefits:
> 
> 	http://lkml.kernel.org/r/52B345A3.6090700@sr71.net

So really the only significant benefit from the cmpxchg16 is with
cache-cold eight-byte kmalloc/kfree?  8% faster in this case?  But with
cache-hot kmalloc/kfree the benefit of cmpxchg16 is precisely zero.

This is really weird and makes me suspect a measurement glitch.

Even if this 8% is real, it's unclear that it's worth all the
complexity the cmpxchg16 adds.

It would be really useful (hint :)) if we were to know exactly where
that 8% is coming from - perhaps it's something which is not directly
related to the cmpxchg16, and we can fix it separately.

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