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

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Date:	Mon, 6 Jan 2014 13:32:37 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Hansen <dave@...1.net>, 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, Jan 03, 2014 at 02:18:16PM -0800, Andrew Morton wrote:
> 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.

Hello,

I guess that cmpxchg16 is not used in this cache-hot kmalloc/kfree test,
because kfree would be done in free fast-path. In this case,
this_cpu_cmpxchg_double() would be called, so you cannot find any effect
of cmpxchg16.

Thanks.

> 
> 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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