linux-kernel - Re: [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on.

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Date:	Mon, 16 Dec 2013 16:45:45 -0800
From:	Dave Hansen <dave@...1.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Pravin B Shelar <pshelar@...ira.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [RFC][PATCH 0/7] re-shrink 'struct page' when SLUB is on.

On 12/16/2013 04:01 PM, Andrew Morton wrote:
> On Fri, 13 Dec 2013 15:59:03 -0800 Dave Hansen <dave@...1.net> wrote:
>> SLUB depends on a 16-byte cmpxchg for an optimization.  For the
>> purposes of this series, I'm assuming that it is a very important
>> optimization that we desperately need to keep around.
> 
> What if we don't do that.

I'll do some testing and see if I can coax out any delta from the
optimization myself.  Christoph went to a lot of trouble to put this
together, so I assumed that he had a really good reason, although the
changelogs don't really mention any.

I honestly can't imagine that a cmpxchg16 is going to be *THAT* much
cheaper than a per-page spinlock.  The contended case of the cmpxchg is
way more expensive than spinlock contention for sure.

fc9bb8c768's commit message says:
>     The doublewords must be properly aligned for cmpxchg_double to work.
>     Sadly this increases the size of page struct by one word on some architectures.
>     But as a resultpage structs are now cacheline aligned on x86_64.

I'm not sure what aligning them buys us though.  I think I just
demonstrated that cache footprint is *way* more important than alignment.
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