linux-kernel - Re: [GIT PULL] Lockless SLUB slowpaths for v3.1-rc1

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Date:	Sun, 31 Jul 2011 11:50:39 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Christoph Lameter <cl@...ux.com>
cc:	Pekka Enberg <penberg@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>, hughd@...gle.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [GIT PULL] Lockless SLUB slowpaths for v3.1-rc1

On Fri, 29 Jul 2011, Christoph Lameter wrote:

> > We haven't come up with a solution to keep struct page size the same but I
> > think it's a reasonable trade-off.
> 

We won't be coming up with a solution to that since the alignment is a 
requirement for cmpxchg16b, unfortunately.

> The change requires the page struct to be aligned to a double word
> boundary. There is actually no variable added to the page struct. Its just
> the alignment requirement that causes padding to be added after each page
> struct.
> 

Well, the counters variable is added although it doesn't increase the size 
of the unaligned struct page because of how it is restructured.  The end 
result of the alignment for CONFIG_CMPXCHG_LOCAL is that struct page will 
increase from 56 bytes to 64 bytes on my config.  That's a cost of 128MB 
on each of my client and server 64GB machines for the netperf benchmark 
for the ~2.3% speedup.
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