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

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Date:	Thu, 16 Jan 2014 09:08:36 -0800
From:	Dave Hansen <dave@...1.net>
To:	Christoph Lameter <cl@...ux.com>
CC:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/9] re-shrink 'struct page' when SLUB is on.

On 01/16/2014 08:44 AM, Christoph Lameter wrote:
> On Tue, 14 Jan 2014, Dave Hansen wrote:
> 
>> On 01/14/2014 12:07 PM, Christoph Lameter wrote:
>>> One easy way to shrink struct page is to simply remove the feature. The
>>> patchset looked a bit complicated and does many other things.
>>
>> Sure.  There's a clear path if you only care about 'struct page' size,
>> or if you only care about making the slub fast path as fast as possible.
>>  We've got three variables, though:
>>
>> 1. slub fast path speed
> 
> The fast path does use this_cpu_cmpxchg_double which is something
> different from a cmpxchg_double and its not used on struct page.

Yeah, I'm confusing the two.  I might as well say: "slub speed when
touching 'struct page'"

>> Arranged in three basic choices:
>>
>> 1. Big 'struct page', fast, medium complexity code
>> 2. Small 'struct page', slow, lowest complexity
> 
> The numbers that I see seem to indicate that a big struct page means slow.

This was a really tight loop where the caches are really hot, but it did
show the large 'struct page' winning:

	http://sr71.net/~dave/intel/slub/slub-perf-20140109.png

As I said in the earlier description, the paravirt code doing interrupt
disabling was what really hurt the two spinlock cases.

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