linux-kernel - Re: [PATCH] dcache: Translating dentry into pathname without taking rename

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Date:	Thu, 5 Sep 2013 09:29:56 -0400
From:	"John Stoffel" <john@...ffel.org>
To:	Waiman Long <waiman.long@...com>
Cc:	John Stoffel <john@...ffel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] dcache: Translating dentry into pathname without taking
 rename_lock

>>>>> "Waiman" == Waiman Long <waiman.long@...com> writes:

Waiman> On 09/04/2013 04:40 PM, John Stoffel wrote:
>>>>>>> "Waiman" == Waiman Long<waiman.long@...com>  writes:
Waiman> In term of AIM7 performance, this patch has a performance boost of
Waiman> about 6-7% on top of Linus' lockref patch on a 8-socket 80-core DL980.
>> 
Waiman> User Range          |   10-100  | 200-10000 | 1100-2000 |
Waiman> Mean JPM w/o patch  | 4,365,114 | 7,211,115 | 6,964,439 |
Waiman> Mean JPM with patch | 3,872,850 | 7,655,958 | 7,422,598 |
Waiman> % Change            |  -11.3%   |   +6.2%   |   +6.6%   |
>> 
>> This -11% impact is worisome to me, because at smaller numbers of
>> users, I would still expect the performance to go up.  So why the big
>> drop?
>> 
>> Also, how is the impact of these changes on smaller 1 socket, 4 core
>> systems?  Just because it helps a couple of big boxes, doesn't mean it
>> won't hurt the more common small case.
>> 
>> John

Waiman> I don't believe the patch will make it slower with less
Waiman> user. It is more a result of run-to-run variation. The short
Waiman> workload typically completed in a very short time. In the
Waiman> 10-100 user range, the completion times range from
Waiman> 0.02-0.11s. With a higher user count, it needs several seconds
Waiman> to run and hence the results are more reliable.

Can you then show the variation over multiple runs?  I think you have
a good justification for larger boxes to make this change, I just
worry about smaller systems getting hit and losing performance.

John

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