linux-ext4 - Re: [RFC, PATCH] Avoid hot statistics cache line in ext4 extent cache

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Date:	Fri, 13 Apr 2012 11:31:16 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>, Ted Ts'o <tytso@....edu>,
	Vivek Haldar <haldar@...gle.com>,
	Andreas Dilger <adilger@...ger.ca>, linux-ext4@...r.kernel.org
Subject: Re: [RFC, PATCH] Avoid hot statistics cache line in ext4 extent
 cache

On Fri, 2012-04-13 at 11:08 -0700, Linus Torvalds wrote:
> On Fri, Apr 13, 2012 at 10:53 AM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> >>
> >> Do you have numbers?
> >
> > In a benchmark that does mmaped-read, there is a 28% speed up
> > after getting rid of the counters.
> 
> Ok, that's big. But is it any actual real workload on a real
> filesystem? It looks like this should only happen for actual IO, so I
> get the feeling that this is some made-up benchmark for a filesystem
> on a RAM-disk?
> 
>                   Linus

Benchmark is working on files on normal hard disk.  
However, I have I have a large number
of processes (80 processes, one for each cpu), each reading
a separate mmaped file.  The files are in the same directory.
That makes cache line bouncing on the counters particularly bad
due to the large number of processes running.

Tim

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