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

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Message-ID: <20120413183722.GG26332@thunk.org>
Date:	Fri, 13 Apr 2012 14:37:22 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	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, Apr 13, 2012 at 11:31:16AM -0700, Tim Chen wrote:
> 
> 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.

OK, so this is with an 80 CPU machine?

And when you say 20% speed up, do you mean to say we are actually
being CPU constrained when reading from files on a normal hard disk?

The reason why I ask this is we're seeing anything like this with Eric
Whitney's 48 CPU scalability testing; we're not CPU bottlenecked, and
I don't even see evidence of a larger than usual CPU utilization
compared to other file systems.

So still I'm trying to understand why your results are so different
from what Eric has been seeing, and I'm still puzzled why this is
super urgent.

Ultimately, this isn't a regression and if Linus is willing to take a
change at this point, I'm willing to send it --- but I really don't
understand the urgency.

Best regards,

						- Ted
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