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

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Message-ID: <20120413184158.GZ17822@one.firstfloor.org>
Date:	Fri, 13 Apr 2012 20:41:58 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ted Ts'o <tytso@....edu>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	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 02:37:22PM -0400, Ted Ts'o wrote:
> 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?

4 sockets, 40 cores, 80 threads.

> 
> 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 files are in memory, but we're still CPU constrained due to various
other issues.

> 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.

I bet Eric didn't test with this statistic counter.

> Ultimately, this isn't a regression and if Linus is willing to take a

The old kernel didn't have that problem, so it's a regression.

> change at this point, I'm willing to send it --- but I really don't
> understand the urgency.

If we don't fix performance regressions before each release then Linux
will get slower and slower. At least I don't want a slow Linux.

-Andi
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