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Re: page fault scalability (ext3, ext4, xfs)

To: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Subject: Re: page fault scalability (ext3, ext4, xfs)
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 15 Aug 2013 14:29:30 +1000
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx, Jan Kara <jack@xxxxxxx>, LKML <linux-kernel@xxxxxxxxxxxxxxx>, Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>, Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130815022401.GQ23412@xxxxxxxxxxxxxxxxxxxx>
References: <520BB9EF.5020308@xxxxxxxxxxxxxxx> <20130815002436.GI6023@dastard> <20130815022401.GQ23412@xxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Aug 14, 2013 at 07:24:01PM -0700, Andi Kleen wrote:
> > And FWIW, it's no secret that XFS has more per-operation overhead
> > than ext4 through the write path when it comes to allocation, so
> > it's no surprise that on a workload that is highly dependent on
> > allocation overhead that ext4 is a bit faster....
> 
> This cannot explain a worse scaling curve though?

The scaling curve is pretty much identical. The difference in
performance will be the overhead of timestamp updates through
the transaction subsystems of the filesystems.

> w-i-s is all about scaling.

Sure, but scaling *what*? It's spending all it's time in the
filesystem through the .page_mkwrite path. It's not a page fault
scaling test - it's a filesystem overwrite test that uses mmap.
Indeed, I bet if you replace the mmap() with a write(fd, buf, 4096)
loop, you'd get almost identical behaviour from the filesystems.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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