linux-ext4 - Re: [RFC PATCH] ext4: fix 50% disk write performance regression

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Message-ID: <4C7C0819.8040505@redhat.com>
Date:	Mon, 30 Aug 2010 14:35:53 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Bill Fink <bill@...ard.sci.gsfc.nasa.gov>
CC:	Bill Fink <billfink@...dspring.com>,
	"tytso@....edu" <tytso@....edu>,
	"adilger@....com" <adilger@....com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Fink, William E. (GSFC-6061)" <william.e.fink@...a.gov>
Subject: Re: [RFC PATCH] ext4: fix 50% disk write performance regression

Bill Fink wrote:
> On Mon, 30 Aug 2010, Eric Sandeen wrote:

...

>> When I get some time (soon I hope) I'll look into the ramifications
>> of this change (i.e. what if wbc->nr_to_write * 8 is more than the dirty
>> pages, do things work out ok?) but it seems pretty reasonable.
> 
> In thinking about that issue, my non-expert thought was that
> if desired_nr_to_write was larger, it hopefully wouldn't be an
> issue since presumably it's only going to actually write at most
> the number of dirty pages anyway.  And on the flip side, there
> doesn't seem to be an issue with desired_nr_to_write possibly
> being smaller that it was without the patch, since the empirical
> evidence is that the performance significantly improved with the
> patch, and the effect of a lower value would presumably be
> reduced performance.
> 
> 					-Bill

Ted's suggestion of capturing blktrace data would be a really great
step towards understanding what changed, too.

Thanks!

-Eric
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