Re: [PATCH] mm: readahead: Increase maximum readahead window

[Jan Kara <jack@suse.cz>]

On Wed 04-10-17 10:41:51, Darrick J. Wong wrote:
> On Wed, Oct 04, 2017 at 11:12:05AM +0200, Jan Kara wrote:
> > Increase default maximum allowed readahead window from 128 KB to 512 KB.
> > This improves performance for some workloads (see below for details) where
> > ability to scale readahead window to larger sizes allows for better total
> > throughput while chances for regression are rather low given readahead
> > window size is dynamically computed based on observation (and thus it never
> > grows large for workloads with a random read pattern).
> > 
> > Note that the same tuning can be done using udev rules or by manually setting
> > the sysctl parameter however we believe the new value is a better default most
> > users will want to use. As a data point we carry this patch in SUSE kernels
> > for over 8 years.
> > 
> > Some data from the last evaluation of this patch (on 4.4-based kernel, I can
> > rerun those tests on a newer kernel but nothing has changed in the readahead
> > area since 4.4). The patch was evaluated on two machines
> 
> This is purely speculating, but I think this is worth at least a quick
> retry on 4.14 to see what's changed in the past 10 kernel release.  For
> one thing, ext3 no longer exists, and XFS' file IO path has changed
> quite a lot since then.

ext3 in this test is actually using ext4 driver already, so that has not
changed. I agree XFS has changed quite a bit so results might differ there.
I can rerun it with current kernel to see whether XFS behavior changed.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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