Re: [Lsf-pc] [dm-devel] [LSF/MM TOPIC] a few storage topics

[Jan Kara <jack@suse.cz>]

On Wed 25-01-12 14:33:54, Steven Whitehouse wrote:
> On Tue, 2012-01-24 at 23:15 -0700, Andreas Dilger wrote:
> > On 2012-01-24, at 8:29 PM, Wu Fengguang wrote:
> > > On Tue, Jan 24, 2012 at 09:39:36PM +0100, Jan Kara wrote:
> > >> On Tue 24-01-12 15:13:40, Jeff Moyer wrote:
> > >>>> Maybe 128 KB is a too small default these days but OTOH noone prevents you
> > >>>> from raising it (e.g. SLES uses 1 MB as a default).
> > >>> 
> > >>> For some reason, I thought it had been bumped to 512KB by default.  Must
> > >>> be that overactive imagination I have...  Anyway, if all of the distros
> > >>> start bumping the default, don't you think it's time to consider bumping
> > >>> it upstream, too?  I thought there was a lot of work put into not being
> > >>> too aggressive on readahead, so the downside of having a larger
> > >>> read_ahead_kb setting was fairly small.
> > >> 
> > >>  Yeah, I believe 512KB should be pretty safe these days except for
> > >> embedded world. OTOH average desktop user doesn't really care so it's
> > >> mostly servers with beefy storage that care... (note that I wrote we raised
> > >> the read_ahead_kb for SLES but not for openSUSE or SLED (desktop enterprise
> > >> distro)).
> > > 
> > > Maybe we don't need to care much about the embedded world when raising
> > > the default readahead size? Because even the current 128KB is too much
> > > for them, and I see Android setting the readahead size to 4KB...
> > > 
> > > Some time ago I posted a series for raising the default readahead size
> > > to 512KB. But I'm open to use 1MB now (shall we vote on it?).
> > 
> > I'm all in favour of 1MB (aligned) readahead.  I think the embedded folks
> > already set enough CONFIG opts that we could trigger on one of those
> > (e.g. CONFIG_EMBEDDED) to avoid stepping on their toes.  It would also be
> > possible to trigger on the size of the device so that the 32MB USB stick
> > doesn't sit busy for a minute with readahead that is useless.
> > 
> > Cheers, Andreas
> > 
> 
> If the reason for not setting a larger readahead value is just that it
> might increase memory pressure and thus decrease performance, is it
> possible to use a suitable metric from the VM in order to set the value
> automatically according to circumstances?
  In theory yes. In practice - do you have such heuristic ;)? There are lot
of factors and it's hard to quantify how increased cache pressure
influences performance of a particular workload. We could introduce some
adaptive logic but so far fixed upperbound worked OK.

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