Re: [PATCH] mm: disallow direct reclaim page writeback

["Sorin Faibish" <sfaibish@emc.com>]

On Sun, 18 Apr 2010 15:05:26 -0400, Christoph Hellwig <hch@infradead.org>  
wrote:

> On Sat, Apr 17, 2010 at 08:32:39PM -0400, Andrew Morton wrote:
>> The poor IO patterns thing is a regression.  Some time several years
>> ago (around 2.6.16, perhaps), page reclaim started to do a LOT more
>> dirty-page writeback than it used to.  AFAIK nobody attempted to work
>> out why, nor attempted to try to fix it.
>
> I just know that we XFS guys have been complaining about it a lot..
I know also that the ext3 and reisefs guys complained about this issue
as well.

>
> But that was mostly a tuning issue - before writeout mostly happened
> from pdflush.  If we got into kswapd or direct reclaim we already
> did get horrible I/O patterns - it just happened far less often.
>
>> Regarding simply not doing any writeout in direct reclaim (Dave's
>> initial proposal): the problem is that pageout() will clean a page in
>> the target zone.  Normal writeout won't do that, so we could get into a
>> situation where vast amounts of writeout is happening, but none of it
>> is cleaning pages in the zone which we're trying to allocate from.
>> It's quite possibly livelockable, too.
>
> As Chris mentioned currently btrfs and ext4 do not actually do delalloc
> conversions from this path, so for typical workloads the amount of
> writeout that can happen from this path is extremly limited.  And unless
> we get things fixed we will have to do the same for XFS.  I'd be much
> more happy if we could just sort it out at the VM level, because this
> means we have one sane place for this kind of policy instead of three
> or more hacks down inside the filesystems.  It's rather interesting
> that all people on the modern fs side completely agree here what the
> problem is, but it seems rather hard to convince the VM side to do
> anything about it.
>
>> To solve the stack-usage thing: dunno, really.  One could envisage code
>> which skips pageout() if we're using more than X amount of stack, but
>> that sucks.
>
> And it doesn't solve other issues, like the whole lock taking problem.
>
>> Another possibility might be to hand the target page over
>> to another thread (I suppose kswapd will do) and then synchronise with
>> that thread - get_page()+wait_on_page_locked() is one way.  The helper
>> thread could of course do writearound.
>
> Allowing the flusher threads to do targeted writeout would be the
> best from the FS POV.  We'll still have one source of the I/O, just
> with another know on how to select the exact region to write out.
> We can still synchronously wait for the I/O for lumpy reclaim if really
> nessecary.
>
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-- 
Best Regards
Sorin Faibish
Corporate Distinguished Engineer
Network Storage Group

        EMC²
where information lives

Phone: 508-435-1000 x 48545
Cellphone: 617-510-0422
Email : sfaibish@emc.com