linux-kernel - Re: merging the per-bdi writeback patchset

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Message-ID: <20090623085505.GU31415@kernel.dk>
Date:	Tue, 23 Jun 2009 10:55:05 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>, hch@...radead.org
Subject: Re: merging the per-bdi writeback patchset

On Tue, Jun 23 2009, Andrew Morton wrote:
> On Tue, 23 Jun 2009 10:11:56 +0200 Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > Things are looking good for this patchset and it's been in -next for
> > almost a week without any reports of problems. So I'd like to merge it
> > for 2.6.31 if at all possible. Any objections?
> 
> erk.  I was rather expecting I'd have time to have a look at it all.

OK, we can wait if we have to, just trying to avoid having to keep this
fresh for one full cycle. I have posted this patchset 11 times though
over months, so it's not like it's a new piece of work :-)

> It's unclear to me actually _why_ the performance changes which were
> observed have actually occurred.  In fact it's a bit unclear (to me)
> why the patchset was written and what it sets out to achieve :(

It started out trying to get rid of the pdflush uneven writeout. If you
look at various pdflush intensive workloads, even on a single disk you
often have 5 or more pdflush threads working the same device. It's just
not optimal. Another issue was starvation with request allocation. Given
that pdflush does non-blocking writes (it has to, by design), pdflush
can potentially be starved if someone else is working the device.

> A long time ago the XFS guys (Dave Chinner iirc) said that XFS needs
> more than one thread per device to keep the device saturated.  Did that
> get addressed?

It supports up to 32-threads per device, but Chinner et all have been
silent. So the support is there and there's a
super_operations->inode_get_wb() to map a dirty inode to a writeback
device. Nobody is doing that yet though.

> (kthread_run() returns an ERR_PTR() on error, btw - not NULL.)

Oh thanks, will fix that up.

-- 
Jens Axboe

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