linux-kernel - Re: [RFC][PATCH 5/7] writeback: use 64MB MAX_WRITEBACK

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Date:	Thu, 10 Sep 2009 08:13:06 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Theodore Tso <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>, jack@...e.cz,
	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/7] writeback: use 64MB MAX_WRITEBACK_PAGES

On Thu, Sep 10, 2009 at 07:29:38AM +0800, Theodore Tso wrote:
> On Wed, Sep 09, 2009 at 10:51:46PM +0800, Wu Fengguang wrote:
> > + * The maximum number of pages to writeout in a single periodic/background
> > + * writeback operation. 64MB means I_SYNC may be hold for up to 1 second.
> > + * This is not a big problem since we normally do kind of trylock on I_SYNC
> > + * for non-data-integrity writes.  Userspace tasks doing throttled writeback
> > + * do not use this value.
> 
> What's your justification for using 64MB?  Where are you getting 1
> second from?  On a fast RAID array 64MB can be written in much less
> than 1 second.

Because this value will be used for desktop and server alike, we have
to consider the worst case - for a laptop, it takes about 1 second to
write 64MB. It's not accurate to say I_SYNC may be hold for up to 1
second, but the same I_SYNC will be taken, dropped because of
congestion, retaken, and this loop could go on for 1 second until a
large file have been written 64MB. In the mean while, in the normal
case of a single kupdate thread per bdi, that file blocks writeout of
other files for 1 second.

> More generally, I assume your patches conflict with Jens' per-bdi patches?

It is based on latest linux-next tree with the vm.max_writeback_mb
patch reverted. The changelog only states the need to increase the
size, but not mentioned why it should be made tunable. So I tend to
just bump up MAX_WRITEBACK_PAGES. It seems that a large value won't
hurt anyone. Or are there demand to further increase it a lot for some
server configurations?

Thanks,
Fengguang
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