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Re: [PATCH 02/27] xfs: remove the unused ilock_nowait codepath in writep

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 02/27] xfs: remove the unused ilock_nowait codepath in writepage
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 30 Jun 2011 10:15:25 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110629140336.717434334@xxxxxxxxxxxxxxxxxxxxxx>
References: <20110629140109.003209430@xxxxxxxxxxxxxxxxxxxxxx> <20110629140336.717434334@xxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Jun 29, 2011 at 10:01:11AM -0400, Christoph Hellwig wrote:
> wbc->nonblocking is never set, so this whole code has been unreachable
> for a long time.  I'm also not sure it would make a lot of sense -
> we'd rather finish our writeout after a short wait for the ilock
> instead of cancelling the whole ioend.

The problem that the non-blocking code is trying to solve is only
obvious when the disk subsystem is fast enough to drive the flusher
thread to being CPU bound.

e.g. when you have a disk subsystem doing background writeback
10GB/s and the flusher thread is put to sleep for 50ms while we wait
for the lock, it can now only push 9.5GB/s. If we just move on, then
we'll spend that 50ms doing useful work on another dirty inode
rather than sleeping onthis one and hence maintaining a 10GB/s
background write rate.

I'd suggest that the only thing that should be dropped is the
wbc->nonblocking check. Numbers would be good to validate that this
is still relevant, but I don't have a storage subsystem with enough
bandwidth to drive a flusher thread to being CPU bound...


Dave Chinner

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