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

Dave Chinner
2011-06-30T00:15:30Z

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...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com