Re: [PATCH 3/3] writeback: Do not congestion sleep when there are no congested BDIs

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 26, 2010 at 04:14:16PM +0100, Mel Gorman wrote:
> If congestion_wait() is called with no BDIs congested, the caller will
> sleep for the full timeout and this is an unnecessary sleep.

That, I think, is an invalid assumption. congestion_wait is used in
some places as a backoff mechanism that waits for some IO work to be
done, with congestion disappearing being a indication that progress
has been made and so we can retry sooner than the entire timeout.

For example, if _xfs_buf_lookup_pages() fails to allocate page cache
pages for a buffer, it will kick the xfsbufd to writeback dirty
buffers (so they can be freed) and immediately enter
congestion_wait(). If there isn't congestion when we enter
congestion_wait(), we still want to give the xfsbufds a chance to
clean some pages before we retry the allocation for the new buffer.
Removing the congestion_wait() sleep behaviour will effectively
_increase_ memory pressure with XFS on fast disk subsystems because
it now won't backoff between failed allocation attempts...

Perhaps a congestion_wait_iff_congested() variant is needed for the
VM? I can certainly see how it benefits the VM from a latency
perspective, but it is the opposite behaviour that is expected in
other places...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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