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

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Aug 27, 2010 at 03:13:16PM +1000, Dave Chinner wrote:
> 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.
> 

As it's write IO rather than some IO, I wonder if that's really the
right thing to do. However, I accept your (and others) point that
converting all congestion_wait() callers may be too much of a change.

> 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...
> 

I'm added a wait_iff_congested() and updated a few of the VM callers. I changed
a fairly minimum number of what appeared to be the obvious ones to change.

Thanks

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab