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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>