On Fri, Aug 27, 2010 at 09:11:06AM +0800, Wu Fengguang wrote: > On Fri, Aug 27, 2010 at 04:23:24AM +0800, Mel Gorman wrote: > > On Thu, Aug 26, 2010 at 08:17:35PM +0200, Johannes Weiner wrote: > > > On Thu, Aug 26, 2010 at 06:42:45PM +0100, Mel Gorman wrote: > > > > On Fri, Aug 27, 2010 at 02:38:43AM +0900, Minchan Kim 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. This patch > > > > > > checks if there are BDIs congested. If so, it goes to sleep as normal. > > > > > > If not, it calls cond_resched() to ensure the caller is not hogging the > > > > > > CPU longer than its quota but otherwise will not sleep. > > > > > > > > > > > > This is aimed at reducing some of the major desktop stalls reported during > > > > > > IO. For example, while kswapd is operating, it calls congestion_wait() > > > > > > but it could just have been reclaiming clean page cache pages with no > > > > > > congestion. Without this patch, it would sleep for a full timeout but after > > > > > > this patch, it'll just call schedule() if it has been on the CPU too long. > > > > > > Similar logic applies to direct reclaimers that are not making enough > > > > > > progress. > > > > > > > > > > > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx> > > > > > > --- > > > > > > mm/backing-dev.c | 20 ++++++++++++++------ > > > > > > 1 files changed, 14 insertions(+), 6 deletions(-) > > > > > > > > > > > > diff --git a/mm/backing-dev.c b/mm/backing-dev.c > > > > > > index a49167f..6abe860 100644 > > > > > > --- a/mm/backing-dev.c > > > > > > +++ b/mm/backing-dev.c > > > > > > > > > > Function's decripton should be changed since we don't wait next write any more. > > > > > > > > > > > > > My bad. I need to check that "next write" thing. It doesn't appear to be > > > > happening but maybe that side of things just broke somewhere in the > > > > distant past. I lack context of how this is meant to work so maybe > > > > someone will educate me. > > > > > > On every retired io request the congestion state on the bdi is checked > > > and the congestion waitqueue woken up. > > > > > > So without congestion, we still only wait until the next write > > > retires, but without any IO, we sleep the full timeout. > > > > > > Check __freed_requests() in block/blk-core.c. > > > > > > > Seems reasonable. Still, if there is no write IO going on and no > > congestion there seems to be no point going to sleep for the full > > timeout. It still feels wrong. > > Yeah the stupid sleeping feels wrong. However there are ~20 > congestion_wait() callers spread randomly in VM, FS and block drivers. > Many of them may be added by rule of thumb, however what if some of > them happen to depend on the old stupid sleeping behavior? Obviously > you've done extensive tests on the page reclaim paths, however that's > far from enough to cover the wider changes made by this patch. > > We may have to do the conversions case by case. Converting to > congestion_wait_check() (see http://lkml.org/lkml/2010/8/18/292) or > other waiting schemes. > I am taking this direction now. -- 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>