On Fri, Aug 27, 2010 at 09:42:54AM +0800, Wu Fengguang wrote: > On Fri, Aug 27, 2010 at 02:17:35AM +0800, 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. > > congestion_wait() is tightly related with pageout() and writeback, > however it may have some intention for the no-IO case as well. > > - if write congested, maybe we are doing too much pageout(), so wait. > it might also reduce some get_request_wait() stalls (the normal way > is to explicitly check for congestion before doing write out). > > - if any write completes, it may free some PG_reclaim pages, so proceed. > (when not congested) > For these cases, would it make sense for wait_iff_congested() to compare nr_writeback to nr_inactive and decide to wait on congestion if more than half the inactive list is in writeback? > - if no IO at all, the 100ms sleep might still prevent a page reclaimer > from stealing lots of slices from a busy computing program that > involves no page allocation at all. > I don't think this is a very strong arguement because cond_reched() is being called. -- 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>