On Fri, 2010-08-27 at 04:31 +0800, Mel Gorman wrote: > On Thu, Aug 26, 2010 at 08:29:04PM +0200, Johannes Weiner wrote: > > On Thu, Aug 26, 2010 at 04:14:15PM +0100, Mel Gorman wrote: > > > If congestion_wait() is called when there is no congestion, the caller > > > will wait for the full timeout. This can cause unreasonable and > > > unnecessary stalls. There are a number of potential modifications that > > > could be made to wake sleepers but this patch measures how serious the > > > problem is. It keeps count of how many congested BDIs there are. If > > > congestion_wait() is called with no BDIs congested, the tracepoint will > > > record that the wait was unnecessary. > > > > I am not convinced that unnecessary is the right word. On a workload > > without any IO (i.e. no congestion_wait() necessary, ever), I noticed > > the VM regressing both in time and in reclaiming the right pages when > > simply removing congestion_wait() from the direct reclaim paths (the > > one in __alloc_pages_slowpath and the other one in > > do_try_to_free_pages). > > > > So just being stupid and waiting for the timeout in direct reclaim > > while kswapd can make progress seemed to do a better job for that > > load. > > > > I can not exactly pinpoint the reason for that behaviour, it would be > > nice if somebody had an idea. > > > > There is a possibility that the behaviour in that case was due to flusher > threads doing the writes rather than direct reclaim queueing pages for IO > in an inefficient manner. So the stall is stupid but happens to work out > well because flusher threads get the chance to do work. If this is the case, we already have queue congested. removing congestion_wait() might cause regression but either your change or the congestion_wait_check() should not have the regression, as we do check if the bdi is congested. Thanks, Shaohua -- 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>