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. So personally I think it's a good idea to get an insight on the use of congestion_wait() [patch 1] but I don't agree with changing its behaviour just yet, or judging its usefulness solely on whether it correctly waits for bdi congestion. -- 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>