On Thu, Dec 20, 2012 at 12:17:07AM +0100, Zlatko Calusic wrote: > On a 4GB RAM machine, where Normal zone is much smaller than > DMA32 zone, the Normal zone gets fragmented in time. This requires > relatively more pressure in balance_pgdat to get the zone above the > required watermark. Unfortunately, the congestion_wait() call in there > slows it down for a completely wrong reason, expecting that there's > a lot of writeback/swapout, even when there's none (much more common). > After a few days, when fragmentation progresses, this flawed logic > translates to a very high CPU iowait times, even though there's no > I/O congestion at all. If THP is enabled, the problem occurs sooner, > but I was able to see it even on !THP kernels, just by giving it a bit > more time to occur. > > The proper way to deal with this is to not wait, unless there's > congestion. Thanks to Mel Gorman, we already have the function that > perfectly fits the job. The patch was tested on a machine which > nicely revealed the problem after only 1 day of uptime, and it's been > working great. > --- > mm/vmscan.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > Acked-by: Mel Gorman <mgorman@xxxxxxx -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>