On Mon, Jan 20, 2014 at 02:21:02PM -0500, riel@xxxxxxxxxx wrote: > From: Rik van Riel <riel@xxxxxxxxxx> > > Excessive migration of pages can hurt the performance of workloads > that span multiple NUMA nodes. However, it turns out that the > p->numa_migrate_deferred knob is a really big hammer, which does > reduce migration rates, but does not actually help performance. > > Now that the second stage of the automatic numa balancing code > has stabilized, it is time to replace the simplistic migration > deferral code with something smarter. > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Mel Gorman <mgorman@xxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: Chegu Vinod <chegu_vinod@xxxxxx> > Signed-off-by: Rik van Riel <riel@xxxxxxxxxx> When I added a tracepoint to track deferred migration I was surprised how often it triggered for some workloads. I agree that we want to do something better because it was a crutch albeit a necessary one at the time. Note that the knob was not about performance as such, it was about avoiding worst-case behaviour. We should keep an eye out for bugs that look like excessive migration on workloads that are not converging. Reintroducing this hammer would be a last resort for working around the problem. Finally, the sysctl is documented in Documentation/sysctl/kernel.txt and this patch should also remove it. Functionally, the patch looks fine and it's time to reinvestigate if it's necessary so assuming the documentation gets removed; 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>