On Fri, Oct 14, 2011 at 07:25:06AM -0500, Dimitri Sivanich wrote: > On Thu, Oct 13, 2011 at 02:24:34PM -0700, Andrew Morton wrote: > > On Thu, 13 Oct 2011 16:02:58 -0500 (CDT) > > Christoph Lameter <cl@xxxxxxxxxx> wrote: > > > > > On Thu, 13 Oct 2011, Andrew Morton wrote: > > > > > > > > If there are no updates occurring for a while (due to increased deltas > > > > > and/or vmstat updates) then the vm_stat cacheline should be able to stay > > > > > in shared mode in multiple processors and the performance should increase. > > > > > > > > > > > > > We could cacheline align vm_stat[]. But the thing is pretty small - we > > > > couild put each entry in its own cacheline. > > > > > > Which in turn would increase the cache footprint of some key kernel > > > functions (because they need multiple vm_stat entries) and cause eviction > > > of other cachelines that then reduce overall system performance again. > > > > Sure, but we gain performance by not having different CPUs treading on > > each other when they update different vmstat fields. Sometimes one > > effect will win and other times the other effect will win. Some > > engineering is needed.. > > I think the first step is to determine the role (if any) that false sharing may be playing in this, since that's a simpler fix (cacheline align and pad the array). > Testing on a smaller machine with 46 writer threads in parallel (my original test used 120). Looks as though cache-aligning and padding the end of the vm_stat array results in a ~150 MB/sec speedup. This is a nice improvement for only 46 writer threads, though it's not the full ~250 MB/sec speedup I get from setting OVERCOMMIT_NEVER. -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>