On Tue, Oct 18, 2011 at 06:16:21PM -0700, David Rientjes wrote: > On Tue, 18 Oct 2011, Andi Kleen wrote: > > > > Would it make sense to have the ZVC delta be tuneable (via /proc/sys/vm?), keeping the > > > same default behavior as what we currently have? > > > > Tunable is bad. We don't really want a "hundreds of lines magic shell script to > > make large systems perform". Please find a way to auto tune. > > > > Agreed, and I think even if we had a tunable that it would result in > potentially erradic VM performance because some areas depend on "fairly > accurate" ZVCs and it wouldn't be clear that you're trading other unknown > VM issues that will affect your workload because you've increased the > deltas. Let's try to avoid having to ask "what is your ZVC delta tunable > set at?" when someone reports a bug about reclaim stopping preemptively. Yes, I'm inclined to agree. > > That said, perhaps we need higher deltas by default and then hints in key > areas in the form of sync_stats_if_delta_above(x) calls that would do > zone_page_state_add() only when that kind of precision is actually needed. > For public interfaces, that would be very easy to audit to see what the > level of precision is when parsing the data. I did some manual tuning to see what deltas would be needed to achieve the greatest tmpfs writeback performance on a system with 640 cpus and 64 nodes: For 120 threads writing in parallel (each to it's own mountpoint), the threshold needs to be on the order of 1000. At a threshold of 750, I start to see a slowdown of 50-60 MB/sec. For 400 threads writing in parallel, the threshold needs to be on the order of 2000 (although we're off by about 40 MB/sec at that point). The necessary deltas in these cases are quite a bit higher than the current 125 maximum (see calculate*threshold in mm/vmstat.c). I like the idea of having certain areas triggering vm_stat sync, as long as we know what those key areas are and how often they might be called. -- 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>