On Tue, Jun 09, 2009 at 10:48:34PM +0900, KOSAKI Motohiro wrote: > Hi > > sorry for late responce. my e-mail reading speed is very slow ;-) > > First, Could you please read past thread? > I think many topic of this mail are already discussed. >
I think I caught them all but the horrible fact of the matter is that whether zone_reclaim_mode should be 1 or 0 on NUMA machines is "it depends". There are arguements for both and no clear winner. > > > On Thu, Jun 04, 2009 at 07:23:15PM +0900, KOSAKI Motohiro wrote: > > > > > > Current linux policy is, zone_reclaim_mode is enabled by default if the > > > machine > > > has large remote node distance. it's because we could assume that large > > > distance > > > mean large server until recently. > > > > > > > We don't make assumptions about the server being large, small or otherwise. > > The > > affinity tables reporting a distance of 20 or more is saying "remote memory > > has twice the latency of local memory". This is true irrespective of > > workload > > and implies that going off-node has a real penalty regardless of workload. > > No. > Now, we talk about off-node allocation vs unnecessary file cache dropping. > IOW, off-node allocation vs disk access. > Even if we used GFP flags to identify the file pages, there is no guarantee that we are taking the correct action to keep "relevant" pages in memory. > Then, the worth doesn't only depend on off-node distance, but also depend on > workload IO tendency and IO speed. > > Fujitsu has 64 core ia64 HPC box, zone-reclaim sometimes made performance > degression although its box. > I bet if it was 0, that the off-node accesses would somewtimes make "performance degression" as well :( > So, I don't think this problem is small vs large machine issue. > nor i7 issue. > high-speed P2P CPU integrated memory controller expose old issue. > > > > > In general, workload depended configration shouldn't put into default > > > settings. > > > > > > However, current code is long standing about two year. Highest POWER and > > > IA64 HPC machine > > > (only) use this setting. > > > > > > Thus, x86 and almost rest architecture change default setting, but Only > > > power and ia64 > > > remain current configuration for backward-compatibility. > > > > > > > What about if it's x86-64-based NUMA but it's not i7 based. There, the > > NUMA distances might really mean something and that zone_reclaim behaviour > > is desirable. > > hmmm.. > I don't hope ignore AMD, I think it's common characterastic of P2P and > integrated memory controller machine. > > Also, I don't hope detect CPU family or similar, because we need update > such code evey when Intel makes new cpu. > > Can we detect P2P interconnect machine? I'm not sure. > I've no idea. It's not just I7 because some of the AMD chips will have integrated memory controllers as well. We were somewhat depending on the affinity information providing the necessary information. > > I think if we're going down the road of setting the default, it shouldn't be > > per-architecture defaults as such. Other choices for addressing this might > > be; > > > > 1. Make RECLAIM_DISTANCE a variable on x86. Set it to 20 by default, and 5 > > (or some other sensible figure) on i7 > > > > 2. There should be a per-arch modifier callback for the affinity > > distances. If the x86 code detects the CPU is an i7, it can reduce the > > reported latencies to be more in line with expected reality. > > > > 3. Do not use zone_reclaim() for file-backed data if more than 20% of memory > > overall is free. The difficulty is figuring out if the allocation is for > > file pages. > > > > 4. Change zone_reclaim_mode default to mean "do your best to figure it > > out". Patch 1 would default large distances to 1 to see what happens. > > Then apply a heuristic when in figure-it-out mode and using reclaim_mode > > == 1 > > > > If we have locally reclaimed 2% of the nodes memory in file pages > > within the last 5 seconds when >= 20% of total physical memory was > > free, then set the reclaim_mode to 0 on the assumption the node is > > mostly caching pages and shouldn't be reclaimed to avoid excessive IO > > > > Option 1 would appear to be the most straight-forward but option 2 > > should be doable. Option 3 and 4 could turn into a rats nest and I would > > consider those approaches a bit more drastic. > > hmhm. > I think the key-point of option 1 and 2 are proper hardware detecting way. > > option 3 and 4 are more prefere idea to me. I like workload adapted heuristic. > but you already pointed out its hard, because page-allocator don't know > allocation purpose ;) > Option 3 may be undoable. Even if the allocations are tagged as "this is a file-backed allocation", we have no way of detecting how important that is to the overall workload. Option 4 would be the preference. It's a heuristic that might let us down, but the administrator can override it and fix the reclaim_mode in the event we get it wrong. > > > > @@ -10,6 +10,12 @@ struct device_node; > > > > > > #include <asm/mmzone.h> > > > > > > +/* > > > + * Distance above which we begin to use zone reclaim > > > + */ > > > +#define RECLAIM_DISTANCE 20 > > > + > > > + > > > > Where is the ia-64-specific modifier to RECAIM_DISTANCE? > > > arch/ia64/include/asm/topology.h has > > /* > * Distance above which we begin to use zone reclaim > */ > #define RECLAIM_DISTANCE 15 > > > I don't think distance==15 is machine independent proper definition. > but there is long lived definition ;) > > > > -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
