On Mon, Jun 9, 2014 at 1:00 PM, Kevin Grittner <kgri...@ymail.com> wrote: > Andres Freund <and...@2ndquadrant.com> wrote: >> On 2014-06-09 08:59:03 -0700, Kevin Grittner wrote: >>>> *) There is a lot of advice floating around (for example here: >>>> http://frosty-postgres.blogspot.com/2012/08/postgresql-numa-and-zone-reclaim-mode.html >>>> ) >>>> to instruct operators to disable zone_reclaim. Will your changes >>>> invalidate any of that advice? >>> >>> I expect that it will make the need for that far less acute, >>> although it is probably still best to disable zone_reclaim (based >>> on the documented conditions under which disabling it makes sense). >> >> I think it'll still be important unless you're running an OLTP workload >> (i.e. minimal per backend allocations) and your entire workload fits >> into shared buffers. What zone_reclaim > 0 essentially does is to never >> allocate memory from remote nodes. I.e. it will throw away all numa node >> local OS cache to satisfy a memory allocation (including >> pagefaults). > > I don't think that cpuset spreading of OS buffers and cache, and > the patch to spread shared memory, will make too much difference > unless the working set is fully cached. Where I have seen the > biggest problems is when the active set > one memory node and < > total machine RAM.
But that's precisely the scenario where vm.zone_reclaim_mode != 0 is a disaster. You'll end up throwing away the cached pages and rereading the data from disk, even though the memory *could* have been kept all in cache. > I would agree that unless this patch is > providing benefit for such a fully-cached load, it won't make any > difference regarding the need for zone_reclaim_mode. Where the > data is heavily cached, zone_reclaim > 0 might discard some cached > pages to allow, say, a RAM sort to be done in faster memory (for > the current process's core), so it might be a wash or even make > zone_reclaim > 0 a win. I will believe that when, and only when, I see benchmarks convincingly demonstrating it. Setting zone_reclaim_mode can only be a win if the performance benefit from using faster memory is greater than the performance cost of any rereading-from-disk that happens. IME, that's a highly unusual situation. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers