Messages in this thread | | | Date | Tue, 23 Jul 2019 12:42:48 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH v3] sched/topology: Improve load balancing on AMD EPYC |
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On Tue, Jul 23, 2019 at 11:48:30AM +0100, Matt Fleming wrote: > SD_BALANCE_{FORK,EXEC} and SD_WAKE_AFFINE are stripped in sd_init() > for any sched domains with a NUMA distance greater than 2 hops > (RECLAIM_DISTANCE). The idea being that it's expensive to balance > across domains that far apart. > > However, as is rather unfortunately explained in > > commit 32e45ff43eaf ("mm: increase RECLAIM_DISTANCE to 30") > > the value for RECLAIM_DISTANCE is based on node distance tables from > 2011-era hardware. > > Current AMD EPYC machines have the following NUMA node distances: > > node distances: > node 0 1 2 3 4 5 6 7 > 0: 10 16 16 16 32 32 32 32 > 1: 16 10 16 16 32 32 32 32 > 2: 16 16 10 16 32 32 32 32 > 3: 16 16 16 10 32 32 32 32 > 4: 32 32 32 32 10 16 16 16 > 5: 32 32 32 32 16 10 16 16 > 6: 32 32 32 32 16 16 10 16 > 7: 32 32 32 32 16 16 16 10 > > where 2 hops is 32. > > The result is that the scheduler fails to load balance properly across > NUMA nodes on different sockets -- 2 hops apart. > > For example, pinning 16 busy threads to NUMA nodes 0 (CPUs 0-7) and 4 > (CPUs 32-39) like so, > > $ numactl -C 0-7,32-39 ./spinner 16 > > causes all threads to fork and remain on node 0 until the active > balancer kicks in after a few seconds and forcibly moves some threads > to node 4. > > Override node_reclaim_distance for AMD Zen. > > Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk> > Cc: "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@amd.com> > Cc: Mel Gorman <mgorman@techsingularity.net> > Cc: "Lendacky, Thomas" <Thomas.Lendacky@amd.com> > Cc: Borislav Petkov <bp@alien8.de>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
The only caveat I can think of is that a future generation of Zen might take a different magic number than 32 as their remote distance. If or when this happens, it'll need additional smarts but lacking a crystal ball, we can cross that bridge when we come to it.
-- Mel Gorman SUSE Labs
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