Matt,
On 7/23/2019 5:48 AM, 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.
I am testing this patch on the Linux-5.2, and I actually do not
notice difference pre vs post patch.
Besides the case above, I have also run an experiment with
a different number of threads across two sockets:
(Note: I only focus on thread0 of each core.)
sXnY = Socket X Node Y
* s0n0 + s0n1 + s1n0 + s1n1
numactl -C 0-15,32-47 ./spinner 32
* s0n2 + s0n3 + s1n2 + s1n3
numactl -C 16-31,48-63 ./spinner 32
* s0 + s1
numactl -C 0-63 ./spinner 64
My observations are:
* I still notice improper load-balance on one of the task initially
for a few seconds before they are load-balanced correctly.
* It is taking longer to load balance w/ more number of tasks.
I wonder if you have tried with a different kernel base?
Regards,
Suravee
