Re: [RFC] sched/numa: don't move tasks to idle numa nodes while src node has very light load?

[Mel Gorman]

On Mon, Sep 07, 2020 at 12:00:10PM +0000, Song Bao Hua (Barry Song) wrote:
> Hi All,
> In case we have a numa system with 4 nodes and in each node we have 24 cpus, and all of the 96 cores are idle.
> Then we start a process with 4 threads in this totally idle system. 
> Actually any one of the four numa nodes should have enough capability to run the 4 threads while they can still have 20 idle CPUS after that.
> But right now the existing code in CFS load balance will spread the 4 threads to multiple nodes.
> This results in two negative side effects:
> 1. more numa nodes are awaken while they can save power in lowest frequency and halt status
> 2. cache coherency overhead between numa nodes
> 
> A proof-of-concept patch I made to "fix" this issue to some extent is like:
> 

This is similar in concept to a patch that did something similar except
in adjust_numa_imbalance(). It ended up being great for light loads like
simple communicating pairs but fell apart for some HPC workloads when
memory bandwidth requirements increased. Ultimately it was dropped until
the NUMA/CPU load balancing was reconciled so may be worth a revisit. At
the time, it was really problematic once a one node was roughly 25% CPU
utilised on a 2-socket machine with hyper-threading enabled. The patch may
still work out but it would need wider testing. Within mmtests, the NAS
workloads for D-class on a 2-socket machine varying the number of parallel
tasks/processes are used should be enough to determine if the patch is
free from side-effects for one machine. It gets problematic for different
machine sizes as the point where memory bandwidth is saturated varies.
group_weight/4 might be fine on one machine as a cut-off and a problem
on a larger machine with more cores -- I hit that particular problem
when one 2 socket machine with 48 logical CPUs was fine but a different
machine with 80 logical CPUs regressed.

I'm not saying the patch is wrong, just that patches in general for this
area (everyone, not just you) need fairly broad testing.

-- 
Mel Gorman
SUSE Labs