Messages in this thread | | | Date | Fri, 7 Sep 2018 13:33:09 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/4] sched/numa: Do not move imbalanced load purely on the basis of an idle CPU |
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On Fri, Sep 07, 2018 at 11:11:39AM +0100, Mel Gorman wrote: > Commit 305c1fac3225 ("sched/numa: Evaluate move once per node") > restructured how task_numa_compare evaluates load but there is an anomaly. > task_numa_find_cpu() checks if the load balance between too nodes is too > imbalanced with the intent of only swapping tasks if it would improve > the balance overall. However, if an idle CPU is encountered, the task is > still moved if it's the best improvement but an idle cpu is always going > to appear to be the best improvement. > > If a machine is lightly loaded such that all tasks can fit on one node then > the idle CPUs are found and the tasks migrate to one socket. From a NUMA > perspective, this seems intuitively great because memory accesses are all > local but there are two counter-intuitive effects. > > First, the load balancer may move tasks so the machine is more evenly > utilised and conflict with automatic NUMA balancing which may respond by > scanning more frequently and increasing overhead. Second, sockets usually > have their own memory channels so using one socket means that fewer > channels are available yielding less memory bandwidth overall. For > memory-bound tasks, it can be beneficial to migrate to another socket and > migrate the data to increase bandwidth even though the accesses are remote > in the short term.
> --- > kernel/sched/fair.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d59d3e00a480..d4c289c11012 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -1560,7 +1560,7 @@ static bool task_numa_compare(struct task_numa_env *env, > goto unlock; > > if (!cur) { > - if (maymove || imp > env->best_imp) > + if (maymove) > goto assign; > else > goto unlock;
Srikar's patch here:
http://lkml.kernel.org/r/1533276841-16341-4-git-send-email-srikar@linux.vnet.ibm.com
Also frobs this condition, but in a less radical way. Does that yield similar results?
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