Messages in this thread | | | Date | Thu, 16 Jan 2020 16:35:29 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH] sched, fair: Allow a small load imbalance between low utilisation SD_NUMA domains v4 |
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On Tue, Jan 14, 2020 at 10:13:20AM +0000, Mel Gorman wrote: > Changelog since V3 > o Allow a fixed imbalance a basic comparison with 2 tasks. This turned out to > be as good or better than allowing an imbalance based on the group weight > without worrying about potential spillover of the lower scheduler domains. > > Changelog since V2 > o Only allow a small imbalance when utilisation is low to address reports that > higher utilisation workloads were hitting corner cases. > > Changelog since V1 > o Alter code flow vincent.guittot > o Use idle CPUs for comparison instead of sum_nr_running vincent.guittot > o Note that the division is still in place. Without it and taking > imbalance_adj into account before the cutoff, two NUMA domains > do not converage as being equally balanced when the number of > busy tasks equals the size of one domain (50% of the sum). > > The CPU load balancer balances between different domains to spread load > and strives to have equal balance everywhere. Communicating tasks can > migrate so they are topologically close to each other but these decisions > are independent. On a lightly loaded NUMA machine, two communicating tasks > pulled together at wakeup time can be pushed apart by the load balancer. > In isolation, the load balancer decision is fine but it ignores the tasks > data locality and the wakeup/LB paths continually conflict. NUMA balancing > is also a factor but it also simply conflicts with the load balancer. > > This patch allows a fixed degree of imbalance of two tasks to exist > between NUMA domains regardless of utilisation levels. In many cases, > this prevents communicating tasks being pulled apart. It was evaluated > whether the imbalance should be scaled to the domain size. However, no > additional benefit was measured across a range of workloads and machines > and scaling adds the risk that lower domains have to be rebalanced. While > this could change again in the future, such a change should specify the > use case and benefit. >
Any thoughts on whether this is ok for tip or are there suggestions on an alternative approach?
-- Mel Gorman SUSE Labs
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