Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 20 Nov 2020 14:33:25 +0100 | Subject | Re: [PATCH 4/4] sched: Limit the amount of NUMA imbalance that can exist at fork time |
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On Fri, 20 Nov 2020 at 10:06, Mel Gorman <mgorman@techsingularity.net> wrote: > > At fork time currently, a local node can be allowed to fill completely > and allow the periodic load balancer to fix the problem. This can be > problematic in cases where a task creates lots of threads that idle until > woken as part of a worker poll causing a memory bandwidth problem. > > However, a "real" workload suffers badly from this behaviour. The workload > in question is mostly NUMA aware but spawns large numbers of threads > that act as a worker pool that can be called from anywhere. These need > to spread early to get reasonable behaviour. > > This patch limits how much a local node can fill before spilling over > to another node and it will not be a universal win. Specifically, > very short-lived workloads that fit within a NUMA node would prefer > the memory bandwidth. > > As I cannot describe the "real" workload, the best proxy measure I found > for illustration was a page fault microbenchmark. It's not representative > of the workload but demonstrates the hazard of the current behaviour. > > pft timings > 5.10.0-rc2 5.10.0-rc2 > imbalancefloat-v2 forkspread-v2 > Amean elapsed-1 46.37 ( 0.00%) 46.05 * 0.69%* > Amean elapsed-4 12.43 ( 0.00%) 12.49 * -0.47%* > Amean elapsed-7 7.61 ( 0.00%) 7.55 * 0.81%* > Amean elapsed-12 4.79 ( 0.00%) 4.80 ( -0.17%) > Amean elapsed-21 3.13 ( 0.00%) 2.89 * 7.74%* > Amean elapsed-30 3.65 ( 0.00%) 2.27 * 37.62%* > Amean elapsed-48 3.08 ( 0.00%) 2.13 * 30.69%* > Amean elapsed-79 2.00 ( 0.00%) 1.90 * 4.95%* > Amean elapsed-80 2.00 ( 0.00%) 1.90 * 4.70%* > > This is showing the time to fault regions belonging to threads. The target > machine has 80 logical CPUs and two nodes. Note the ~30% gain when the > machine is approximately the point where one node becomes fully utilised. > The slower results are borderline noise. > > Kernel building shows similar benefits around the same balance point. > Generally performance was either neutral or better in the tests conducted. > The main consideration with this patch is the point where fork stops > spreading a task so some workloads may benefit from different balance > points but it would be a risky tuning parameter. > > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> --- > kernel/sched/fair.c | 17 +++++++++++++++-- > 1 file changed, 15 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index e17e6c5da1d5..6d1c24708664 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8761,6 +8761,16 @@ static bool update_pick_idlest(struct sched_group *idlest, > return true; > } > > +/* > + * Allow a NUMA imbalance if busy CPUs is less than 25% of the domain. > + * This is an approximation as the number of running tasks may not be > + * related to the number of busy CPUs due to sched_setaffinity. > + */ > +static inline bool allow_numa_imbalance(int dst_running, int dst_weight) > +{ > + return (dst_running < (dst_weight >> 2)); > +} > + > /* > * find_idlest_group() finds and returns the least busy CPU group within the > * domain. > @@ -8893,7 +8903,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu) > * a real need of migration, periodic load balance will > * take care of it. > */ > - if (local_sgs.idle_cpus) > + if (allow_numa_imbalance(local_sgs.sum_nr_running, sd->span_weight)) > return NULL; > } > > @@ -9000,11 +9010,14 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > static inline long adjust_numa_imbalance(int imbalance, > int dst_running, int dst_weight) > { > + if (!allow_numa_imbalance(dst_running, dst_weight)) > + return imbalance; > + > /* > * Allow a small imbalance based on a simple pair of communicating > * tasks that remain local when the destination is lightly loaded. > */ > - if (dst_running < (dst_weight >> 2) && imbalance <= NUMA_IMBALANCE_MIN) > + if (imbalance <= NUMA_IMBALANCE_MIN) > return 0; > > return imbalance; > -- > 2.26.2 >
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