Messages in this thread Patch in this message | | | Date | Fri, 8 Nov 2019 17:35:01 +0100 | From | Vincent Guittot <> | Subject | Re: [PATCH v4 04/11] sched/fair: rework load_balance |
| |
Le Thursday 31 Oct 2019 à 11:40:20 (+0000), Mel Gorman a écrit : > On Thu, Oct 31, 2019 at 12:13:09PM +0100, Vincent Guittot wrote: > > > > > On the last one, spreading tasks evenly across NUMA domains is not > > > > > necessarily a good idea. If I have 2 tasks running on a 2-socket machine > > > > > with 24 logical CPUs per socket, it should not automatically mean that > > > > > one task should move cross-node and I have definitely observed this > > > > > happening. It's probably bad in terms of locality no matter what but it's > > > > > especially bad if the 2 tasks happened to be communicating because then > > > > > load balancing will pull apart the tasks while wake_affine will push > > > > > them together (and potentially NUMA balancing as well). Note that this > > > > > also applies for some IO workloads because, depending on the filesystem, > > > > > the task may be communicating with workqueues (XFS) or a kernel thread > > > > > (ext4 with jbd2). > > > > > > > > This rework doesn't touch the NUMA_BALANCING part and NUMA balancing > > > > still gives guidances with fbq_classify_group/queue. > > > > > > I know the NUMA_BALANCING part is not touched, I'm talking about load > > > balancing across SD_NUMA domains which happens independently of > > > NUMA_BALANCING. In fact, there is logic in NUMA_BALANCING that tries to > > > override the load balancer when it moves tasks away from the preferred > > > node. > > > > Yes. this patchset relies on this override for now to prevent moving task away. > > Fair enough, netperf hits the corner case where it does not work but > that is also true without your series.
I run mmtest/netperf test on my setup. It's a mix of small positive or negative differences (see below)
netperf-udp 5.3-rc2 5.3-rc2 tip +rwk+fix Hmean send-64 95.06 ( 0.00%) 94.12 * -0.99%* Hmean send-128 191.71 ( 0.00%) 189.94 * -0.93%* Hmean send-256 379.05 ( 0.00%) 370.96 * -2.14%* Hmean send-1024 1485.24 ( 0.00%) 1476.64 * -0.58%* Hmean send-2048 2894.80 ( 0.00%) 2887.00 * -0.27%* Hmean send-3312 4580.27 ( 0.00%) 4555.91 * -0.53%* Hmean send-4096 5592.99 ( 0.00%) 5517.31 * -1.35%* Hmean send-8192 9117.00 ( 0.00%) 9497.06 * 4.17%* Hmean send-16384 15824.59 ( 0.00%) 15824.30 * -0.00%* Hmean recv-64 95.06 ( 0.00%) 94.08 * -1.04%* Hmean recv-128 191.68 ( 0.00%) 189.89 * -0.93%* Hmean recv-256 378.94 ( 0.00%) 370.87 * -2.13%* Hmean recv-1024 1485.24 ( 0.00%) 1476.20 * -0.61%* Hmean recv-2048 2893.52 ( 0.00%) 2885.25 * -0.29%* Hmean recv-3312 4580.27 ( 0.00%) 4553.48 * -0.58%* Hmean recv-4096 5592.99 ( 0.00%) 5517.27 * -1.35%* Hmean recv-8192 9115.69 ( 0.00%) 9495.69 * 4.17%* Hmean recv-16384 15824.36 ( 0.00%) 15818.36 * -0.04%* Stddev send-64 0.15 ( 0.00%) 1.17 (-688.29%) Stddev send-128 1.56 ( 0.00%) 1.15 ( 25.96%) Stddev send-256 4.20 ( 0.00%) 5.27 ( -25.63%) Stddev send-1024 20.11 ( 0.00%) 5.68 ( 71.74%) Stddev send-2048 11.06 ( 0.00%) 21.74 ( -96.50%) Stddev send-3312 61.10 ( 0.00%) 48.03 ( 21.39%) Stddev send-4096 71.84 ( 0.00%) 31.99 ( 55.46%) Stddev send-8192 165.14 ( 0.00%) 159.99 ( 3.12%) Stddev send-16384 81.30 ( 0.00%) 188.65 (-132.05%) Stddev recv-64 0.15 ( 0.00%) 1.15 (-673.42%) Stddev recv-128 1.58 ( 0.00%) 1.14 ( 28.27%) Stddev recv-256 4.29 ( 0.00%) 5.19 ( -21.05%) Stddev recv-1024 20.11 ( 0.00%) 5.70 ( 71.67%) Stddev recv-2048 10.43 ( 0.00%) 21.41 (-105.22%) Stddev recv-3312 61.10 ( 0.00%) 46.92 ( 23.20%) Stddev recv-4096 71.84 ( 0.00%) 31.97 ( 55.50%) Stddev recv-8192 163.90 ( 0.00%) 160.88 ( 1.84%) Stddev recv-16384 81.41 ( 0.00%) 187.01 (-129.71%)
5.3-rc2 5.3-rc2 tip +rwk+fix Duration User 38.90 39.13 Duration System 1311.29 1311.10 Duration Elapsed 1892.82 1892.86 netperf-tcp 5.3-rc2 5.3-rc2 tip +rwk+fix Hmean 64 871.30 ( 0.00%) 860.90 * -1.19%* Hmean 128 1689.39 ( 0.00%) 1679.31 * -0.60%* Hmean 256 3199.59 ( 0.00%) 3241.98 * 1.32%* Hmean 1024 9390.47 ( 0.00%) 9268.47 * -1.30%* Hmean 2048 13373.95 ( 0.00%) 13395.61 * 0.16%* Hmean 3312 16701.30 ( 0.00%) 17165.96 * 2.78%* Hmean 4096 15831.03 ( 0.00%) 15544.66 * -1.81%* Hmean 8192 19720.01 ( 0.00%) 20188.60 * 2.38%* Hmean 16384 23925.90 ( 0.00%) 23914.50 * -0.05%* Stddev 64 7.38 ( 0.00%) 4.23 ( 42.67%) Stddev 128 11.62 ( 0.00%) 10.13 ( 12.85%) Stddev 256 34.33 ( 0.00%) 7.94 ( 76.88%) Stddev 1024 35.61 ( 0.00%) 116.34 (-226.66%) Stddev 2048 285.30 ( 0.00%) 80.50 ( 71.78%) Stddev 3312 304.74 ( 0.00%) 449.08 ( -47.36%) Stddev 4096 668.11 ( 0.00%) 569.30 ( 14.79%) Stddev 8192 733.23 ( 0.00%) 944.38 ( -28.80%) Stddev 16384 553.03 ( 0.00%) 299.44 ( 45.86%)
5.3-rc2 5.3-rc2 tip +rwk+fix Duration User 138.05 140.95 Duration System 1210.60 1208.45 Duration Elapsed 1352.86 1352.90
> > > I agree that additional patches are probably needed to improve load > > balance at NUMA level and I expect that this rework will make it > > simpler to add. > > I just wanted to get the output of some real use cases before defining > > more numa level specific conditions. Some want to spread on there numa > > nodes but other want to keep everything together. The preferred node > > and fbq_classify_group was the only sensible metrics to me when he > > wrote this patchset but changes can be added if they make sense. > > > > That's fair. While it was possible to address the case before your > series, it was a hatchet job. If the changelog simply notes that some > special casing may still be required for SD_NUMA but it's outside the > scope of the series, then I'd be happy. At least there is a good chance > then if there is follow-up work that it won't be interpreted as an > attempt to reintroduce hacky heuristics. >
Would the additional comment make sense for you about work to be done for SD_NUMA ?
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0ad4b21..7e4cb65 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6960,11 +6960,34 @@ enum fbq_type { regular, remote, all }; * group. see update_sd_pick_busiest(). */ enum group_type { + /* + * The group has spare capacity that can be used to process more work. + */ group_has_spare = 0, + /* + * The group is fully used and the tasks don't compete for more CPU + * cycles. Nevetheless, some tasks might wait before running. + */ group_fully_busy, + /* + * One task doesn't fit with CPU's capacity and must be migrated on a + * more powerful CPU. + */ group_misfit_task, + /* + * One local CPU with higher capacity is available and task should be + * migrated on it instead on current CPU. + */ group_asym_packing, + /* + * The tasks affinity prevents the scheduler to balance the load across + * the system. + */ group_imbalanced, + /* + * The CPU is overloaded and can't provide expected CPU cycles to all + * tasks. + */ group_overloaded }; @@ -8563,7 +8586,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s /* * Try to use spare capacity of local group without overloading it or - * emptying busiest + * emptying busiest. + * XXX Spreading tasks across numa nodes is not always the best policy + * and special cares should be taken for SD_NUMA domain level before + * spreading the tasks. For now, load_balance() fully relies on + * NUMA_BALANCING and fbq_classify_group/rq to overide the decision. */ if (local->group_type == group_has_spare) { if (busiest->group_type > group_fully_busy) { -- 2.7.4 > > > > > > > > But the latter could also take advantage of the new type of group. For > > > > example, what I did in the fix for find_idlest_group : checking > > > > numa_preferred_nid when the group has capacity and keep the task on > > > > preferred node if possible. Similar behavior could also be beneficial > > > > in periodic load_balance case. > > > > > > > > > > And this is the catch -- numa_preferred_nid is not guaranteed to be set at > > > all. NUMA balancing might be disabled, the task may not have been running > > > long enough to pick a preferred NID or NUMA balancing might be unable to > > > pick a preferred NID. The decision to avoid unnecessary migrations across > > > NUMA domains should be made independently of NUMA balancing. The netperf > > > configuration from mmtests is great at illustrating the point because it'll > > > also say what the average local/remote access ratio is. 2 communicating > > > tasks running on an otherwise idle NUMA machine should not have the load > > > balancer move the server to one node and the client to another. > > > > I'm going to make it a try on my setup to see the results > > > > Thanks. > > -- > Mel Gorman > SUSE Labs
| |