Messages in this thread | | | Date | Tue, 28 Mar 2023 14:09:39 +0200 | Subject | Re: [RFC PATCH] sched/fair: Make tg->load_avg per node | From | Dietmar Eggemann <> |
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On 27/03/2023 07:39, Aaron Lu wrote: > When using sysbench to benchmark Postgres in a single docker instance > with sysbench's nr_threads set to nr_cpu, it is observed there are times > update_cfs_group() and update_load_avg() shows noticeable overhead on > cpus of one node of a 2sockets/112core/224cpu Intel Sapphire Rapids: > > 10.01% 9.86% [kernel.vmlinux] [k] update_cfs_group > 7.84% 7.43% [kernel.vmlinux] [k] update_load_avg > > While cpus of the other node normally sees a lower cycle percent: > > 4.46% 4.36% [kernel.vmlinux] [k] update_cfs_group > 4.02% 3.40% [kernel.vmlinux] [k] update_load_avg > > Annotate shows the cycles are mostly spent on accessing tg->load_avg > with update_load_avg() being the write side and update_cfs_group() being > the read side. > > The reason why only cpus of one node has bigger overhead is: task_group > is allocated on demand from a slab and whichever cpu happens to do the > allocation, the allocated tg will be located on that node and accessing > to tg->load_avg will have a lower cost for cpus on the same node and > a higer cost for cpus of the remote node. > > Tim Chen told me that PeterZ once mentioned a way to solve a similar > problem by making a counter per node so do the same for tg->load_avg. > After this change, the worst number I saw during a 5 minutes run from > both nodes are: > > 2.77% 2.11% [kernel.vmlinux] [k] update_load_avg > 2.72% 2.59% [kernel.vmlinux] [k] update_cfs_group > > Another observation of this workload is: it has a lot of wakeup time > task migrations and that is the reason why update_load_avg() and > update_cfs_group() shows noticeable cost. Running this workload in N > instances setup where N >= 2 with sysbench's nr_threads set to 1/N nr_cpu, > task migrations on wake up time are greatly reduced and the overhead from > the two above mentioned functions also dropped a lot. It's not clear to > me why running in multiple instances can reduce task migrations on > wakeup path yet. > > Reported-by: Nitin Tekchandani <nitin.tekchandani@intel.com> > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
I'm so far not seeing this issue on my Arm64 server.
$ numactl -H available: 4 nodes (0-3) node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 node 1 cpus: 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 node 2 cpus: 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 node 3 cpus: 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 node distances: node 0 1 2 3 0: 10 12 20 22 1: 12 10 22 24 2: 20 22 10 12 3: 22 24 12 10
sysbench --table-size=100000 --tables=24 --threads=96 ... /usr/share/sysbench/oltp_read_write.lua run
perf report | grep kernel | head
9.12% sysbench [kernel.vmlinux] [k] _raw_spin_unlock_irqrestore 5.26% sysbench [kernel.vmlinux] [k] finish_task_switch 1.56% sysbench [kernel.vmlinux] [k] __do_softirq 1.22% sysbench [kernel.vmlinux] [k] arch_local_irq_restore 1.12% sysbench [kernel.vmlinux] [k] __arch_copy_to_user 1.12% sysbench [kernel.vmlinux] [k] el0_svc_common.constprop.1 0.95% sysbench [kernel.vmlinux] [k] __fget_light 0.94% sysbench [kernel.vmlinux] [k] rwsem_spin_on_owner 0.85% sysbench [kernel.vmlinux] [k] tcp_ack 0.56% sysbench [kernel.vmlinux] [k] do_sys_poll
Is your postgres/sysbench running in a cgroup with cpu controller attached? Mine isn't.
Maybe I'm doing something else differently?
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