* Vincent Guittot <vincent.guittot@xxxxxxxxxx> [2019-12-10 10:43:46]: > On Tue, 10 Dec 2019 at 06:43, Srikar Dronamraju > <srikar@xxxxxxxxxxxxxxxxxx> wrote: > > > > This is more prone to happen if the current running task is CPU > > intensive and the sched_wake_up_granularity is set to larger value. > > When the sched_wake_up_granularity was relatively small, it was observed > > that the bound thread would complete before the load balancer would have > > chosen to move the cache hot task to a different CPU. > > > > To deal with this situation, the current running task would yield to a > > per CPU bound kthread, provided kthread is not CPU intensive. > > > > /pboffline/hwcct_prg_old/lib/fsperf -t overwrite --noclean -f 5g -b 4k /pboffline > > > > (With sched_wake_up_granularity set to 15ms) > > So you increase sched_wake_up_granularity to a high level to ensure > that current is no preempted by waking thread but then you add a way > to finally preempt it which is somewhat weird IMO > Yes, setting to a smaller value will help mitigate/solve the problem. There may be folks out who have traditionally set a high wake_up_granularity (and have seen better performance with it), who may miss out that when using blk-mq, such settings will cause more harm. And they may continue to see some performance regressions when they move to a lower wake_up_granularity. > Have you tried to increase the priority of workqueue thread (decrease > nice priority) ? This is the right way to reduce the impact of the > sched_wake_up_granularity on the wakeup of your specific kthread. > Because what you want at the end is keeping a low wakeup granularity > for these io workqueues > Yes, people can tune the priority of workqueue threads and infact it may be easier to set wake_up_granularity to a lower value. However the point is how do we make everyone aware that they are running into a performance issue with a higher wakeup_granularity? -- Thanks and Regards Srikar Dronamraju
