Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

[Peter Zijlstra]

On Fri, Nov 01, 2024 at 08:47:15AM -0400, Phil Auld wrote:

> This one is causing a 10-20% performance hit on our filesystem tests.
> 
> On 6.12-rc5 (so with the latest follow ons) we get:
> 
> with DELAY_DEQUEUE the bandwidth is 510 MB/s
> with NO_DELAY_DEQUEUE the bandwidth is 590 MB/s
> 
> The test is fio, something like this:
> 
> taskset -c 1,2,3,4,5,6,7,8 fio --rw randwrite --bs 4k --runtime 1m --fsync 0 --iodepth 32 --direct 1 --ioengine libaio --numjobs 8 --size 30g --nrfiles 1 --loops 1 --name default --randrepeat 1 --time_based --group_reporting --directory /testfs
> 
> In this case it's ext4, but I'm not sure it will be FS specific.
> 
> I should have the machine and setup next week to poke further but I wanted
> to mention it now just in case any one has an "aha" moment.
> 
> It seems to only effect these FS loads. Other perf tests are not showing any
> issues that I am aware of.

There's a number of reports -- mostly it seems to be a case of excessive
preemption hurting things.

What happens if you use:

  schedtool -B -a 1-8 -e fio ....