Re: Observing higher CPU utilization during random IO fio testing

[Wen Xiong <wenxiong@linux.ibm.com>]

On 2026-05-25 00:28, Yu Kuai wrote:
> Hi,
> 
> 在 2026/5/22 5:52, Jens Axboe 写道:
>>> - More IO scheduler interaction, forces requests through scheduler 
>>> instead of direct dispatch(direct dispatch to hardware queue)
> I don't understand this point. Can you explain more? I think plug
> should not matter if request go through scheduler or not.

My understanding is:
Random IO tests are more CPU intensive.
Plug delays the dispatch IOs to hardware queue(quick way) directly.
Plug submits multiple IO requests in a batch to defer submitting IO 
until calling blk_flush_plug(dispatch to hardware queue) or task gets 
scheduling.
> 

> And I assume you're testing raw disk, because filesystems should
> always enable plug.
> 
Yes. FIO random IO tests over raw disks.

> Yes, perf data will be helpful. And please show your test in details 
> and I'll
> check if I can reproduce it.

System config:
47 dedicate cores
120 GB memory
PCIe4 2-Port 64Gb FC Adapter
64Gb FC switch
FlashSystem: FS9500, 12 LUNs/FC port

Below is fio config for rwmixread=100:
[global]
randrepeat=0
buffered=0
direct=1
norandommap=1
group_reporting=1
size=80g
ioengine=libaio
rw=randrw
bs=4k
iodepth=1
rwmixread=100
runtime=600
ramp_time=5
time_based=1
numjobs=20

[job1]
filename=/dev/dm-2

[job2]
filename=/dev/dm-3
...
24 jobs in total.

We collected some perf data. What kind of perf data you want? Let me 
know.
Thanks,
Wendy