Re: [PATCH] Revert "block: don't reorder requests in blk_add_rq_to_plug"

[Jens Axboe]

On 6/11/25 9:10 AM, Ming Lei wrote:
> On Wed, Jun 11, 2025 at 12:14:54PM +0000, Hazem Mohamed Abuelfotoh wrote:
>> This reverts commit e70c301faece15b618e54b613b1fd6ece3dd05b4.
>>
>> Commit <e70c301faece> ("block: don't reorder requests in
>> blk_add_rq_to_plug") reversed how requests are stored in the blk_plug
>> list, this had significant impact on bio merging with requests exist on
>> the plug list. This impact has been reported in [1] and could easily be
>> reproducible using 4k randwrite fio benchmark on an NVME based SSD without
>> having any filesystem on the disk.
>>
>> My benchmark is:
>>
>>     fio --time_based --name=benchmark --size=50G --rw=randwrite \
>> 	--runtime=60 --filename="/dev/nvme1n1" --ioengine=psync \
>> 	--randrepeat=0 --iodepth=1 --fsync=64 --invalidate=1 \
>> 	--verify=0 --verify_fatal=0 --blocksize=4k --numjobs=4 \
>> 	--group_reporting
>>
>> On 1.9TiB SSD(180K Max IOPS) attached to i3.16xlarge AWS EC2 instance.
>>
>> Kernel        |  fio (B.W MiB/sec)  | I/O size (iostat)
>> --------------+---------------------+--------------------
>> 6.15.1        |   362               |  2KiB
>> 6.15.1+revert |   660 (+82%)        |  4KiB
>> --------------+---------------------+--------------------
> 
> I just run one quick test in my test VM, but can't reproduce it.
> 
> Also be curious, why does writeback produce so many 2KiB bios?

I was pondering that too, sounds like a misconfiguration of sorts. But
even without that, in a quick synthetic test here locally, I do see a
lot of missed merges that is solved with the alternative patch I sent
out. I strongly suspect it'll fix this issue too.

-- 
Jens Axboe