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

[Ming Lei]

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?


Thanks,
Ming