[Question] testing results of support async buffered reads feature

[Hao_Xu <haoxu@linux.alibaba.com>]

Hi Jens,
I've done some testing for io_uring async buffered reads with fio. But I 
found something strange to me.
- when readahead is exactly turned off, the async buffered reads feature 
appears to be worse than the io-wq method in terms of IOPS.
- when readahead is on, async buffered reads works better but the 
optimization rate seems to be related with the size of readahead.
I'm wondering why.

my environment is:
	server: physical server
	kernel: mainline 5.9.0-rc8+ latest commit 6f2f486d57c4d562cdf4
	fs: ext4
	device: nvme
	fio: 3.20
	
I did the tests by setting and commenting the code:
	filp->f_mode |= FMODE_BUF_RASYNC;
in fs/ext4/file.c ext4_file_open()

the IOPS in different condition is below:

when blockdev setra 0 /mnt/nvme0n1
QD/Test		FMODE_BUF_RASYNC set	FMODE_BUF_RASYNC not set
1		12.9k			11.0k
2		32.4k			29.7k
4		65.8k			62.1k
8		123k			116k
16		211k			208k
32		235k			296k
64		241k			328k
128		229k			312k

the async buffered reads feature has a smaller IOPS.

when blockdev setra 64 /mnt/nvme0n1
QD/Test		FMODE_BUF_RASYNC set	FMODE_BUF_RASYNC not set
1		11.4k			12.2k
2		23.8k			30.0k
4		52.7k			61.7k
8		122k			114k
16		208k			181k
32		237k			199k
64		260k			185k
128		231k			201k

for QD=64	(260-185)/185 = 40.5%

when blockdev setra 128 /mnt/nvme0n1
QD/Test		FMODE_BUF_RASYNC set	FMODE_BUF_RASYNC not set
1		11.4k			10.8k
2		23.9k			22.7k
4		53.1k			46.5k
8		122k			106k
16		204k			182k
32		212k			200k
64		242k			202k
128		229k			188k

for QD=64	(242-202)/202 = 20.0%

when blockdev setra 256 /mnt/nvme0n1
QD/Test		FMODE_BUF_RASYNC set	FMODE_BUF_RASYNC not set
1		11.5k			12.2k
2		23.8k			29.7k
4		52.9k			61.9k
8		121k			117k
16		207k			186k
32		229k			204k
64		230k			211k
128		240k			203k

for QD=64	(230-211)/211 = 9.0%

the arguments of fio I use are:
fio_test.sh:
blockdev --setra $2 /dev/nvme0n1
fio -filename=/mnt/nvme0n1/fio_read_test.txt \
     -buffered=1 \
     -iodepth $1 \
     -rw=randread \
     -ioengine=io_uring \
     -randseed=89 \
     -runtime=10s \
     -norandommap \
     -direct=0 \
     -bs=4k \
     -size=4G \
     -name=rand_read_4k