Re: [PATCH] blk-mq: allow hardware queue to get more tag while sharing a tag set

From: yukuai (C)
Date: Mon Aug 02 2021 - 22:57:30 EST

On 2021/08/03 0:17, Bart Van Assche wrote:
On 8/2/21 6:34 AM, yukuai (C) wrote:
I run a test on both null_blk and nvme, results show that there are no
performance degradation:

test platform: x86
test cpu: 2 nodes, total 72
test scheduler: none
test device: null_blk / nvme

test cmd: fio -filename=/dev/xxx -name=test -ioengine=libaio -direct=1
-numjobs=72 -iodepth=16 -bs=4k -rw=write -offset_increment=1G
-cpus_allowed=0:71 -cpus_allowed_policy=split -group_reporting

test results: iops
1) null_blk before this patch: 280k
2) null_blk after this patch: 282k
3) nvme before this patch: 378k
4) nvme after this patch: 384k

Please use io_uring for performance tests.

The null_blk numbers seem way too low to me. If I run a null_blk performance test inside a VM with 6 CPU cores (Xeon W-2135 CPU) I see about 6 million IOPS for synchronous I/O and about 4.4 million IOPS when using libaio. The options I used and that are not in the above command line are: --thread --gtod_reduce=1 --ioscheduler=none.

Hi, Bart

The cpu I'm testing is Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz, and
after switching to io_uring with "--thread --gtod_reduce=1
--ioscheduler=none", the numbers can increase to 330k, yet still
far behind 6000k.

The new atomic operations in the hot path is atomic_read() from
hctx_may_queue(), and the atomic variable will change in two

a. fail to get driver tag with dbusy not set, increase and set dbusy.
b. if dbusy is set when queue switch from busy to dile, decrease and
clear dbusy.

During the period a device "idle -> busy -> idle", the new atomic
variable can be writen twice at most, which means this is almost
readonly in the above test situation. So I guess the impact on
performance is minimal ?