[Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2

[Ming Lei]

Hi Stefan,

I found VM block I/O thoughput is decreased by more than 40%
on my laptop, and looks much worsen in my server environment,
and it is caused by your commit 580b6b2aa2:

          dataplane: use the QEMU block layer for I/O

I run fio with below config to test random read:

[global]
direct=1
size=4G
bsrange=4k-4k
timeout=20
numjobs=4
ioengine=libaio
iodepth=64
filename=/dev/vdc
group_reporting=1

[f]
rw=randread

Together with throughput drop, the latency is improved a little.

With this commit, I/O block submitted to fs becomes much smaller
than before, and more io_submit() need to be called to kernel, that
means iodepth may become much less.

I am not surprised with the result since I did compare VM I/O
performance between qemu and lkvm before, which has no big qemu
lock problem and handle I/O in a dedicated thread, but lkvm's block
IO is still much worse than qemu from view of throughput, because
lkvm doesn't submit block I/O at batch like the way of previous
dataplane, IMO.

But now you change the way of submitting I/O, could you share
the motivation about the change? Is the throughput drop you expect?



Thanks,
-- 
Ming Lei