Re: [RFC patch 0/1] block: vhost-blk backend

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Oct 05, 2022 at 01:28:14PM +0300, Andrey Zhadchenko wrote:
> 
> 
> On 10/4/22 21:26, Stefan Hajnoczi wrote:
> > On Mon, Jul 25, 2022 at 11:55:26PM +0300, Andrey Zhadchenko wrote:
> > > Although QEMU virtio-blk is quite fast, there is still some room for
> > > improvements. Disk latency can be reduced if we handle virito-blk requests
> > > in host kernel so we avoid a lot of syscalls and context switches.
> > > 
> > > The biggest disadvantage of this vhost-blk flavor is raw format.
> > > Luckily Kirill Thai proposed device mapper driver for QCOW2 format to attach
> > > files as block devices: https://www.spinics.net/lists/kernel/msg4292965.html
> > > 
> > > Also by using kernel modules we can bypass iothread limitation and finaly scale
> > > block requests with cpus for high-performance devices. This is planned to be
> > > implemented in next version.
> > > 
> > > Linux kernel module part:
> > > https://lore.kernel.org/kvm/20220725202753.298725-1-andrey.zhadchenko@virtuozzo.com/
> > > 
> > > test setups and results:
> > > fio --direct=1 --rw=randread  --bs=4k  --ioengine=libaio --iodepth=128
> > 
> > > QEMU drive options: cache=none
> > > filesystem: xfs
> > 
> > Please post the full QEMU command-line so it's clear exactly what this
> > is benchmarking.
> 
> The full command for vhost is this:
> qemu-system-x86_64 \
> -kernel bzImage -nographic -append "console=ttyS0 root=/dev/sdb rw
> systemd.unified_cgroup_hierarchy=0 nokaslr" \
> -m 1024 -s --enable-kvm -smp $2 \
> -drive id=main_drive,file=debian_sid.img,media=disk,format=raw \
> -drive id=vhost_drive,file=$1,media=disk,format=raw,if=none \

No cache=none because vhost-blk directly submits bios in the kernel?

> -device vhost-blk-pci,drive=vhost_drive,num-threads=$3
> 
> (num-threads option for vhost-blk-pci was not used)
> 
> For virtio I used this:
> qemu-system-x86_64 \
> -kernel bzImage -nographic -append "console=ttyS0 root=/dev/sdb rw
> systemd.unified_cgroup_hierarchy=0 nokaslr" \
> -m 1024 -s --enable-kvm -smp $2 \
> -drive file=debian_sid.img,media=disk \
> -drive file=$1,media=disk,if=virtio,cache=none,if=none,id=d1,aio=threads\
> -device virtio-blk-pci,drive=d1
> 
> > 
> > A preallocated raw image file is a good baseline with:
> > 
> >    --object iothread,id=iothread0 \
> >    --blockdev file,filename=test.img,cache.direct=on,aio=native,node-name=drive0 >    --device virtio-blk-pci,drive=drive0,iothread=iothread0
> The image I used was preallocated qcow2 image set up with dm-qcow2 because
> this vhost-blk version directly uses bio interface and can't work with
> regular files.

I see. 

> 
> > 
> > (BTW QEMU's default vq size is 256 descriptors and the number of vqs is
> > the number of vCPUs.)
> > 
> > > 
> > > SSD:
> > >                 | randread, IOPS  | randwrite, IOPS |
> > > Host           |      95.8k	 |	85.3k	   |
> > > QEMU virtio    |      57.5k	 |	79.4k	   |
> 
> Adding iothread0 and using raw file instead of qcow2 + dm-qcow2 setup brings
> the numbers to
>                   |      60.4k   |      84.3k      |
> 
> > > QEMU vhost-blk |      95.6k	 |	84.3k	   |
> > > 
> > > RAMDISK (vq == vcpu):
> > 
> > With fio numjobs=vcpu here?
> 
> Yes
> 
> > 
> > >                   | randread, IOPS | randwrite, IOPS |
> > > virtio, 1vcpu    |	123k	  |	 129k       |
> > > virtio, 2vcpu    |	253k (??) |	 250k (??)  |
> > 
> > QEMU's aio=threads (default) gets around the single IOThread. It beats
> > aio=native for this reason in some cases. Were you using aio=native or
> > aio=threads?
> 
> At some point of time I started to specify aio=threads (and before that I
> did not use this option). I am not sure when exactly. I will re-measure all
> cases for the next submission.

aio=native is usually recommended. aio=threads is less optimized.

aio=native should have lower latency than aio=threads although it scales
worse on hosts with free CPUs because it's limited to a single thread.

> 
> > 
> > > virtio, 4vcpu    |	158k	  |	 154k       |
> > > vhost-blk, 1vcpu |	110k	  |	 113k       |
> > > vhost-blk, 2vcpu |	247k	  |	 252k       |
> 

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