Re: QEMU 5.0 virtio-blk performance regression with high queue depths

[Stefan Hajnoczi]

On Thu, Aug 27, 2020 at 3:24 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> Hi Denis,
> A performance regression was found after the virtio-blk queue-size
> property was increased from 128 to 256 in QEMU 5.0 in commit
> c9b7d9ec21dfca716f0bb3b68dee75660d86629c ("virtio: increase virtqueue
> size for virtio-scsi and virtio-blk"). I wanted to let you know if case
> you have ideas or see something similar.

Ping, have you noticed performance regressions after switching to
virtio-blk queue-size 256?

>
> Throughput and IOPS of the following fio benchmarks dropped by 30-40%:
>
>   # mkfs.xfs /dev/vdb
>   # mount /dev/vdb /mnt
>   # fio --rw=%s --bs=%s --iodepth=64 --runtime=1m --direct=1 
> --filename=/mnt/%s --name=job1 --ioengine=libaio --thread --group_reporting 
> --numjobs=16 --size=512MB --time_based --output=/tmp/fio_result &> /dev/null
>     - rw: read write
>     - bs: 4k 64k
>
> Note that there are 16 threads submitting 64 requests each! The guest
> block device queue depth will be maxed out. The virtqueue should be full
> most of the time.
>
> Have you seen regressions after virtio-blk queue-size was increased in
> QEMU 5.0?
>
> Here are the details of the host storage:
>
>   # mkfs.xfs /dev/sdb # 60GB SSD drive
>   # mount /dev/sdb /mnt/test
>   # qemu-img create -f qcow2 /mnt/test/storage2.qcow2 40G
>
> The guest command-line is:
>
>   # MALLOC_PERTURB_=1 numactl \
>     -m 1  /usr/libexec/qemu-kvm \
>     -S  \
>     -name 'avocado-vt-vm1'  \
>     -sandbox on  \
>     -machine q35 \
>     -device 
> pcie-root-port,id=pcie-root-port-0,multifunction=on,bus=pcie.0,addr=0x1,chassis=1
>  \
>     -device 
> pcie-pci-bridge,id=pcie-pci-bridge-0,addr=0x0,bus=pcie-root-port-0  \
>     -nodefaults \
>     -device VGA,bus=pcie.0,addr=0x2 \
>     -m 4096  \
>     -smp 2,maxcpus=2,cores=1,threads=1,dies=1,sockets=2  \
>     -cpu 'IvyBridge',+kvm_pv_unhalt \
>     -chardev 
> socket,server,id=qmp_id_qmpmonitor1,nowait,path=/var/tmp/avocado_bapfdqao/monitor-qmpmonitor1-20200721-014154-5HJGMjxW
>   \
>     -mon chardev=qmp_id_qmpmonitor1,mode=control \
>     -chardev 
> socket,server,id=qmp_id_catch_monitor,nowait,path=/var/tmp/avocado_bapfdqao/monitor-catch_monitor-20200721-014154-5HJGMjxW
>   \
>     -mon chardev=qmp_id_catch_monitor,mode=control \
>     -device pvpanic,ioport=0x505,id=id31BN83 \
>     -chardev 
> socket,server,id=chardev_serial0,nowait,path=/var/tmp/avocado_bapfdqao/serial-serial0-20200721-014154-5HJGMjxW
>  \
>     -device isa-serial,id=serial0,chardev=chardev_serial0  \
>     -chardev 
> socket,id=seabioslog_id_20200721-014154-5HJGMjxW,path=/var/tmp/avocado_bapfdqao/seabios-20200721-014154-5HJGMjxW,server,nowait
>  \
>     -device 
> isa-debugcon,chardev=seabioslog_id_20200721-014154-5HJGMjxW,iobase=0x402 \
>     -device 
> pcie-root-port,id=pcie-root-port-1,port=0x1,addr=0x1.0x1,bus=pcie.0,chassis=2 
> \
>     -device qemu-xhci,id=usb1,bus=pcie-root-port-1,addr=0x0 \
>     -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 \
>     -blockdev 
> node-name=file_image1,driver=file,aio=threads,filename=rootfs.qcow2,cache.direct=on,cache.no-flush=off
>  \
>     -blockdev 
> node-name=drive_image1,driver=qcow2,cache.direct=on,cache.no-flush=off,file=file_image1
>  \
>     -device 
> pcie-root-port,id=pcie-root-port-2,port=0x2,addr=0x1.0x2,bus=pcie.0,chassis=3 
> \
>     -device 
> virtio-blk-pci,id=image1,drive=drive_image1,bootindex=0,write-cache=on,bus=pcie-root-port-2,addr=0x0
>  \
>     -blockdev 
> node-name=file_disk1,driver=file,aio=threads,filename=/mnt/test/storage2.qcow2,cache.direct=on,cache.no-flush=off
>  \
>     -blockdev 
> node-name=drive_disk1,driver=qcow2,cache.direct=on,cache.no-flush=off,file=file_disk1
>  \
>     -device 
> pcie-root-port,id=pcie-root-port-3,port=0x3,addr=0x1.0x3,bus=pcie.0,chassis=4 
> \
>     -device 
> virtio-blk-pci,id=disk1,drive=drive_disk1,bootindex=1,write-cache=on,bus=pcie-root-port-3,addr=0x0
>  \
>     -device 
> pcie-root-port,id=pcie-root-port-4,port=0x4,addr=0x1.0x4,bus=pcie.0,chassis=5 
> \
>     -device 
> virtio-net-pci,mac=9a:37:37:37:37:4e,id=idBMd7vy,netdev=idLb51aS,bus=pcie-root-port-4,addr=0x0
>   \
>     -netdev tap,id=idLb51aS,fd=14  \
>     -vnc :0  \
>     -rtc base=utc,clock=host,driftfix=slew  \
>     -boot menu=off,order=cdn,once=c,strict=off \
>     -enable-kvm \
>     -device 
> pcie-root-port,id=pcie_extra_root_port_0,multifunction=on,bus=pcie.0,addr=0x3,chassis=6
>
> Stefan