Re: virtio-fs performance

[Stefan Hajnoczi]

> I'm trying and testing the virtio-fs feature in QEMU v5.0.0.
> My host and guest OS are both ubuntu 18.04 with kernel 5.4, and the
> underlying storage is one single SSD.
> 
> The configuations are:
> (1) virtiofsd
> ./virtiofsd -o 
> source=/mnt/ssd/virtiofs,cache=auto,flock,posix_lock,writeback,xattr
> --thread-pool-size=1 --socket-path=/tmp/vhostqemu
> 
> (2) qemu
> qemu-system-x86_64 \
> -enable-kvm \
> -name ubuntu \
> -cpu Westmere \
> -m 4096 \
> -global kvm-apic.vapic=false \
> -netdev 
> tap,id=hn0,vhost=off,br=br0,helper=/usr/local/libexec/qemu-bridge-helper
> \
> -device e1000,id=e0,netdev=hn0 \
> -blockdev '{"node-name": "disk0", "driver": "qcow2",
> "refcount-cache-size": 1638400, "l2-cache-size": 6553600, "file": {
> "driver": "file", "filename": "'${imagefolder}\/ubuntu.qcow2'"}}' \
> -device virtio-blk,drive=disk0,id=disk0 \
> -chardev socket,id=ch0,path=/tmp/vhostqemu \
> -device vhost-user-fs-pci,chardev=ch0,tag=myfs \
> -object memory-backend-memfd,id=mem,size=4G,share=on \
> -numa node,memdev=mem \
> -qmp stdio \
> -vnc :0
> 
> (3) guest
> mount -t virtiofs myfs /mnt/virtiofs
> 
> I tried to change virtiofsd's --thread-pool-size value and test the
> storage performance by fio.
> Before each read/write/randread/randwrite test, the pagecaches of
> guest and host are dropped.
> 
> ```
> RW="read" # or write/randread/randwrite
> fio --name=test --rw=$RW --bs=4k --numjobs=1 --ioengine=libaio
> --runtime=60 --direct=0 --iodepth=64 --size=10g
> --filename=/mnt/virtiofs/testfile
> done
> ```
> 
> --thread-pool-size=64 (default)
>     seq read: 305 MB/s
>     seq write: 118 MB/s
>     rand 4KB read: 2222 IOPS
>     rand 4KB write: 21100 IOPS
> 
> --thread-pool-size=1
>     seq read: 387 MB/s
>     seq write: 160 MB/s
>     rand 4KB read: 2622 IOPS
>     rand 4KB write: 30400 IOPS
> 
> The results show the performance using default-pool-size (64) is
> poorer than using single thread.
> Is it due to the lock contention of the multiple threads?
> When can virtio-fs get better performance using multiple threads?
> 
> 
> I also tested the performance that guest accesses host's files via
> NFSv4/CIFS network filesystem.
> The "seq read" and "randread" performance of virtio-fs are also worse
> than the NFSv4 and CIFS.
> 
> NFSv4:
>   seq write: 244 MB/s
>   rand 4K read: 4086 IOPS
> 
> I cannot figure out why the perf of NFSv4/CIFS with the network stack
> is better than virtio-fs.
> Is it expected? Or, do I have an incorrect configuration?

No, I remember benchmarking the thread pool and did not see such a big
difference.

Please use direct=1 so that each I/O results in a virtio-fs request.
Otherwise the I/O pattern is not directly controlled by the benchmark
but by the page cache (readahead, etc).

Using numactl(8) or taskset(1) to launch virtiofsd allows you to control
NUMA and CPU scheduling properties. For example, you could force all 64
threads to run on the same host CPU using taskset to see if that helps
this I/O bound workload.

fio can collect detailed statistics on queue depths and a latency
histogram. It would be interesting to compare the --thread-pool-size=64
and --thread-pool-size=1 numbers.

Comparing the "perf record -e kvm:kvm_exit" counts between the two might
also be interesting.

Stefan

signature.asc
Description: PGP signature