[Qemu-devel] virtio-blk-dataplane performance numbers

[Diana Madalina Craciun]

Hi,

I made some measurements guest vs bare metal (using virtio-data-plane)
and got some results I cannot fully explain.

First some details about the setup:

- I have an ARM v8 hardware + 1 SSD connected to SATA.

I have run FIO using multiple block sizes and IO depths:

for i in 1 2 4 8 16 32
do
    for j in 4 8 16 32 64 128 256 512
    do
        echo "Test ${i}_${j}"
        fio -filename=/dev/sda1 -direct=1 -iodepth $i  -rw=write 
-ioengine=libaio -bs=${j}k -size=8G -numjobs=4 -group_reporting
-name=mytest_write_${i}_${j} > /dev/out_write_${i}_${j}
    done
done


I run the same script for both baremetal and guest.

QEMU (QEMU 2.4 and kernel 4.1) command line is the following:

qemu-system-aarch64 -enable-kvm -nographic -machine type=virt -cpu host
-kernel /boot/Image -append "root=/dev/ram rw console=ttyAMA0,115200
ramdisk_size=1000000" -serial tcp::4444,server,telnet -initrd
/boot/rootfs.ext2.gz -m 1024 -mem-path /var/lib/hugetlbfs/pagesize-1GB
-object iothread,id=iothread0 -drive
if=none,id=drive0,cache=none,format=raw,file=/dev/sda,aio=native -device
virtio-blk-pci,drive=drive0,scsi=off,iothread=iothread0

I have pinned the I/O thread to physical CPU 0 and the VCPU thread to
physical CPU 1.

When comparing bare metal vs guest I have noticed that in some
situations I get better results in guest. The table contains the results
for sequential read (lines: io depth and columns: block sizes, the
numbers are guest vs bare metal degradation percentage). For random read
and write the results do not show large variations, but for sequential
read and write I see important variations.

   4k      8K      16K     32K     64K    128K    256K     512K
1  50.28   37.19   36.08  -0.4    4.09   5.18    3.22     1.71
2  46.22   22.63   24.41  -0.45   1.72   2.17    2.37    -4.64
4 -10.82   15.60   11.64   5.21   0.09   2.86   -3.52     6.71
8 -18.05   5.96    8.82    0.26   0.95   4.30   -13.53    17.9  
16 12.78   11.76   6.29    3.42   7.00   18.14  -0.4      5.59
32 16.99   7.98    4.70    7.67  -9.78   3.66    9.48    -3.55   

The negative numbers may come from the benchmark variation, probably I
would have to run the benchmark multiple times to see the variation even
in host.

However if somebody have an explanation of why I might get better
results in guest vs bare metal or at least in what direction to investigate.

Thank you,

Diana