[Bug 1847861] Re: Guest stuttering under high disk IO (virtio)

[Launchpad Bug Tracker <1847861@bugs.launchpad.net>]

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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https://bugs.launchpad.net/bugs/1847861

Title:
  Guest stuttering under high disk IO (virtio)

Status in QEMU:
  Expired

Bug description:
  Performing io intensive tasks on virtualized Windows causes the system
  to visually stutter. I can often reproduce the problem by running fio
  on windows:

  fio --randrepeat=1 --ioengine=windowsaio --direct=1 --gtod_reduce=1
  --name=test --filename=\\.\PhysicalDrive0 --bs=4k --iodepth=128
  --size=4G --readwrite=randread

  While the fio command is running, moving the mouse pointer will be be
  laggy. The stuttering does not appear with iodepth <= 32 . The
  stuttering also manifests while playing games, the music and video
  pauses for a fraction of second in a playable but disturbing way.

  Here are my system specs:

  Host OS: archlinux
  Guest OS: Windows 10 Enterprise
  qemu version: qemu-git 8:v4.1.0.r1378.g98b2e3c9ab-1 (from AUR, compiled with -march=native)
  CPU: AMD Ryzen Threadripper 1900X 8-Core Processor
  Huge Pages: vm.nr_hugepages=4128
  Disk: nvme type=raw, io=threads bus=virtio
  GPU (passthrough): Radeon RX 570

  Here are some fio test results on my windows guest:

  [size=512M,iodepth=1 -> min=30k,avg=31k,stddev=508]
  [size=2G,iodepth=8 -> min=203k,avg=207k,stddev=2.3k]
  [size=2G,iodepth=16 -> min=320k,avg=330k,stddev=4.3k]
  [size=4G,iodepth=32 -> min=300k,avg=310k,stddev=4.8k]
  [size=4G,iodepth=64 -> min=278k,avg=366k,stddev=68.6k] -> STUTTER
  [size=4G,iodepth=64 -> min=358k,avg=428k,stddev=52.6k] -> STUTTER
  [size=4G,iodepth=128 -> min=92k,avg=217k,stddev=185k] -> STUTTER
  [size=4G,iodepth=128 -> min=241k,avg=257k,stddev=14k] -> same config as above, but no stuttering

  The min and avg values are the bandwidth values reported in KB/s by
  fio. You can see that, when the stuttering occurs, the stardard
  deviation is high and the minimum bandwidth is way below the average.

  Additional note: the bandwidth reported with `fio` on my linux host is
  about 2x the one reported in the guest:

  sudo fio --randrepeat=1 --ioengine=libaio --direct=1 --gtod_reduce=1
  --name=test --filename=/dev/nvme0n1 --bs=4k --iodepth=64 --size=512M
  --readwrite=randread

  read: IOPS=279k, BW=1092MiB/s (1145MB/s)(512MiB/469msec)

  Moreover, during the fio tests on the guest I've noticed that the CPU
  usage on the host is very high (around 450%) whereas on the guest is
  only 10%

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