Re: [Qemu-devel] [PATCH 0/4] virtiofsd: multithreading preparation part 3

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Aug 01, 2019 at 05:54:05PM +0100, Stefan Hajnoczi wrote:
> Performance
> -----------
> Please try these patches out and share your results.

Here are the performance numbers:

  Threadpool | iodepth | iodepth
     size    |    1    |   64
  -----------+---------+--------
  None       |   4451  |  4876
  1          |   4360  |  4858
  64         |   4359  | 33,266

A graph is available here:
https://vmsplice.net/~stefan/virtiofsd-threadpool-performance.png

Summary:

 * iodepth=64 performance is increased by 6.8 times.
 * iodepth=1 performance degrades by 2%.
 * DAX is bottlenecked by QEMU's single-threaded
   VHOST_USER_SLAVE_FS_MAP/UNMAP handler.

Threadpool size "none" is virtiofsd commit 813a824b707 ("virtiofsd: use
fuse_lowlevel_is_virtio() in fuse_session_destroy()") without any of the
multithreading preparation patches.  I benchmarked this to check whether
the patches introduce a regression for iodepth=1.  They do, but it's
only around 2%.

I also ran with DAX but found there was not much difference between
iodepth=1 and iodepth=64.  This might be because the host mmap(2)
syscall becomes the bottleneck and a serialization point.  QEMU only
processes one VHOST_USER_SLAVE_FS_MAP/UNMAP at a time.  If we want to
accelerate DAX it may be necessary to parallelize mmap, assuming the
host kernel can do them in parallel on a single file.  This performance
optimization is future work and not directly related to this patch
series.

The following fio job was run with cache=none and no DAX:

  [global]
  runtime=60
  ramp_time=30
  filename=/var/tmp/fio.dat
  direct=1
  rw=randread
  bs=4k
  size=4G
  ioengine=libaio
  iodepth=1

  [read]

Guest configuration:
1 vCPU
4 GB RAM
Linux 5.1 (vivek-aug-06-2019)

Host configuration:
Intel(R) Core(TM) i7-5600U CPU @ 2.60GHz (2 cores x 2 threads)
8 GB RAM
Linux 5.1.20-300.fc30.x86_64
XFS + dm-thin + dm-crypt
Toshiba THNSFJ256GDNU (256 GB SATA SSD)

Stefan

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