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

[Vivek Goyal]

On Wed, Aug 07, 2019 at 07:03:55PM +0100, Stefan Hajnoczi wrote:
> 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.

Good to see nice improvement with higher queue depth.

Kernel also serializes MAP/UNMAP on one inode. So you will need to run
multiple jobs operating on different inodes to see parallel MAP/UNMAP
(atleast from kernel's point of view).

Thanks
Vivek