Re: virtiofs vs 9p performance(Re: tools/virtiofs: Multi threading seems to hurt performance)

[Christian Schoenebeck]

On Freitag, 25. September 2020 20:51:47 CEST Dr. David Alan Gilbert wrote:
> * Christian Schoenebeck (qemu_oss@crudebyte.com) wrote:
> > On Freitag, 25. September 2020 15:05:38 CEST Dr. David Alan Gilbert wrote:
> > > > > 9p ( mount -t 9p -o trans=virtio kernel /mnt
> > > > > -oversion=9p2000.L,cache=mmap,msize=1048576 ) test: (g=0):
> > > > > rw=randrw,
> > > > 
> > > > Bottleneck ------------------------------^
> > > > 
> > > > By increasing 'msize' you would encounter better 9P I/O results.
> > > 
> > > OK, I thought that was bigger than the default;  what number should I
> > > use?
> > 
> > It depends on the underlying storage hardware. In other words: you have to
> > try increasing the 'msize' value to a point where you no longer notice a
> > negative performance impact (or almost). Which is fortunately quite easy
> > to test on> 
> > guest like:
> >     dd if=/dev/zero of=test.dat bs=1G count=12
> >     time cat test.dat > /dev/null
> > 
> > I would start with an absolute minimum msize of 10MB. I would recommend
> > something around 100MB maybe for a mechanical hard drive. With a PCIe
> > flash
> > you probably would rather pick several hundred MB or even more.
> > 
> > That unpleasant 'msize' issue is a limitation of the 9p protocol: client
> > (guest) must suggest the value of msize on connection to server (host).
> > Server can only lower, but not raise it. And the client in turn obviously
> > cannot see host's storage device(s), so client is unable to pick a good
> > value by itself. So it's a suboptimal handshake issue right now.
> 
> It doesn't seem to be making a vast difference here:
> 
> 
> 
> 9p mount -t 9p -o trans=virtio kernel /mnt
> -oversion=9p2000.L,cache=mmap,msize=104857600
> 
> Run status group 0 (all jobs):
>    READ: bw=62.5MiB/s (65.6MB/s), 62.5MiB/s-62.5MiB/s (65.6MB/s-65.6MB/s),
> io=3070MiB (3219MB), run=49099-49099msec WRITE: bw=20.9MiB/s (21.9MB/s),
> 20.9MiB/s-20.9MiB/s (21.9MB/s-21.9MB/s), io=1026MiB (1076MB),
> run=49099-49099msec
> 
> 9p mount -t 9p -o trans=virtio kernel /mnt
> -oversion=9p2000.L,cache=mmap,msize=1048576000
> 
> Run status group 0 (all jobs):
>    READ: bw=65.2MiB/s (68.3MB/s), 65.2MiB/s-65.2MiB/s (68.3MB/s-68.3MB/s),
> io=3070MiB (3219MB), run=47104-47104msec WRITE: bw=21.8MiB/s (22.8MB/s),
> 21.8MiB/s-21.8MiB/s (22.8MB/s-22.8MB/s), io=1026MiB (1076MB),
> run=47104-47104msec
> 
> 
> Dave

Is that benchmark tool honoring 'iounit' to automatically run with max. I/O 
chunk sizes? What's that benchmark tool actually? And do you also see no 
improvement with a simple

        time cat largefile.dat > /dev/null

?

Best regards,
Christian Schoenebeck