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

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Freitag, 25. September 2020 18:05:17 CEST Christian Schoenebeck 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 forgot: you should execute that 'dd' command and host side, and the 'cat' 
command on guest side, to avoid any caching making the benchmark result look 
better than it actually is. Because for finding a good 'msize' value you only 
care about actual 9p data really being transmitted between host and guest.

Best regards,
Christian Schoenebeck