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.
Many users don't even know this 'msize' parameter exists and hence run with
the Linux kernel's default value of just 8kB. For QEMU 5.2 I addressed this by
logging a performance warning on host side for making users at least aware
about this issue. The long-term plan is to pass a good msize value from host
to guest via virtio (like it's already done for the available export tags) and
the Linux kernel would default to that instead.
Best regards,
Christian Schoenebeck
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