* Christian Schoenebeck (qemu_...@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 > 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 > > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
