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

[Vivek Goyal]

On Tue, Sep 29, 2020 at 03:28:06PM +0200, Christian Schoenebeck wrote:
> On Dienstag, 29. September 2020 15:03:25 CEST Vivek Goyal wrote:
> > On Sun, Sep 27, 2020 at 02:14:43PM +0200, Christian Schoenebeck wrote:
> > > 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
> > 
> > I am assuming that msize only helps with sequential I/O and not random
> > I/O.
> > 
> > Dave is running random read and random write mix and probably that's why
> > he is not seeing any improvement with msize increase.
> > 
> > If we run sequential workload (as "cat largefile.dat"), that should
> > see an improvement with msize increase.
> > 
> > Thanks
> > Vivek
> 
> Depends on what's randomized. If read chunk size is randomized, then yes, you 
> would probably see less performance increase compared to a simple
> 'cat foo.dat'.

We are using "fio" for testing and read chunk size is not being
randomized. chunk size (block size) is fixed at 4K size for these tests.

> 
> If only the read position is randomized, but the read chunk size honors 
> iounit, a.k.a. stat's st_blksize (i.e. reading with the most efficient block 
> size advertised by 9P), then I would assume still seeing a performance 
> increase.

Yes, we are randomizing read position. But there is no notion of looking
at st_blksize. Its fixed at 4K. (notice option --bs=4k in fio
commandline).

> Because seeking is a no/low cost factor in this case. The guest OS 
> seeking does not transmit a 9p message. The offset is rather passed with any 
> Tread message instead:
> https://github.com/chaos/diod/blob/master/protocol.md
> 
> I mean, yes, random seeks reduce I/O performance in general of course, but in 
> direct performance comparison, the difference in overhead of the 9p vs. 
> virtiofs network controller layer is most probably the most relevant aspect 
> if 
> large I/O chunk sizes are used.
> 

Agreed that large I/O chunk size will help with the perfomance numbers.
But idea is to intentonally use smaller I/O chunk size with some of
the tests to measure how efficient communication path is.

Thanks
Vivek