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_...@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
