On Freitag, 25. September 2020 15:13:56 CEST Vivek Goyal wrote: > On Fri, Sep 25, 2020 at 10:06:41AM +0200, Christian Schoenebeck wrote: > > On Freitag, 25. September 2020 00:10:23 CEST Vivek Goyal wrote: > > > In my testing, with cache=none, virtiofs performed better than 9p in > > > all the fio jobs I was running. For the case of cache=auto for virtiofs > > > (with xattr enabled), 9p performed better in certain write workloads. I > > > have identified root cause of that problem and working on > > > HANDLE_KILLPRIV_V2 patches to improve WRITE performance of virtiofs > > > with cache=auto and xattr enabled. > > > > Please note, when it comes to performance aspects, you should set a > > reasonable high value for 'msize' on 9p client side: > > https://wiki.qemu.org/Documentation/9psetup#msize > > Interesting. I will try that. What does "msize" do?
Simple: it's the "maximum message size" ever to be used for communication between host and guest, in both directions that is. So if that 'msize' value is too small, a potential large 9p message would be split into several smaller 9p messages, and each message adds latency which is the main problem. Keep in mind: The default value with Linux clients for msize is still only 8kB! Think of doing 'dd bs=8192 if=/src.dat of=/dst.dat count=...' as analogy, which probably makes its impact on performance clear. However the negative impact of a small 'msize' value is not just limited to raw file I/O like that; calling readdir() for instance on a guest directory with several hundred files or more, will likewise slow down in the same way tremendously as both sides have to transmit a large amount of 9p messages back and forth instead of just 2 messages (Treaddir and Rreaddir). > > I'm also working on performance optimizations for 9p BTW. There is plenty > > of headroom to put it mildly. For QEMU 5.2 I started by addressing > > readdir requests: > > https://wiki.qemu.org/ChangeLog/5.2#9pfs > > Nice. I guess this performance comparison between 9p and virtiofs is good. > Both the projects can try to identify weak points and improve performance. Yes, that's indeed handy being able to make comparisons. Best regards, Christian Schoenebeck
