Re: virtiofs vs 9p performance

[Christian Schoenebeck]

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