Re: [Qemu-devel] [PATCH v2 0/3] io-thread optimizations

[Avi Kivity]

On 04/14/2011 04:59 PM, Anthony Liguori wrote:
> On 04/14/2011 08:45 AM, Avi Kivity wrote:
> > On 04/14/2011 10:14 AM, Jan Kiszka wrote:
> > > On 2011-04-13 22:16, Aurelien Jarno wrote:
> > > >  On Mon, Apr 11, 2011 at 10:27:41PM +0200, Jan Kiszka wrote:
> > > >>  These patches were posted before. They bring down the overhead 
> > > of the
> > > >>  io-thread mode for TCG here, specifically when emulating SMP.
> > > >>
> > > >>  The major change in this version, besides rebasing, is the 
> > > exclusion of
> > > >>  KVM from the main loop polling optimization.
> > > >>
> > > >>
> > > >>
> > > >>  Jan Kiszka (3):
> > > >>    Do not drop global mutex for polled main loop runs
> > > >>    Poll main loop after I/O events were received
> > > >>    Do not kick vcpus in TCG mode
> > > >>
> > > >>   cpus.c   |    2 +-
> > > >>   sysemu.h |    2 +-
> > > >>   vl.c     |   22 ++++++++++++++++------
> > > >>   3 files changed, 18 insertions(+), 8 deletions(-)
> > > >>
> > > >
> > > >  Thanks for working on improving the io-thread with TCG. Your patches
> > > >  make sense, but they don't seems to fix the slowdown observed when
> > > >  enabling the io-thread. Well maybe they were not supposed to. 
> > > This is
> > > >  for example the results of netperf between guest and host using 
> > > virtio:
> > > >
> > > >  no io-thread            122 MB/s
> > > >  io-thread                97 MB/s
> > > >  io-thread + patches      98 MB/s
> > > >
> > >
> > > Can you capture ftraces of io-thread enabled&  disabled runs? They just
> > > need to cover a hand full of frames.
> >
> > Also interesting would be the context switch rates on the host.
> >
> > If they're large, perhaps using user-space threading instead of 
> > native threads would help.
>
> I still suspect mitigation as the culprit here.  Select is going to 
> get to run more often which means more interrupt generation.
>
> I bet if you count the number of packets per interrupt/notify you'll 
> find that less batching is occurring.

Can you clarify?  Which mitigation? virtio-net interrupt mitigation?

virtio-net interrupt mitigation is time-based, no? so why should 
threading affect it?  and why would select() run more often? since we 
make all fds generate a signal, we ought to run a similar number same 
number of select()s.

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