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

[Anthony Liguori]

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.

Regards,

Anthony Liguori