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

[Avi Kivity <avi@redhat.com>]

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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