* Pekka Enberg <penb...@kernel.org> wrote:
> On Sat, 2011-05-07 at 16:47 +0200, Ingo Molnar wrote:
> > Can you anything in the virtio protocol implementation that would explain
> > networking lags, which seem to be caused by guest notifications either not
> > be
> > sent or being missed?
> >
> > In particular this sequence:
> >
> > > while pending_requests:
> > > a = get_next_request();
> > > process_next_request(a);
> >
> > is apparently not what Qemu uses - so maybe there's some latent bug or some
> > silly oversight somewhere.
> >
> > It is suboptimal and i agree with you that the better sequence should be
> > implemented, but the above *should* work, yet it does not.
>
> Yes, so the performance benefits of Asias' patch aren't the interesting
> part but the fact that it fixes a real bug in our tool.
It could be the same like the mutex_lock() change: that too seemed to 'fix' the
latency bug but we still do not understand the root cause of the 'stuck
ring-buffer' situation.
I.e. some sort of timing related condition which goes away spuriously when
unrelated but timing-relevant changes are done to the code.
And we'll continue to see these problems on and off, in probably all virtio
drivers. virtio-console might be suffering from it, virtio-blk, etc. etc.
I'd suggest freezing changes to this driver until this bug is analyzed
correctly...
Thanks,
Ingo
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