Re: [Qemu-devel] [PATCH 6/6] RFH: We lost "connect" events

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Aug 19, 2019 at 12:46:20PM +0200, Juan Quintela wrote:
> Daniel P. Berrangé <berrange@redhat.com> wrote:
> > On Wed, Aug 14, 2019 at 04:02:18AM +0200, Juan Quintela wrote:
> >> When we have lots of channels, sometimes multifd migration fails
> >> with the following error:
> >> 
> >> Any good ideas?
> >
> > In inet_listen_saddr() we call
> >
> >     if (!listen(slisten, 1)) {
> >
> > note the second parameter sets the socket backlog, which is the max
> > number of pending socket connections we allow. My guess is that the
> > target QEMU is not accepting incoming connections quickly enough and
> > thus you hit the limit & the kernel starts dropping the incoming
> > connections.
> >
> > As a quick test, just hack this code to pass a value of 100 and see
> > if it makes your test reliable. If it does, then we'll need to figure
> > out a nice way to handle backlog instead of hardcoding it at 1.
> 
> Nice.
> 
> With this change I can create 100 channels on a 4 core machine without
> any trouble.
> 
> How can we proceed from here?

I don't think we want to expose this in the QAPI schema for the socket
address, since the correct value is really something that QEMU should
figure out based on usage context.

Thus, I think we'll have to make it an explicit parameter to the
qio_channel_socket_listen_{sync,async} APIs, and socket_listen()
and inet_listen_saddr(), etc. Then the migration code can pass in
a sensible value based on multifd usage.

Regards,
Daniel
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