Re: [Qemu-devel] [PATCH v2 1/2] net: introduce MAC_TABLE_CHANGED event

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 23, 2013 at 11:54:03AM -0400, Luiz Capitulino wrote:
> On Thu, 16 May 2013 18:17:23 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > > The
> > > existing throttling approach ensures that if the event includes latest
> > > guest information, then the host doesn't even have to do do a query, and
> > > is guaranteed that reacting to the final event will always see the most
> > > recent request.  But most importantly, if the existing throttling works,
> > > why do we have to invent a one-off approach for this event instead of
> > > reusing existing code?
> 
> Sorry to restart this week old discussion, but I'm now reviewing the patch
> in question and I dislike how we're coupling the event and the query
> command.
> 
> > Because of the 1st issue above. A large delay because we
> 
> Has this been measured? How long is this large delay?
> 
> Also, is it impossible for management to issue query-rx-filter
> on a reasonable rate that would also cause the same problems?
> IOW, how can we be sure we're fixing anything without trying it
> on a real use-case scenario?

Play with priorities, you can make management arbitrarily slow.  It's
just not sane to assume any timing guarantees for tasks running on
Linux.

> > exceed an arbitrary throttling rate would be bad
> > for the guest. Contrast with delay in e.g.
> > device delete event.
> > The throttling mechanism is good for events that host cares
> > about, not for events that guest cares about.
> > 
> > > -- 
> > > Eric Blake   eblake redhat com    +1-919-301-3266
> > > Libvirt virtualization library http://libvirt.org
> > > 
> > 
> > 
> >