[Spice-devel] Performance of Xspice - some results, and a potential patch

[John A. Sullivan III]

On Mon, 2012-08-06 at 15:52 -0500, Jeremy White wrote:
> > <snip>
> > I'm very encouraged to hear this as we are very, very interested in
> > SPICE as a WAN protocol.  We have noticed that the end user experience
> > is almost as dependent upon latency as bandwidth so I was a little
> > concerned that you are sending pixmaps changes on a periodic basis.
> > Does that introduce any noticeable latency?
> > 
> > I've been quite surprised at how much users are affected by latency I
> > would not have thought an issue.  Thus, I would define noticeable as
> > somewhere between 50 and 100 ms.  Thanks very much - John
> > 
> 
> Actually, there is a hard coded behavior in the Spice server that can
> make it function poorly with any kind of latency.  That is, by default,
> it requires that the client supply an acknowledgment either every 20th
> or 40th packet.  So if you do something that generates more than 40
> display messages in a short period of time (which isn't that hard to do)
> you have to wait for a full round trip before proceeding on.
> 
> The attached patch hacks its way around that issue by mangling the sizes
> up by a factor of 10.  I have a todo on my list to ask smarter people
> here how better to address this issue.
> 
> Cheers,
> 
> Jeremy
> 
Interesting.  That could explain some of the latency issues we uncovered
in our testing which convinced us to stay with NX and RDP for now rather
than moving to SPICE.  Thanks - John