[Spice-devel] Clients, Nagle and net test

[Frediano Ziglio]

> 
> Hey,
> 
> On Tue, Dec 22, 2015 at 11:03:47AM -0500, Frediano Ziglio wrote:
> > Hi,
> >   today I was trying to do some (quick) tests for the Glib code trying to
> >   play with
> > bandwidth and latency.
> > 
> > I noted that the net test was giving very strange results. With a delay of
> > 1ms and a
> > bandwidth of 400kb/s was returning 325ms as roundtrip and a very high
> > bandwidth :(
> > 
> > After a bit of digging I realized that net test is done sending 3 pings
> > (using spice
> > protocol)
> > - 1 ping (warmup) of 0 byte data
> > - 1 ping (latency) of 0 bytes
> > - 1 ping (bandwidth) or 250 kb
> > Ping messages contains a timestamp (usec) returned verbatim from the
> > client.
> > Doing a strace I noted that all ping replies were returned at the same
> > time.
> > Taking into account that items are queued in the stream quite fast and that
> > roundtrip is compiled using current time - timestamp from ping reply this
> > make
> > basically the roundtrip computation equal to the total roundtrip of all
> > pings
> > so code thinks that latency is high and bandwidth (computed as data per
> > extra time
> > after roundtrip) too.
> > More strace, involving remote-viewer (Fedora 22 one) reveals that Nagle
> > algorithm
> > on the client is not disabled to client queue replies to socket but kernel
> > send
> > after all replies are queued giving the huge roundtrip!
> > So at least while sending the ping replies Nagle algorithm should be
> > disabled.
> > I'm trying to mitigate this issue using Linux tcp information (see TCP_INFO
> > on
> > tcp(7) man page), the tcpi_rtt field. If a bit higher (133ms instead of
> > 1ms)
> > but better than 325ms.
> > 
> > Now, I'm not that used to client code and just enabling this flag could
> > lead
> > to excessive network packets sent to the server (potentially every byte).
> > Somebody skilled with client code?
> 
> Client and server are both setting TCP_NODELAY, which should disable the
> Nagle algorithm, so maybe the delays are caused by something else?
> 
> Christophe
> 

I don't know but surely remote-viewer on Fedora 22 does not set TCP_NODELAY.

Frediano