[Spice-devel] Clients, Nagle and net test

[Frediano Ziglio]

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?

Frediano