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
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