Very Low Throughputs (at very low RTTs, less than 1ms)

[oana-barbu]

Hi,

I'm trying to run Vivace between two machines (running Ubuntu 16.04) and the throughputs are around 7Mbps. If the flow runs for a long time, it achieves throughputs of over 200Mbps, but that's still very low - in comparison, iperf gets more than 700Mbps. But even when it reaches higher throughputs, it will still revert to 10-20 Mbps. Do you have any idea why this behavior occurs?

Update: The problem seems to occur only if the RTT between the two machines is very low (~0.2ms), otherwise, everything looks fine (e.g., with RTT ~190ms).

[nathanhjay]

I think low throughput at very small RTT values on standard Linux machines is related to latency noise. Vivace uses control that behaves badly when latency noise (perhaps up to 1 or 2ms in the software stack) is high relative to the base RTT (in this case, it's quite a bit larger). Our experiments seem to suggest that performance is substantially better when latency noise is less than 10% of the base RTT, though this hasn't been investigated thoroughly. I'll re-title the (if possible) issue to reflect the conditions and leave it open.