cc_cubic: Use units of micro seconds (usecs) instead of ticks in rtt.

[cc]

Summary

This improves TCP friendly cwnd in cases of low latency high drop rate
networks.
Tests show +42% and +37% better performance in 1Gpbs and 10Gbps cases.

test result in detail: testD40185

[tuexen]

sys/netinet/cc/cc_cubic.h

113

Why isn't this converted?

232

Should there be a space between * and tick?

[cc]

sys/netinet/cc/cc_cubic.h

113

I am not sure if there is a high resolution timer to use for the base stack. So this is a workaround.

[cc]

correct the lower bound for min_rtt_usecs and correct some space and comments

[tuexen]

You might want to wait also for Richards' approval, since he is much more familiar with the code than I am.

[cc]

sys/netinet/cc/cc_cubic.h

113

Secondly, cubic_data->t_last_cong as an intermediate variable is used for usecs_since_cong calculation in cubic_ack_received(). The usecs_since_cong is converted by using usec there. So I don't think we need to convert cubic_data->t_last_cong by TICKS_2_USEC(ticks).

[cc]

In D40185#917355, @tuexen wrote:

You might want to wait also for Richards' approval, since he is much more familiar with the code than I am.

Sounds good to me. :)

[rscheff]

Not changing the generic rtt base stack unit / measuring, just improviing the granularity/precision of the calculations within Cubic. since this improves cubic performance, let's put this in prior 14 branching...

[cc]

This patch is submitted in a3aa6f652904