描述
以下是提交者Sangtae Ha对这个patch的描述:
HyStart sets the initial exit point of slow start.
Suppose that HyStart exits at 0.5BDP in a BDP network and no history exists.
If the BDP of a network is large, CUBIC's initial cwnd growth may be too conservative
to utilize the link.
CUBIC increases the cwnd 20% per RTT in this case.(这里有处笔误,是5%)
代码
--- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -270,6 +270,13 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd) ca->cnt = 100 * cwnd; /* very small increment*/ } + /* + * The initial growth of cubic function may be too conservative + * when the available bandwidth is still unknown. + */ + if (ca->loss_cwnd == 0 && ca->cnt > 20) + ca->cnt = 20; /* increase cwnd 5% per RTT */ + /* TCP Friendly */ if (tcp_friendliness) { u32 scale = beta_scale;
评价
一些因素会导致HyStart的提前退出,从而不能充分利用可用带宽。实际上HyStart的测量结果很可能会偏小。
所以当退出HyStart后,进入拥塞避免状态时,如果发现之前没有丢包过,并且cwnd的增长幅度小于5%,
那么就把cwnd每RTT的增长幅度调整为5%。
这样一来如果前提退出慢启动时,保证拥塞窗口的增速不会太低。
Author
zhangskd @ csdn blog