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tcp: do not slow start when cwnd equals ssthresh

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In the original design slow start is only used to raise cwnd
when cwnd is stricly below ssthresh. It makes little sense
to slow start when cwnd == ssthresh: especially
when hystart has set ssthresh in the initial ramp, or after
recovery when cwnd resets to ssthresh. Not doing so will
also help reduce the buffer bloat slightly.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Nandita Dukkipati <nanditad@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yuchung Cheng 2015-07-09 13:16:30 -07:00 committed by David S. Miller
parent 071d5080e3
commit 76174004a0
4 changed files with 4 additions and 6 deletions
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2
include/net/tcp.h
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@ -991,7 +991,7 @@ static inline unsigned int tcp_packets_in_flight(const struct tcp_sock *tp)
static inline bool tcp_in_slow_start(const struct tcp_sock *tp) static inline bool tcp_in_slow_start(const struct tcp_sock *tp)
{ {
return tp->snd_cwnd <= tp->snd_ssthresh; return tp->snd_cwnd < tp->snd_ssthresh;
} }
static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp) static inline bool tcp_in_initial_slowstart(const struct tcp_sock *tp)

2
net/ipv4/tcp_cdg.c
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@ -264,7 +264,7 @@ static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
u32 prior_snd_cwnd; u32 prior_snd_cwnd;
u32 incr; u32 incr;
if (tp->snd_cwnd < tp->snd_ssthresh && hystart_detect) if (tcp_in_slow_start(tp) && hystart_detect)
tcp_cdg_hystart_update(sk); tcp_cdg_hystart_update(sk);
if (after(ack, ca->rtt_seq) && ca->rtt.v64) { if (after(ack, ca->rtt_seq) && ca->rtt.v64) {

4
net/ipv4/tcp_cong.c
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@ -365,10 +365,8 @@ int tcp_set_congestion_control(struct sock *sk, const char *name)
*/ */
u32 tcp_slow_start(struct tcp_sock *tp, u32 acked) u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
{ {
u32 cwnd = tp->snd_cwnd + acked; u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);
if (cwnd > tp->snd_ssthresh)
cwnd = tp->snd_ssthresh + 1;
acked -= cwnd - tp->snd_cwnd; acked -= cwnd - tp->snd_cwnd;
tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp); tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);

2
net/ipv4/tcp_hybla.c
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@ -112,7 +112,7 @@ static void hybla_cong_avoid(struct sock *sk, u32 ack, u32 acked)
rho_fractions = ca->rho_3ls - (ca->rho << 3); rho_fractions = ca->rho_3ls - (ca->rho << 3);
if (tp->snd_cwnd < tp->snd_ssthresh) { if (tcp_in_slow_start(tp)) {
/* /*
* slow start * slow start
* INC = 2^RHO - 1 * INC = 2^RHO - 1