From b0983d3c9b132c33b6fb2e28d157a1edc18a173c Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Fri, 20 Sep 2013 13:56:58 -0700 Subject: [PATCH] tcp: fix dynamic right sizing Dynamic Right Sizing (DRS) is supposed to open TCP receive window automatically, but suffers from two bugs, presented by order of importance. 1) tcp_rcv_space_adjust() fix : Using twice the last received amount is very pessimistic, because it doesn't allow fast recovery or proper slow start ramp up, if sender wants to increase cwin by 100% every RTT. copied = bytes received in previous RTT 2*copied = bytes we expect to receive in next RTT 4*copied = bytes we need to advertise in rwin at end of next RTT DRS is one RTT late, it needs a 4x factor. If sender is not using ABC, and increases cwin by 50% every rtt, then we needed 1.5*1.5 = 2.25 factor. This is probably why this bug was not really noticed. 2) There is no window adjustment after first RTT. DRS triggers only after the second RTT. DRS needs two RTT to initialize, so tcp_fixup_rcvbuf() should setup sk_rcvbuf to allow proper window grow for first two RTT. This patch increases TCP efficiency particularly for large RTT flows when autotuning is used at the receiver, and more particularly in presence of packet losses. Signed-off-by: Eric Dumazet Signed-off-by: Neal Cardwell Signed-off-by: Yuchung Cheng Cc: Van Jacobson Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 84 ++++++++++++++++++++++++++++---------------- 1 file changed, 53 insertions(+), 31 deletions(-) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 25a89eaa669d..5d083855c111 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -355,6 +355,12 @@ static void tcp_fixup_rcvbuf(struct sock *sk) rcvmem = 2 * SKB_TRUESIZE(mss + MAX_TCP_HEADER) * tcp_default_init_rwnd(mss); + /* Dynamic Right Sizing (DRS) has 2 to 3 RTT latency + * Allow enough cushion so that sender is not limited by our window + */ + if (sysctl_tcp_moderate_rcvbuf) + rcvmem <<= 2; + if (sk->sk_rcvbuf < rcvmem) sk->sk_rcvbuf = min(rcvmem, sysctl_tcp_rmem[2]); } @@ -373,6 +379,8 @@ void tcp_init_buffer_space(struct sock *sk) tcp_fixup_sndbuf(sk); tp->rcvq_space.space = tp->rcv_wnd; + tp->rcvq_space.time = tcp_time_stamp; + tp->rcvq_space.seq = tp->copied_seq; maxwin = tcp_full_space(sk); @@ -512,48 +520,62 @@ void tcp_rcv_space_adjust(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); int time; - int space; - - if (tp->rcvq_space.time == 0) - goto new_measure; + int copied; time = tcp_time_stamp - tp->rcvq_space.time; if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 0) return; - space = 2 * (tp->copied_seq - tp->rcvq_space.seq); + /* Number of bytes copied to user in last RTT */ + copied = tp->copied_seq - tp->rcvq_space.seq; + if (copied <= tp->rcvq_space.space) + goto new_measure; + + /* A bit of theory : + * copied = bytes received in previous RTT, our base window + * To cope with packet losses, we need a 2x factor + * To cope with slow start, and sender growing its cwin by 100 % + * every RTT, we need a 4x factor, because the ACK we are sending + * now is for the next RTT, not the current one : + * + */ + + if (sysctl_tcp_moderate_rcvbuf && + !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) { + int rcvwin, rcvmem, rcvbuf; - space = max(tp->rcvq_space.space, space); + /* minimal window to cope with packet losses, assuming + * steady state. Add some cushion because of small variations. + */ + rcvwin = (copied << 1) + 16 * tp->advmss; - if (tp->rcvq_space.space != space) { - int rcvmem; + /* If rate increased by 25%, + * assume slow start, rcvwin = 3 * copied + * If rate increased by 50%, + * assume sender can use 2x growth, rcvwin = 4 * copied + */ + if (copied >= + tp->rcvq_space.space + (tp->rcvq_space.space >> 2)) { + if (copied >= + tp->rcvq_space.space + (tp->rcvq_space.space >> 1)) + rcvwin <<= 1; + else + rcvwin += (rcvwin >> 1); + } - tp->rcvq_space.space = space; + rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER); + while (tcp_win_from_space(rcvmem) < tp->advmss) + rcvmem += 128; - if (sysctl_tcp_moderate_rcvbuf && - !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) { - int new_clamp = space; + rcvbuf = min(rcvwin / tp->advmss * rcvmem, sysctl_tcp_rmem[2]); + if (rcvbuf > sk->sk_rcvbuf) { + sk->sk_rcvbuf = rcvbuf; - /* Receive space grows, normalize in order to - * take into account packet headers and sk_buff - * structure overhead. - */ - space /= tp->advmss; - if (!space) - space = 1; - rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER); - while (tcp_win_from_space(rcvmem) < tp->advmss) - rcvmem += 128; - space *= rcvmem; - space = min(space, sysctl_tcp_rmem[2]); - if (space > sk->sk_rcvbuf) { - sk->sk_rcvbuf = space; - - /* Make the window clamp follow along. */ - tp->window_clamp = new_clamp; - } + /* Make the window clamp follow along. */ + tp->window_clamp = rcvwin; } } + tp->rcvq_space.space = copied; new_measure: tp->rcvq_space.seq = tp->copied_seq; @@ -5674,8 +5696,8 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb, tcp_init_congestion_control(sk); tcp_mtup_init(sk); - tcp_init_buffer_space(sk); tp->copied_seq = tp->rcv_nxt; + tcp_init_buffer_space(sk); } smp_mb(); tcp_set_state(sk, TCP_ESTABLISHED); -- 2.20.1