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Commit a337531b authored by Yuchung Cheng's avatar Yuchung Cheng Committed by David S. Miller
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tcp: up initial rmem to 128KB and SYN rwin to around 64KB 



Previously TCP initial receive buffer is ~87KB by default and
the initial receive window is ~29KB (20 MSS). This patch changes
the two numbers to 128KB and ~64KB (rounding down to the multiples
of MSS) respectively. The patch also simplifies the calculations s.t.
the two numbers are directly controlled by sysctl tcp_rmem[1]:

  1) Initial receiver buffer budget (sk_rcvbuf): while this should
     be configured via sysctl tcp_rmem[1], previously tcp_fixup_rcvbuf()
     always override and set a larger size when a new connection
     establishes.

  2) Initial receive window in SYN: previously it is set to 20
     packets if MSS <= 1460. The number 20 was based on the initial
     congestion window of 10: the receiver needs twice amount to
     avoid being limited by the receive window upon out-of-order
     delivery in the first window burst. But since this only
     applies if the receiving MSS <= 1460, connection using large MTU
     (e.g. to utilize receiver zero-copy) may be limited by the
     receive window.

With this patch TCP memory configuration is more straight-forward and
more properly sized to modern high-speed networks by default. Several
popular stacks have been announcing 64KB rwin in SYNs as well.

Signed-off-by: default avatarYuchung Cheng <ycheng@google.com>
Signed-off-by: default avatarWei Wang <weiwan@google.com>
Signed-off-by: default avatarNeal Cardwell <ncardwell@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 3ff6cde8

net/ipv4/tcp.c

+2 −2
Original line number Diff line number Diff line
@@ -3896,8 +3896,8 @@ void __init tcp_init(void) 
	init_net.ipv4.sysctl_tcp_wmem[2] = max(64*1024, max_wshare);



	init_net.ipv4.sysctl_tcp_rmem[0] = SK_MEM_QUANTUM;

	init_net.ipv4.sysctl_tcp_rmem[1] = 87380;

	init_net.ipv4.sysctl_tcp_rmem[2] = max(87380, max_rshare);

	init_net.ipv4.sysctl_tcp_rmem[1] = 131072;

	init_net.ipv4.sysctl_tcp_rmem[2] = max(131072, max_rshare);



	pr_info("Hash tables configured (established %u bind %u)\n",

		tcp_hashinfo.ehash_mask + 1, tcp_hashinfo.bhash_size);

net/ipv4/tcp_input.c

+2 −23
Original line number Diff line number Diff line
@@ -426,26 +426,7 @@ static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb) 
	}

}



/* 3. Tuning rcvbuf, when connection enters established state. */

static void tcp_fixup_rcvbuf(struct sock *sk)

{

	u32 mss = tcp_sk(sk)->advmss;

	int rcvmem;



	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 (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf)

		rcvmem <<= 2;



	if (sk->sk_rcvbuf < rcvmem)

		sk->sk_rcvbuf = min(rcvmem, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);

}



/* 4. Try to fixup all. It is made immediately after connection enters

/* 3. Try to fixup all. It is made immediately after connection enters

 *    established state.

 */

void tcp_init_buffer_space(struct sock *sk)
@@ -454,8 +435,6 @@ void tcp_init_buffer_space(struct sock *sk) 
	struct tcp_sock *tp = tcp_sk(sk);

	int maxwin;



	if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))

		tcp_fixup_rcvbuf(sk);

	if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))

		tcp_sndbuf_expand(sk);
@@ -485,7 +464,7 @@ void tcp_init_buffer_space(struct sock *sk) 
	tp->snd_cwnd_stamp = tcp_jiffies32;

}



/* 5. Recalculate window clamp after socket hit its memory bounds. */

/* 4. Recalculate window clamp after socket hit its memory bounds. */

static void tcp_clamp_window(struct sock *sk)

{

	struct tcp_sock *tp = tcp_sk(sk);

net/ipv4/tcp_output.c

+4 −21
Original line number Diff line number Diff line
@@ -195,21 +195,6 @@ static inline void tcp_event_ack_sent(struct sock *sk, unsigned int pkts, 
	inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);

}





u32 tcp_default_init_rwnd(u32 mss)

{

	/* Initial receive window should be twice of TCP_INIT_CWND to

	 * enable proper sending of new unsent data during fast recovery

	 * (RFC 3517, Section 4, NextSeg() rule (2)). Further place a

	 * limit when mss is larger than 1460.

	 */

	u32 init_rwnd = TCP_INIT_CWND * 2;



	if (mss > 1460)

		init_rwnd = max((1460 * init_rwnd) / mss, 2U);

	return init_rwnd;

}



/* Determine a window scaling and initial window to offer.

 * Based on the assumption that the given amount of space

 * will be offered. Store the results in the tp structure.
@@ -244,7 +229,10 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss, 
	if (sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows)

		(*rcv_wnd) = min(space, MAX_TCP_WINDOW);

	else

		(*rcv_wnd) = space;

		(*rcv_wnd) = min_t(u32, space, U16_MAX);



	if (init_rcv_wnd)

		*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);



	(*rcv_wscale) = 0;

	if (wscale_ok) {
@@ -257,11 +245,6 @@ void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss, 
			(*rcv_wscale)++;

		}

	}



	if (!init_rcv_wnd) /* Use default unless specified otherwise */

		init_rcv_wnd = tcp_default_init_rwnd(mss);

	*rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);



	/* Set the clamp no higher than max representable value */

	(*window_clamp) = min_t(__u32, U16_MAX << (*rcv_wscale), *window_clamp);

}