Commit 6b58e0a5 authored by Fan Du's avatar Fan Du Committed by David S. Miller
Browse files

ipv4: Use binary search to choose tcp PMTU probe_size 



Current probe_size is chosen by doubling mss_cache,
the probing process will end shortly with a sub-optimal
mss size, and the link mtu will not be taken full
advantage of, in return, this will make user to tweak
tcp_base_mss with care.

Use binary search to choose probe_size in a fine
granularity manner, an optimal mss will be found
to boost performance as its maxmium.

In addition, introduce a sysctl_tcp_probe_threshold
to control when probing will stop in respect to
the width of search range.

Test env:
Docker instance with vxlan encapuslation(82599EB)
iperf -c 10.0.0.24  -t 60

before this patch:
1.26 Gbits/sec

After this patch: increase 26%
1.59 Gbits/sec

Signed-off-by: default avatarFan Du <fan.du@intel.com>
Acked-by: default avatarJohn Heffner <johnwheffner@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent dcd8fb85

include/net/netns/ipv4.h

+1 −0
Original line number Diff line number Diff line
@@ -87,6 +87,7 @@ struct netns_ipv4 { 
	int sysctl_tcp_fwmark_accept;

	int sysctl_tcp_mtu_probing;

	int sysctl_tcp_base_mss;

	int sysctl_tcp_probe_threshold;



	struct ping_group_range ping_group_range;

include/net/tcp.h

+3 −0
Original line number Diff line number Diff line
@@ -67,6 +67,9 @@ void tcp_time_wait(struct sock *sk, int state, int timeo); 
/* The least MTU to use for probing */

#define TCP_BASE_MSS		1024



/* Specify interval when tcp mtu probing will stop */

#define TCP_PROBE_THRESHOLD	8



/* After receiving this amount of duplicate ACKs fast retransmit starts. */

#define TCP_FASTRETRANS_THRESH 3

net/ipv4/sysctl_net_ipv4.c

+7 −0
Original line number Diff line number Diff line
@@ -883,6 +883,13 @@ static struct ctl_table ipv4_net_table[] = { 
		.mode		= 0644,

		.proc_handler	= proc_dointvec,

	},

	{

		.procname	= "tcp_probe_threshold",

		.data		= &init_net.ipv4.sysctl_tcp_probe_threshold,

		.maxlen		= sizeof(int),

		.mode		= 0644,

		.proc_handler	= proc_dointvec,

	},

	{ }

};

net/ipv4/tcp_ipv4.c

+1 −0
Original line number Diff line number Diff line
@@ -2460,6 +2460,7 @@ static int __net_init tcp_sk_init(struct net *net) 
	}

	net->ipv4.sysctl_tcp_ecn = 2;

	net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS;

	net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD;

	return 0;



fail:

net/ipv4/tcp_output.c

+11 −3
Original line number Diff line number Diff line
@@ -1842,11 +1842,13 @@ static int tcp_mtu_probe(struct sock *sk) 
	struct tcp_sock *tp = tcp_sk(sk);

	struct inet_connection_sock *icsk = inet_csk(sk);

	struct sk_buff *skb, *nskb, *next;

	struct net *net = sock_net(sk);

	int len;

	int probe_size;

	int size_needed;

	int copy;

	int mss_now;

	int interval;



	/* Not currently probing/verifying,

	 * not in recovery,
@@ -1859,11 +1861,17 @@ static int tcp_mtu_probe(struct sock *sk) 
	    tp->rx_opt.num_sacks || tp->rx_opt.dsack)

		return -1;



	/* Very simple search strategy: just double the MSS. */

	/* Use binary search for probe_size between tcp_mss_base,

	 * and current mss_clamp. if (search_high - search_low)

	 * smaller than a threshold, backoff from probing.

	 */

	mss_now = tcp_current_mss(sk);

	probe_size = 2 * tp->mss_cache;

	probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +

				    icsk->icsk_mtup.search_low) >> 1);

	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;

	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {

	interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;

	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||

	    interval < max(1, net->ipv4.sysctl_tcp_probe_threshold)) {

		/* TODO: set timer for probe_converge_event */

		return -1;

	}

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