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Date: Tue, 3 Mar 2015 11:51:39 -0500
From: John Heffner <johnwheffner@...il.com>
To: Fan Du <fan.du@...el.com>
Cc: Eric Dumazet <edumazet@...gle.com>,
David Miller <davem@...emloft.net>,
Netdev <netdev@...r.kernel.org>,
Fan Du <fengyuleidian0615@...il.com>
Subject: Re: [PATCHv4 net-next 2/3] ipv4: Use binary search to choose tcp PMTU probe_size
On Tue, Mar 3, 2015 at 4:19 AM, Fan Du <fan.du@...el.com> wrote:
> 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: Fan Du <fan.du@...el.com>
> ---
> v4:
> - Convert probe_size to mss
> - Clamp probe_threshold
> v3:
> - Fix commit message
> v2:
> - Use sysctl_tcp_probe_threshold to control when probing
> will stop wrt interval between search high and search low.
> ---
> include/net/netns/ipv4.h | 1 +
> include/net/tcp.h | 3 +++
> net/ipv4/sysctl_net_ipv4.c | 7 +++++++
> net/ipv4/tcp_ipv4.c | 1 +
> net/ipv4/tcp_output.c | 14 +++++++++++---
> 5 files changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h
> index 1b26c6c..374bf3f 100644
> --- a/include/net/netns/ipv4.h
> +++ b/include/net/netns/ipv4.h
> @@ -85,6 +85,7 @@ 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;
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 7b57e5b..d269c91 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -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
>
> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
> index d151539..d3c09c1 100644
> --- a/net/ipv4/sysctl_net_ipv4.c
> +++ b/net/ipv4/sysctl_net_ipv4.c
> @@ -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,
> + },
> { }
> };
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 5a2dfed..35790d9 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -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:
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index a2a796c..46acddc 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1837,11 +1837,13 @@ 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,
> @@ -1854,11 +1856,17 @@ 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;
> }
> --
> 1.7.1
>
I suspect there's plenty of room for further improvement in the
probing heuristic, but this seems like a reasonable improvement.
Acked-by: John Heffner <johnwheffner@...il.com>
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