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Date: Wed, 4 Mar 2015 08:39:00 -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 11:51 AM, John Heffner <johnwheffner@...il.com> wrote:
> 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>
Actually, one final suggestion here. The cost of a failed probe is
much higher than a successful probe. I'd suggest still bounding the
probe segment size to no more than 2*cur_mss. (Think of a common case
where mss_clamp = 9000 - headers, but the actual path MTU = 1500.)
-John
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