| Message ID | 1380277734.30872.25.camel@edumazet-glaptop.roam.corp.google.com |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
On Fri, Sep 27, 2013 at 6:28 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > From: Eric Dumazet <edumazet@google.com> > > When TCP Small Queues was added, we used a sysctl to limit amount of > packets queues on Qdisc/device queues for a given TCP flow. > > Problem is this limit is either too big for low rates, or too small > for high rates. > > Now TCP stack has rate estimation in sk->sk_pacing_rate, and TSO > auto sizing, it can better control number of packets in Qdisc/device > queues. > > New limit is two packets or at least 1 to 2 ms worth of packets. > > Low rates flows benefit from this patch by having even smaller > number of packets in queues, allowing for faster recovery, > better RTT estimations. > > High rates flows benefit from this patch by allowing more than 2 packets > in flight as we had reports this was a limiting factor to reach line > rate. [ In particular if TX completion is delayed because of coalescing > parameters ] > > Example for a single flow on 10Gbp link controlled by FQ/pacing > > 14 packets in flight instead of 2 > > $ tc -s -d qd > qdisc fq 8001: dev eth0 root refcnt 32 limit 10000p flow_limit 100p > buckets 1024 quantum 3028 initial_quantum 15140 > Sent 1168459366606 bytes 771822841 pkt (dropped 0, overlimits 0 > requeues 6822476) > rate 9346Mbit 771713pps backlog 953820b 14p requeues 6822476 > 2047 flow, 2046 inactive, 1 throttled, delay 15673 ns > 2372 gc, 0 highprio, 0 retrans, 9739249 throttled, 0 flows_plimit > > Note that sk_pacing_rate is currently set to twice the actual rate, but > this might be refined in the future when a flow is in congestion > avoidance. > > Additional change : skb->destructor should be set to tcp_wfree(). > > A future patch (for linux 3.13+) might remove tcp_limit_output_bytes > > Signed-off-by: Eric Dumazet <edumazet@google.com> > Cc: Wei Liu <wei.liu2@citrix.com> > Cc: Cong Wang <xiyou.wangcong@gmail.com> > Cc: Yuchung Cheng <ycheng@google.com> > Cc: Neal Cardwell <ncardwell@google.com> > --- Acked-by: Neal Cardwell <ncardwell@google.com> neal -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Fri, Sep 27, 2013 at 3:28 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote: > > Additional change : skb->destructor should be set to tcp_wfree(). > > A future patch (for linux 3.13+) might remove tcp_limit_output_bytes Yeah, now it is only used in tcp_xmit_size_goal()... -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
From: Eric Dumazet <eric.dumazet@gmail.com> Date: Fri, 27 Sep 2013 03:28:54 -0700 > From: Eric Dumazet <edumazet@google.com> > > When TCP Small Queues was added, we used a sysctl to limit amount of > packets queues on Qdisc/device queues for a given TCP flow. > > Problem is this limit is either too big for low rates, or too small > for high rates. > > Now TCP stack has rate estimation in sk->sk_pacing_rate, and TSO > auto sizing, it can better control number of packets in Qdisc/device > queues. > > New limit is two packets or at least 1 to 2 ms worth of packets. > > Low rates flows benefit from this patch by having even smaller > number of packets in queues, allowing for faster recovery, > better RTT estimations. > > High rates flows benefit from this patch by allowing more than 2 packets > in flight as we had reports this was a limiting factor to reach line > rate. [ In particular if TX completion is delayed because of coalescing > parameters ] > > Example for a single flow on 10Gbp link controlled by FQ/pacing > > 14 packets in flight instead of 2 > > $ tc -s -d qd > qdisc fq 8001: dev eth0 root refcnt 32 limit 10000p flow_limit 100p > buckets 1024 quantum 3028 initial_quantum 15140 > Sent 1168459366606 bytes 771822841 pkt (dropped 0, overlimits 0 > requeues 6822476) > rate 9346Mbit 771713pps backlog 953820b 14p requeues 6822476 > 2047 flow, 2046 inactive, 1 throttled, delay 15673 ns > 2372 gc, 0 highprio, 0 retrans, 9739249 throttled, 0 flows_plimit > > Note that sk_pacing_rate is currently set to twice the actual rate, but > this might be refined in the future when a flow is in congestion > avoidance. > > Additional change : skb->destructor should be set to tcp_wfree(). > > A future patch (for linux 3.13+) might remove tcp_limit_output_bytes > > Signed-off-by: Eric Dumazet <edumazet@google.com> Applied, thanks Eric. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 7c83cb8..c20e406 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -895,8 +895,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it, skb_orphan(skb); skb->sk = sk; - skb->destructor = (sysctl_tcp_limit_output_bytes > 0) ? - tcp_wfree : sock_wfree; + skb->destructor = tcp_wfree; atomic_add(skb->truesize, &sk->sk_wmem_alloc); /* Build TCP header and checksum it. */ @@ -1840,7 +1839,6 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, while ((skb = tcp_send_head(sk))) { unsigned int limit; - tso_segs = tcp_init_tso_segs(sk, skb, mss_now); BUG_ON(!tso_segs); @@ -1869,13 +1867,20 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, break; } - /* TSQ : sk_wmem_alloc accounts skb truesize, - * including skb overhead. But thats OK. + /* TCP Small Queues : + * Control number of packets in qdisc/devices to two packets / or ~1 ms. + * This allows for : + * - better RTT estimation and ACK scheduling + * - faster recovery + * - high rates */ - if (atomic_read(&sk->sk_wmem_alloc) >= sysctl_tcp_limit_output_bytes) { + limit = max(skb->truesize, sk->sk_pacing_rate >> 10); + + if (atomic_read(&sk->sk_wmem_alloc) > limit) { set_bit(TSQ_THROTTLED, &tp->tsq_flags); break; } + limit = mss_now; if (tso_segs > 1 && !tcp_urg_mode(tp)) limit = tcp_mss_split_point(sk, skb, mss_now,