Re: [PATCH] tcp: TSQ can use a dynamic limit

[David Miller <davem@davemloft.net>]

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.