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

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Date:	Fri, 27 Sep 2013 11:08:28 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	David Miller <davem@...emloft.net>,
	Wei Liu <wei.liu2@...rix.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH] tcp: TSQ can use a dynamic limit

On Fri, Sep 27, 2013 at 6:28 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> From: Eric Dumazet <edumazet@...gle.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@...gle.com>
> Cc: Wei Liu <wei.liu2@...rix.com>
> Cc: Cong Wang <xiyou.wangcong@...il.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Cc: Neal Cardwell <ncardwell@...gle.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal
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