netdev - Re: [RFC PATCHv2 net-next 1/2] tcp: RTO Restart (RTOR)

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Date:	Thu, 10 Dec 2015 22:11:25 +0100
From:	Per Hurtig <per.hurtig@....se>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Tom Herbert <tom@...bertland.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Florian Westphal <fw@...len.de>,
	Marcelo Ricardo Leitner <mleitner@...hat.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Pasi Sarolahti <pasi.sarolahti@....fi>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Netdev <netdev@...r.kernel.org>,
	Anna Brunström <anna.brunstrom@....se>,
	Andreas Petlund <apetlund@...ula.no>,
	Michael Welzl <michawe@....uio.no>,
	Mohammad Rajiullah <mohammad.rajiullah@....se>
Subject: Re: [RFC PATCHv2 net-next 1/2] tcp: RTO Restart (RTOR)



> 10 dec. 2015 kl. 16:37 skrev Neal Cardwell <ncardwell@...gle.com>:
> 
>> On Thu, Dec 10, 2015 at 1:51 AM, Per Hurtig <per.hurtig@....se> wrote:
>> 
>>> On 08 Dec 2015, at 14:47, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> 
>>> On Tue, 2015-12-08 at 10:19 +0100, Per Hurtig wrote:
>>> 
>>>> +static u32 tcp_unsent_pkts(const struct sock *sk, u32 ulimit)
>>>> +{
>>>> +    struct sk_buff *skb = tcp_send_head(sk);
>>>> +    u32 pkts = 0;
>>>> +
>>>> +    if (skb)
>>>> +            tcp_for_write_queue_from(skb, sk) {
>>>> +                    pkts += tcp_skb_pcount(skb);
>>>> +
>>>> +                    if (ulimit && pkts >= ulimit)
>>>> +                            return ulimit;
>>>> +            }
>>>> +
>>>> +    return pkts;
>>>> +}
>>> 
>>> 
>>> Considering Yuchung feedback, have you looked at using an approximation
>>> instead ?
>>> 
>>> (ie using tp->write_seq - tp->snd_nxt)
>> 
>> Well, an approximation is rather “dangerous” as missing a single packet
>> could inhibit the desired behaviour. If looping is undesired, I think a
>> better solution is to actually *not* do this check at all and instead rely
>> solely on the
>> 
>> tp->packets_out < TCP_RTORESTART_THRESH
> 
> Yes, this simpler version seems very much preferable, IMHO. I agree
> that it does not seem worth the complexity to try to cover the kind of
> corner cases you outline.
> 
> I would also suggest a TCP_RTORESTART_THRESH value higher than 4.
> 
> In the ID at https://tools.ietf.org/html/draft-ietf-tcpm-rtorestart-10 it says:
> 
>   The RECOMMENDED value of rrthresh is four, as this value will ensure
>   that RTOR is only used when fast retransmit cannot be triggered.
> 
> But my sense is that fast retransmit is often not triggered at
> in-flight counts of much higher than 4, due to drop-tail queues, TSO
> bursts, the initial IW10 being unpaced, etc. It would be interesting
> to see A/B experiments for a few TCP_RTORESTART_THRESH values, say, 4
> vs 10.
> 
> neal
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Sure. One idea could also be to use the "reordering" value as a dynamic threshhold?

-- Per
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