netdev - Re: [PATCH net-next] tcp: remove bad timeout logic in fast recovery

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Message-ID: <CAB_+Fg57tpcBzsjZQHQm9dpF+LoBVTZauc+UGrwA6U=Xqbq+fA@mail.gmail.com>
Date:	Fri, 17 May 2013 22:52:34 -0700
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Ilpo Jarvinen <ilpo.jarvinen@...helsinki.fi>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] tcp: remove bad timeout logic in fast recovery

On Fri, May 17, 2013 at 8:17 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Fri, May 17, 2013 at 9:46 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> On Fri, 2013-05-17 at 16:45 -0700, Yuchung Cheng wrote:
>>> tcp_timeout_skb() was intended to trigger fast recovery on timeout,
>>> unfortunately in reality it often causes spurious retransmission
>>> storms during fast recovery. The particular sign is fast retransmit
>>> over highest sacked sequence (SND.FACK).
>>>
>>> Currently the RTO timer re-arming (as in RFC6298) offers a nice cushion
>>> to avoid spurious timeout: when SND.UNA advances the sender re-arms
>>> RTO and extends the timeout by icsk_rto. The sender does not offset
>>> the time elapsed since the packet at SND.UNA was sent.
>>>
>>> But if the next (DUP)ACK arrives later than ~RTTVAR and triggers
>>> tcp_fastretrans_alert(), then tcp_timeout_skb() will mark any packet
>>> sent before icsk_rto interval lost, including one that's above the
>>> highest sacked sequence. Most likely a large part of scorebard will
>>> be marked.
>>>
>>> If most packets are not lost then the subsequence DUPACKs with new
>>> SACK blockes will cause the sender to continue retransmit packets
>>> beyond SND.FACK spuriously right. Even only one packet is lost the
>>> sender may falsely retransmit almost the entire window.
>>>
>>> The situation becomes common in the world of bufferbloat: the RTT
>>> continues to grow as the queue builds up but RTTVAR remains small and
>>> close to the minimum 200ms. If a data packet is lost and the DUPACK
>>> triggered by the next data packet is slightly delayed, then a spurious
>>> retransmission storm forms.
>>>
>>> As the original comment on tcp_timeout_skb() suggests: the usefulness
>>> of this feature is questionable. It also wastes cycles walking the
>>> sack scoreboard and is actually harmful because of the false recovery.
>>> It's time to remove this.
>>>
>>> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
>>> ---
>>>  include/linux/tcp.h  |  1 -
>>>  include/net/tcp.h    |  1 -
>>>  net/ipv4/tcp_input.c | 65 +---------------------------------------------------
>>>  3 files changed, 1 insertion(+), 66 deletions(-)
>>
>> Acked-by: Eric Dumazet <edumazet@...gle.com>
>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>

Acked-by: Nandita Dukkipati <nanditad@...gle.com>
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