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

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Message-ID: <CADVnQyny+AXxR6YgyCdzFk3TYbwJ8BXGgvSzB4rHD7HA-v210w@mail.gmail.com>
Date:	Fri, 17 May 2013 23:17:39 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Yuchung Cheng <ycheng@...gle.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Nandita Dukkipati <nanditad@...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 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>

neal
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