netdev - Re: [PATCH v2 net-next 1/2] tcp: adjust window probe timers to safer values

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Date:	Wed, 6 May 2015 18:15:26 -0400
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Yuchung Cheng <ycheng@...gle.com>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net-next 1/2] tcp: adjust window probe timers to safer values

On Wed, May 6, 2015 at 5:26 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> With the advent of small rto timers in datacenter TCP,
> (ip route ... rto_min x), the following can happen :
>
> 1) Qdisc is full, transmit fails.
>
>    TCP sets a timer based on icsk_rto to retry the transmit, without
>    exponential backoff.
>    With low icsk_rto, and lot of sockets, all cpus are servicing timer
>    interrupts like crazy.
>    Intent of the code was to retry with a timer between 200 (TCP_RTO_MIN)
>    and 500ms (TCP_RESOURCE_PROBE_INTERVAL)
>
> 2) Receivers can send zero windows if they don't drain their receive queue.
>
>    TCP sends zero window probes, based on icsk_rto current value, with
>    exponential backoff.
>    With /proc/sys/net/ipv4/tcp_retries2 being 15 (or even smaller in
>    some cases), sender can abort in less than one or two minutes !
>    If receiver stops the sender, it obviously doesn't care of very tight
>    rto. Probability of dropping the ACK reopening the window is not
>    worth the risk.
>
> Lets change the base timer to be at least 200ms (TCP_RTO_MIN) for these
> events (but not normal RTO based retransmits)
>
> A followup patch adds a new SNMP counter, as it would have helped a lot
> diagnosing this issue.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>

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

neal
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