linux-kernel - Re: TCP connection issues against Amazon S3

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Date:	Tue, 06 Jan 2015 09:20:37 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Erik Grinaker <erik@...gler.no>
Cc:	linux-kernel@...r.kernel.org, Yuchung Cheng <ycheng@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: TCP connection issues against Amazon S3

On Tue, 2015-01-06 at 16:11 +0000, Erik Grinaker wrote:
> > On 06 Jan 2015, at 16:04, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Tue, 2015-01-06 at 15:14 +0000, Erik Grinaker wrote:
> >> (CCing Yuchung, as his name comes up in the relevant commits)
> >> 
> >> After upgrading from Ubuntu 12.04.5 to 14.04.1 we have begun seeing
> >> intermittent TCP connection hangs for HTTP image requests against
> >> Amazon S3. 3-5% of requests will suddenly stall in the middle of the
> >> transfer before timing out. We see this problem across a range of
> >> servers, in several data centres and networks, all located in Norway.
> >> 
> >> A packet dump [1] shows repeated ACK retransmits for some of the
> >> requests. Using Ubuntu mainline kernels, we found the problem to have
> >> been introduced between 3.11.10 and 3.12.0, possibly in
> >> 0f7cc9a3c2bd89b15720dbf358e9b9e62af27126. The problem is also present
> >> in 3.18.1. Disabling tcp_window_scaling seems to solve it, but has
> >> obvious drawbacks for transfer speeds. Other sysctls do not seem to
> >> affect it.
> >> 
> >> I am not sure if this is fundamentally a kernel bug or a network
> >> issue, but we did not see this problem with older kernels.
> >> 
> >> [1] http://abstrakt.bengler.no/tcp-issues-s3.pcap.bz2
> > 
> > 
> > CC netdev
> > 
> > This looks like the bug we fixed here :
> > 
> > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=39bb5e62867de82b269b07df900165029b928359
> 
> Has that patch gone into a release? Because the problem persists with 3.18.1.

Patch is in 3.18.1 yes.

So thats a separate issue. 

Can you confirm pcap was taken at receiver (195.159.221.106), not sender
(54.231.136.74) , and on which host is running the 'buggy kernel' ?

If the sender is broken, changing the kernel on receiver wont help.

BTW not using sack (on 54.231.132.98) is terrible for performance in
lossy environments.



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