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

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Message-Id: <34CE0233-F881-4F40-B119-AA9D8F7D500F@bengler.no>
Date:	Wed, 7 Jan 2015 13:31:28 +0000
From:	Erik Grinaker <erik@...gler.no>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>
Subject: Re: TCP connection issues against Amazon S3

On 06 Jan 2015, at 22:00, Yuchung Cheng <ycheng@...gle.com> wrote:
> On Tue, Jan 6, 2015 at 1:04 PM, Erik Grinaker <erik@...gler.no> wrote:
>> 
>>> On 06 Jan 2015, at 20:26, Erik Grinaker <erik@...gler.no> wrote:
>> This still doesn’t explain why it works with older kernels, but not newer ones. I’m thinking it’s
> probably some minor change, which gets amplified by the lack of SACKs
> on the loadbalancer. Anyway, I’ll bring it up with Amazon.
> can you post traces with the older kernels?

Here is a dump using 3.11.10 against a non-SACK-enabled loadbalancer:

http://abstrakt.bengler.no/tcp-issues-s3-nosack-3.11.10.pcap.bz2

The transfer shows lots of DUPACKs and retransmits, but this does not seem to have as bad an effect as it did with the failing transfer we saw on newer kernels:

http://abstrakt.bengler.no/tcp-issues-s3-failure.pcap.bz2

One big difference, which Rick touched on earlier, is that the newer kernels keep sending TCP window updates as it’s going through the retransmits. The older kernel does not do this.


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