netdev - Re: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

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Date:	Wed, 20 Nov 2013 18:12:27 +0100
From:	Willy Tarreau <w@....eu>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Arnaud Ebalard <arno@...isbad.org>,
	Cong Wang <xiyou.wangcong@...il.com>, edumazet@...gle.com,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [BUG,REGRESSION?] 3.11.6+,3.12: GbE iface rate drops to few KB/s

Hi guys,

On Sun, Nov 17, 2013 at 09:41:38AM -0800, Eric Dumazet wrote:
> On Sun, 2013-11-17 at 15:19 +0100, Willy Tarreau wrote:
> 
> > 
> > So it is fairly possible that in your case you can't fill the link if you
> > consume too many descriptors. For example, if your server uses TCP_NODELAY
> > and sends incomplete segments (which is quite common), it's very easy to
> > run out of descriptors before the link is full.
> 
> BTW I have a very simple patch for TCP stack that could help this exact
> situation...
> 
> Idea is to use TCP Small Queue so that we dont fill qdisc/TX ring with
> very small frames, and let tcp_sendmsg() have more chance to fill
> complete packets.
> 
> Again, for this to work very well, you need that NIC performs TX
> completion in reasonable amount of time...

Eric, first I would like to confirm that I could reproduce Arnaud's issue
using 3.10.19 (160 kB/s in the worst case).

Second, I confirm that your patch partially fixes it and my performance
can be brought back to what I had with 3.10-rc7, but with a lot of
concurrent streams. In fact, in 3.10-rc7, I managed to constantly saturate
the wire when transfering 7 concurrent streams (118.6 kB/s). With the patch
applied, performance is still only 27 MB/s at 7 concurrent streams, and I
need at least 35 concurrent streams to fill the pipe. Strangely, after
2 GB of cumulated data transferred, the bandwidth divided by 11-fold and
fell to 10 MB/s again.

If I revert both "0ae5f47eff tcp: TSQ can use a dynamic limit" and
your latest patch, the performance is back to original.

Now I understand there's a major issue with the driver. But since the
patch emphasizes the situations where drivers take a lot of time to
wake the queue up, don't you think there could be an issue with low
bandwidth links (eg: PPPoE over xDSL, 10 Mbps ethernet, etc...) ?
I'm a bit worried about what we might discover in this area I must
confess (despite generally being mostly focused on 10+ Gbps).

Best regards,
Willy

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