Re: tcp wifi upload performance and lots of ACKs

On 06/07/2012 05:20 AM, David Laight wrote:

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-----Original Message-----
From: netdev-owner@vger.kernel.org
[mailto:netdev-owner@vger.kernel.org] On Behalf Of Ben Greear
Sent: 07 June 2012 05:16
To: Daniel Baluta
Cc: netdev
Subject: Re: tcp wifi upload performance and lots of ACKs

On 06/04/2012 12:22 PM, Daniel Baluta wrote:

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On Mon, Jun 4, 2012 at 9:29 PM, Ben Greear[off-list ref]

wrote:

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I'm going some TCP performance testing on wifi ->   LAN interface

connections.

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   With
UDP, we can get around 250Mbps of payload throughput.  With TCP,

max is

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about 80Mbps.

I think the problem is that there are way too many ACK packets, and
bi-directional
traffic on wifi interfaces really slows things down.  (About 7000

pkts per

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second in
upload direction, 2000 pps download.  And the vast majority of the

download

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pkts
are 66 byte ACK pkts from what I can tell.)

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[1] http://marc.info/?l=linux-netdev&m=131983649130350&w=2

After a bit more playing, I did notice a reliable 5% increase in
traffic (200Mbps ->  210Mbps) from changing the delack segments
to 20 from the default of 1.  That is enough to be useful to me,
and there may be more significant gains to be found...
I haven't done a full matrix of testing yet.

Does this delaying of acks have a detrimental effect on the
sending end?
I've seen very bad interactions between delayed acks and
(I believe) the 'slow start' code on connections with
one-directional data, Nagle disabled and very low RTT.

What I saw was the sender sending 4 data packets, then
sitting waiting for an ack - in spite of accumulating
several kB of data to send.

Delaying acks further will only make this worse.

I'm sure it's not for everyone in all cases.  In my case, I'm
sending long-term bulk transfer, at high speeds, over wifi network
which has some latency.  Tested one-way traffic so far.

With the patch and delayed acks, I get more sender throughput than
without (200Mbps -> 210Mbps).

Thanks,
Ben

	David

-- 
Ben Greear [off-list ref]
Candela Technologies Inc  http://www.candelatech.com