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In this Document

Symptoms

Cause

Solution

Created from <SR 3-7792689041>

Applies to:

Symptoms

SMB transfers are slow between Mac clients running OSX 10.7 or 10.8 when they are connected via the 10Gbps link.

Those connected via a 1Gbps link do not show this problem.

Cause

Mac OSX has a tunable parameter, net.inet.tcp.delayed_ack which controls when TCP acknowledgements are sent.

The default setting, net.inet.tcp.delayed_ack=3, causes the OS to check for and throttle back high speed streaming data that can negatively impact interactive network I/O.

If high speed streaming is detected, acknowledgements are delayed.  Snoops show the delay is 200ms. This can result in an idle connection for significant periods of time on a high speed link.

In our test, we saw about a 200 ms delay once every transfer of 61440 bytes. That limited the transfer speed to 300K bytes per second as the data transfer time was a fraction of the delay.

The delayed acknowledgements introduce a pacing mechanism that seriously degrades throughput on file transfers.

Solution

The customer tested the 4 possible settings, 0-3,  and settled on net.inet.tcp.delayed_ack=2. This results in an acknowledgement every other packet.

Commands

To set the value in the running kernel:

To have the system set the value on subsequent boots:

Based on notes found on the internet, the following are the possible values and effects for net.inet.tcp.delayed_ack.  I was unable to get a definitive answer.

0 = acknowledge each packet

1 = send 1 ack for each 6 packets

2 = send immediate ack after the 2nd packet, 2 packets per ack

3 = auto detect streaming (DEFAULT)

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