Readynas OS6 TX - Uplink speed Problem // Slow Performance on iperf

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kixm wrote:

Flow Control needs to be turned off

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Not necessarily, but adjusting flow control should be part of the troubleshooting.

The original ethernet flow control is actually a "pause".  So if too much traffic is going to a device, flow control will block all new traffic going to the device until the queue drains.  No packets would be lost.  

If ethernet flow control is disabled, then the queue will overflow and packets will be lost.  TCP's congestion control will back off the rate significantly in response to the packet loss, and it will slowly increase the rate that.  There are several TCP congestion control algorithms in the wild - it's been enhanced/tuned over the years.  So different senders will behave differently when there is packet loss. 

In my own experience, enabling ethernet flow control end-to-end (switches and clients) often helps performance, especially if you have some older devices on slower links.  But this isn't a "one size fits all" thing - so you basically need to try it both ways (similar to Jumbo Frames).