[Bridge] Real time issues when using a bridge

[Stephen Hemminger]

On Tue, 12 Oct 2010 16:31:36 +0200
Jean-Michel Hautbois <jhautbois at gmail.com> wrote:

> Hi there !
> 
> I am facing a jitter issue that I will try to explain :
> I have the following configuration :
> 
> (A) eth0 <--> br0 <--> eth1 (B)
> 
> I have a small and custom kernel (I removed netfilter, for instance) and I
> am measuring the time between A and B (using hardware taps, etc.).
> My bandwidth is quite small (each packet is about 200 bytes and there is a
> delay of 10ms between each packet).
> I am measuring both the latency and the jitter (max_latency - min_latency)
> and this one is quite large.
> 
> I added ftrace option, and after some (long) investigations, I am currently
> limiting my study to the bridge part.
> 
> When I trace only br_handle_frame and br_forward_finish and do my test on
> 6500 frames I have :
> - average latency of 39µs
> - minimum latency of 34 µs
> - maximum latency of 81µs
> 
> Thus, my jitter is 47µs.
> This may seem low, but this is about two to three times what I expect and I
> cannot see why a bridge which is only forwarding a frame would have jitter ?
> If you have any idea, everything is interesting for me.
> I thought maybe to skb_clone ?
> 
> Best Regards,
> JM

You probably have NAPI and/or interrupt coalescing setting
enabled on one or both of the devices.

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