[Bridge] Real time issues when using a bridge

Jean-Michel Hautbois jhautbois at gmail.com
Tue Oct 12 07:31:36 PDT 2010


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
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