From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 12 Oct 2010 10:10:51 -0700 From: Stephen Hemminger Message-ID: <20101012101051.43dfd499@nehalam> In-Reply-To: References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Subject: Re: [Bridge] Real time issues when using a bridge List-Id: Linux Ethernet Bridging List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jean-Michel Hautbois Cc: bridge@lists.linux-foundation.org On Tue, 12 Oct 2010 16:31:36 +0200 Jean-Michel Hautbois wrote: > Hi there ! >=20 > I am facing a jitter issue that I will try to explain : > I have the following configuration : >=20 > (A) eth0 <--> br0 <--> eth1 (B) >=20 > 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. >=20 > I added ftrace option, and after some (long) investigations, I am current= ly > limiting my study to the bridge part. >=20 > When I trace only br_handle_frame and br_forward_finish and do my test on > 6500 frames I have : > - average latency of 39=B5s > - minimum latency of 34 =B5s > - maximum latency of 81=B5s >=20 > Thus, my jitter is 47=B5s. > 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 jitte= r ? > If you have any idea, everything is interesting for me. > I thought maybe to skb_clone ? >=20 > Best Regards, > JM You probably have NAPI and/or interrupt coalescing setting enabled on one or both of the devices. --=20