Re: [LARTC] Latency of Linux Bridge

[Carl-Daniel Hailfinger <c-d.hailfinger.devel.2005@gmx.net>]

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Christian Konecny (VI/SEA) schrieb:
> 
> Carl-Daniel wrote:
>> That's strange. On my bridge with 4 network interfaces,
>> the additional latency is always below 0.5 ms, even if
>> I'm pushing 400 MBit/s through the machine and a kernel
>> compile is running at 100% CPU. Network interfaces are
>> PCIe GBit from Syskonnect, the machine is an Athlon64
>> at 2 GHz. Even if the clock speed is halved by powersave
>> the additional latency will not go above 0.9 ms.
>> Kernel is vanilla 2.6.11.x.
> 
> Did you use certain specific compile options?

Depends. I chose to use a slightly modified .config
from SUSE 9.3 (attached).

> I have changed now from (Knoppix) Debian to (Slax)
> Slackware running now 2.6.12.2 and have exactly the
> same on each machine.
> top shows me a CPU usage less than 0.5%, load average
> 0.1 while the bridge is handling roughly 64kBit/sec.
> I can still measure a variable delta between packets
> of 5ms-10ms.
> How did you measure your latency values?

(with bridge in between)
linux:~ # ping -f -c 1000 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.

--- 192.168.0.1 ping statistics ---
1000 packets transmitted, 1000 received, 0% packet loss, time 963ms
rtt min/avg/max/mdev = 0.642/0.914/2.487/0.277 ms, ipg/ewma 0.964/1.023 ms

(same config without bridge in between)
linux:~ # ping -f -c 1000 192.168.0.1
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.

--- 192.168.0.1 ping statistics ---
1000 packets transmitted, 1000 received, 0% packet loss, time 883ms
rtt min/avg/max/mdev = 0.767/0.834/4.925/0.173 ms, ipg/ewma 0.884/0.814 ms


> I am using 3 PCs, one is the Linux-Box acting as bridge
> (brctl addbr br0; brctl addif br0 eth0; brctl addif br0
> eth1), the other 2 PCs are connected to either side of
> the bridge, running ethereal.

I'm using 2 PCs (one as bridge, one as local client) and a Cisco Pix
(slow, insecure, unstable) as remote router (192.168.0.1). The
measaurements were done from the local client.

> In my case I have 2 Phone systems generating continous traffic. Each
> system is sending out packets every 30ms.
> If I compare the 2 traces I can see that after the bridge the timings
> are different then before the bridge.

It is very possible that a bridge changes the timing distribtion
(but the drastic effect you're seeing shouldn't happen).

> The difference is always in steps of 5ms. 
> so, on sending side is always 30ms difference between each packet
> on receiving side - after the bridge - the delta is then 25,30,35, or 40ms.
> 
> Is this really different in your setup?

Since the remote side is not a linux box, my chances to measure the
timing distribution are somewhat limited. Please try the flood ping
I did above (ping -f -c 1000 re.mo.te.ip) and report your results.
If you can see latencies above 2 ms something is definitely going
wrong. Could be the nic, the nic settings (NAPI), the timing source
or packet type (ICMP vs. IP).

1. Check ping
2a. If ping looks wrong -> try my .config and check again
2b. If ping looks OK -> try the phone while running (ping -f re.mo.te.ip)
3. Report back.


Grüße aus Tübingen
Carl-Daniel
-- 
http://www.hailfinger.org/

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