Re: [Bloat] Kirkwood BQL?

[Alan Jenkins <alan.christopher.jenkins@gmail.com>]

On 29/07/15 05:32, Rosen Penev wrote:
> Anyone know what the situation is with kirkwood and BQL? I found a
> patch for it but have no idea if there are any issues.
>
> I have such a system but have no idea how to ascertain the efficacy of BQL.

To the latter:

BQL works for transmissions that reach the full line rate (e.g. for 
1000MB ethernet).  It limits the queue that builds in the driver/device 
to the minimum they need.  Then queue mostly builds in the generic 
networking stack, where it can be managed effectively e.g. by fq_codel.

So a simple efficacy test is to run a transmission at full speed, and 
monitor latency (ping) at the same time.  Just make sure the device 
qdisc is set to fq_codel.  fq_codel effectively prioritizes ping, so the 
difference will be very easy to see.

I don't know if there's any corner cases that want testing as well.

BQL can be disabled at runtime for comparison testing:
http://lists.openwall.net/netdev/2011/12/01/112

There's a BQL tool to see it working graphically (using readouts from 
the same sysfs directory):
https://github.com/ffainelli/bqlmon

My Kirkwood setup at home is weak, I basically never reach full link 
speed.  So this might be somewhat academic unless you set the link speed 
to 100 or 10 using the ethtool command.  (It seems like a good idea to 
test those speeds even if you can do better though).  You probably also 
want to start with offloads (tso, gso, gro) disabled using ethtool, 
because they aggregate packets.

Flent can do this test and generate pretty graphs, including a time 
series (plot type "all_scaled") and frequency distribution for the ping 
("ping_cdf").  Flent is a frontend to the netperf network performance 
tester.  You could use a directly connected laptop and run your own 
netperf server (netserver command).  You'll need to set up static IPs on 
both ends for the duration... if headless then make sure you have an 
alternative console access :).

The normal Flent test is RRUL, which is two-way.  tcp_2up would be 
better, to avoid testing both end's BQL at the same time.  If you want 
to run tcp_2up the other way round, so you only need netserver on the 
ARM, try using '--swap-up-down'.

Alan