Re: [Bloat] Kirkwood BQL?

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

On 29/07/15 18:42, Dave Taht wrote:
> On Wed, Jul 29, 2015 at 7:07 PM, David Lang <david@lang.hm> wrote:
>> On Wed, 29 Jul 2015, Alan Jenkins wrote:
>>
>>> On 29/07/15 12:24, Alan Jenkins wrote:
>>>> 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 adjusts the number of packets that can be queued based on their size, so
>> you can have far more 64 byte packets queued than you can have 1500 byte
>> packets.
>>
>> do a ping flood of your network with different packet sizes and look at the
>> queue lengths that are allowed, the queue length should be much higher with
>> small packets.
>>
>>>> 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.
>>>>
>>> a quick test with a 100M setting, connected to gigabit switch, and flent
>>> tcp_download, shows ping under load increases to about 8ms. Conclusion: the
>>> Debian kirkwood kernel probably isn't doing BQL for me :).
> Wrong way I think. Try tcp_upload.

"flent tcp_download" running on the connected x86 laptop.  So I didn't 
have to use Flent on the Kirkwood device, only netperf's netserver.