[Codel] [PATCH v5] pkt_sched: codel: Controlled Delay AQM

[dave taht]

On 05/05/2012 03:48 PM, dave taht wrote:
> On 05/05/2012 03:39 PM, Eric Dumazet wrote:
>> On Sat, 2012-05-05 at 15:34 -0700, dave taht wrote:
>>> On 05/05/2012 03:09 PM, Eric Dumazet wrote:
>>>> On Sat, 2012-05-05 at 15:03 -0700, dave taht wrote:
>>>>
>>>>> Maybe on your arch, but highly doubtful on a 680Mhz mips that 
>>>>> isn't even
>>>>> superscalar.
>>>>>
>>>> CPU are fast, memory is slow.
>>>>
>>>>> I'd prefer to leave it in and be able to compile it out, and actually
>>>>> measure the difference.
>>>> You optimize the case where there is no need to optimize (small queue)
>>>>
>>>> I can see count bigger than 100000 with 20 concurrent netperf
>>>>
>>>> This makes no sense to have a cache so big.
>>>>
>>>> Or there is a bug in codel
>>> The original reciprocol approximation test code rapidly goes AWOL after
>>> exceeding 2^8.
>>>
>>> I went looking for butterflies and didn't see any in the scaled code in
>>> the range 0-100000,
>>> and they would only take flight briefly, so...
>>>
>>> However I have not corrected it for BITS_PER_LONG as per our 4AM
>>> discussion.
>> You should use the exact code in kernel. (using BITS_PER_LONG)
>>
>>> ...
>>>
>>> interval/sqrt(99999)=316229 approx :6250190 19.76475908 
>>> interval/scaled:
>>> 316236 1.00002214
>>>
>>>>
>> If you read the code , there is no possible overflow, even with very
>> large 'u32 count'
>>
>> anyway the problem is q->count keeps increasing under load.
>>
>> Only when load is stopped for a while, count is reset to 1
>>
> Stalking butterflies. ( 
> http://en.wikipedia.org/wiki/File:Lorenz_attractor_yb.svg )
>
> I suspected also we would have issues as we hit some natural quantums 
> (clock rate/interrupt rate/bql estimator etc) but for all I know it's 
> just a plain bug. I need a reboot. Goin to dinner.
>

If we just compare us to us rather than ns to ns you get chunkier drops, 
by a lot...

interval/sqrt(370)=5198752 approx :6250190 1.20224815 interval/scaled: 
5198761 1.00000173
interval/sqrt(371)=5191741 approx :6250190 1.20387169 interval/scaled: 
5191776 1.00000674


secondly adding a fudge factor to the calculation would bring it closer 
to inline with an actual sqrt.


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