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

[dave taht <dave.taht@gmail.com>]

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.

>
>
>
>
>