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

[Eric Dumazet <eric.dumazet@gmail.com>]

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