On Mon, 10 Aug 2015, Jeff Weeks wrote:
The CoDel control_law is defined as 't + interval/sqrt(count)'
The sample implementation (http://queue.acm.org/appendices/codel.html) suggests
that sqrt(count) can be calculated using only integer multiplication, but I'm
wondering if it even needs to be calculated.
Would it not be more efficient to pre-calculate a set of 1/sqrt(count) for some
range -- probably (1..interval), and for any count > interval, the next
interval can just be set to t+1.
Is there any reason to not use this approach (assuming sufficient memory
exists, of course)?
I think this should be left up to individual implementation. I have some
experience from assembler programming on the C64 and Amiga in the 80ties,
and back then it was common to have these kinds of lookup tables because
memory accesses were "cheap" and calculations were "expensive". I don't
know if this is still the case in modern hardware, it might even be that
it's cheaper to do sqrt on an internal register compared to doing memory
access to relatively slow DRAM (if the lookup table isn't kept in cache)
to fetch some data from a precalculated lookup table.
--
Mikael Abrahamsson email: [email protected]
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