O(1) sum

[Stefan Koch via Digitalmars-d-learn]

On Monday, 12 June 2017 at 06:15:07 UTC, Biotronic wrote:
> On Monday, 12 June 2017 at 01:36:04 UTC, Stefan Koch wrote:
>> On Monday, 12 June 2017 at 01:02:58 UTC, helxi wrote:
>>> Is it possible to sum an array in O(1)?
>>
>> No.
>> If you want to sum the elements you have to at-least look at 
>> all the elements.
>> So it'll always be O(N).
>> it's the best you can do.
>
> On a multi-core system we can do better:
>
> auto nums = iota(10_000_000.0f);
> auto sum = taskPool.reduce!"a + b"(nums);
>
> Given arbitrary parallelism (yeah, right!), this will be 
> O(log(N)). For real-world systems, it might give a speed-up for 
> large arrays, but won't reduce the big-O complexity. Of course, 
> there will also be overhead to such a solution, so there is a 
> limit to how much one'd actually benefit from it.
>
> --
>   Biotronic

Biotronic how do you arrive at  O(log(N)) ??
And which logarithm  ?