Re: [boost] [future] Early draft of wait for multiple futures interface

From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2008-05-30 16:24:03

Frank Mori Hess <frank.hess_at_[hidden]> writes:

> On Friday 30 May 2008 11:45 am, Anthony Williams wrote:
>> >
>> > One problem is wait_for_any is not sufficient to implement an efficient
>> > scheduler.  You have to copy all the futures into wait_for_any so each
>> > wait_for_any call is O(N).  So something like the future_selecter (should
>> > be spelled future_selector?) class I mentioned earlier would still be
>> > needed.
>>
>> What do you mean by O(N) in this context?
>
> I mean there are N method requests in my scheduler, and I want to wait until 
> one of them is ready.
>
>> You have N futures to wait for. You have to somehow register that
>> you're waiting for each one => you need O(N) operations.
>
> Yes.  But the scheduler is going to repeatedly wait on the same N futures 
> (plus or minus one).  

Aha: it's the repeated waits that matter. For one wait it's O(N) in
all cases. If you wait again on (almost) the same set, until the set
is empty, it's an issue if you have to redo the set up, as that is
then O(N^2) in total.

That's a different use case to the one I imagined. I would call it a
"future dispatcher" and have it invoke a callback on each future as it
became ready.

Anthony
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