Subject: Re: [boost] [afio] Formal review of Boost.AFIO
From: AgustÃn K-ballo BergÃ© (kaballo86_at_[hidden])
Date: 2015-08-31 15:21:18
On 8/31/2015 4:17 PM, Thomas Heller wrote:
> On 08/31/2015 06:53 PM, Niall Douglas wrote:
>> On 31 Aug 2015 at 16:50, Thomas Heller wrote:
>> None of those get wait() called on them, and therefore you very
>> rarely block. As an example, if you executed 1000 promise + future +
>> continuations but blocked on just one of those, my condition
>> "future.wait() very rarely blocks in your use scenario" is fulfilled
>> by virtue that future.wait() is simply never called.
> Looks like a problem with my english, sorry, I am not a native speaker.
> Let's see if I got it correct now: "Your code rarely calls
> future<T>::wait to avoid blocking because that is usually costly".
> From my experience that holds true for any future implementation (be it
> based on OS synchronization primitives or user level context based
> synchronization), and is probably the number one reason why one would
> want something like future<T>::then, when_all and friends.
> So I take it that this requirement is nothing special to your
> lightweight futures.
My interpretation (which could of course be wrong) is that the
"lightweight future" implementation of `wait` does busy-waiting because
there is nowhere to store a `condition_variable`, so a spinlock is used
instead. There is no technical reason for that to be the case.
A spinlock is still a perfectly adequate choice for a future
implementation, given that there's very little concurrent access to the
shared state. That doesn't rule out yielding the thread/cpu while
waiting. An implementation could use a thread-local `condition_variable`
and have the future point to it while it waits. This is not necessarily
a good approach, but it is simple and should do as a proof of concept.
Again, I could just be misinterpreting things.
-- Agustín K-ballo Bergé.- http://talesofcpp.fusionfenix.com
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