Subject: Re: [boost] [thread] Timed waits in Boost.Thread potentially fundamentally broken on Windows (possibly rest of Boost too)
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-01-25 05:21:12
Le 24/01/15 15:18, Niall Douglas a écrit :
> On 23 Jan 2015 at 18:30, Andrey Semashev wrote:
>
>>> Vista made these changes to scheduling for efficiency purposes. I
>>> suspect Boost.Thread was written for an XP or earlier target.
>> I'm confused. Boost.Thread always implemented the standard behavior, with
>> possibility of spurious wakeups. Windows before Vista did not exhibit spurious
>> wakeups (which was ok) and since Vista it started doing this, and this
>> improved efficiency (I assume, the estimate of the improvement had included
>> the negative effect on the applications dealing with the wakeups).
>> Boost.Thread is still behaving correctly wrt the standard. So, why would you
>> want to change Boost.Thread and conceal spurious wakeups, making it less
>> efficient? I'll reiterate that any current use of cv must deal with spurious
>> wakeups already.
> Firstly, my thanks to both you and Peter for your thoughts on this.
>
> After sleeping on it for a night, this is what I'll do: I'm going to
> update Thread's clamping of timeouts to better match Windows since
> Vista onwards. This basically means asking Windows what its current
> scheduling quantum is (it can vary from moment to moment according to
> what applications request) and clamping the timeout sent to Windows
> APIs to that quantum. Therefore, if you asked for a 10ms timeout,
> Thread will clamp that probably to 15 ms before sending it onto
> Windows. I'll always round upwards, so there is a greater potential
> for a timed wait to take one quantum longer than it should before
> timing out, but then that can happen anyway at any time.
>
> None of this will remove spurious wakeups, but it will reduce their
> frequency considerably. It means that the CPU spends more time
> sleeping and less being woken up and being put back to sleep by a
> predicate check loop.
>
> If anyone has a problem with this solution, now is the time to speak.
>
>
Niall, could we first fix the incoherences you have identified
respecting the standard specification and then see if we need to do
something else.
Best,
Vicente