Re: [boost] Boost.Fiber mini-review September 4-13

Subject: Re: [boost] Boost.Fiber mini-review September 4-13
From: Peter Dimov (lists_at_[hidden])
Date: 2015-09-05 09:47:27

Agustín K-ballo Bergé wrote:

> Alas pthread specifies different semantics than the standard library, and 
> there you are actually expected to hold the lock if you want predictable 
> scheduling.

I don't think that's true. The ability to notify without holding the lock 
comes from pthreads. It's not a C++ invention, it's a pthread invention; 
you're most definitely not expected to hold the lock.

It's also more efficient because otherwise the awakened thread would 
immediately block on the mutex.

> I hear pthread won't actually wake up any threads then (which wouldn't be 
> able to make progress otherwise), but rather switch them from waiting on 
> the cv to waiting on the mutex to avoid useless context switches; when the 
> mutex is finally unlocked the thread will finally wake up.

Some pthreads implementations supposedly have this optimization, but not 
all; and if you notify without holding the mutex, the thread would proceed 
immediately.