Thin lock vs. Futex

[Sean Kelly]

Jb wrote:
> "Bartosz Milewski" <bartosz at relisoft.com> wrote in message 
> news:g9jvui$nbs$1 at digitalmars.com...
>> As promised, I posted a blog comparing the thin lock to futex. Please vote 
>> it on reddit: http://www.reddit.com/comments/6z4sv/ .
> 
> Early on you say this...
> 
> "Unlock tests if the futex variable is equal to one (we are the owner, and 
> nobody is waiting). If true, it sets it to zero. This is the fast-track 
> common-case execution that doesn't make any futex calls whatsoever."
> 
> But the futex code you list later in the blog does this....
> 
> // we own the lock, so it's either 1 or 2
>       if (atomic_decrement(&_word) != 1)
> 
> So does the futex actualy need/use an atomic for unlocking?

That implementation does.  However, I think it could be replaced by an 
unordered atomic load followed by a set with "release" semantics on most 
modern architectures.  On (non-AMD) x86, this should mean no explicit 
synchronization for the unlock() routine.

> If it does then that makes it a great deal slower than a thin lock as that 
> typicaly doesnt require an atomic for the unlock. As im sure you're aware, 
> CAS and such like cost around 120..160 cycles on most cpus, so having an 
> algorythm that cuts their usage in half is a big avantage.

Definitely.


Sean