From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-09-20 07:18:35
Anthony Williams wrote:
> The case given in Alexander's email above is:
>
> A: lock [lockcount == 0]
> B: lock [lockcount == 1, slow path]
> A: unlock [lockcount == 0, slow path/post sema]
> A: lock - [lockcount == 1, slow path]
>
> He claims that the second A: lock takes the slow path, when it could
> have taken the fast path because the lock is free. I disagree. The
> lock is NOT free, since B is waiting on it. If A can take the fast
> path here, that's opening the door for B to be starved.
This is correct. The mutex does not guarantee fairness (for the above
definition of fairness) at the expense of performance. It is indeed true
that in heavily contended scenarios thread A can finish well ahead of thread
B; however, the total time taken by threads A+B can be (and is)
significantly less than that with the "fairer" CS design.
As far as I know, most contemporary mutexes do the same, they give priority
to thread A not only because it can take the fast path, but also because (a)
its cache is hot and (b) it's better to minimize context switches.