From: MenTaLguY <mental@...>
Date: 2008-02-21T07:44:46+09:00
Subject: Re: Ruby 1.9, threads and FreeBSD 5

On Thu, 21 Feb 2008 06:51:18 +0900, Eric Jacoboni <eric.jacoboni@gmail.com> wrote:
> I've had wrote a general semaphore implementation using IO.pipe

That can still be useful sometimes -- for example, I wrote a
concurrent-selectable gem which provides latch, semaphore, and
channel (queue) implementations which can be passed as arguments to
IO.select, libev, etc. because they use IO.pipe underneath.

> PortableSemaphore is way more elegant... thanks a lot (i just wonder
> if #up/#down honor the FIFO policy)

It depends upon the implementation of ConditionVariable#wait.  Some
Ruby implementations will wake threads in the order the threads called
#wait, and some will not.  Most are roughly FIFO but not 100% "fair".

Fairness actually involves a tradeoff:  while unfair blocking
primitives can sometimes lead to starvation (as sufficiently
greedy threads could keep "jumping the queue"), fair primitives
are more likely to have problems with convoying[1].

-mental

[1] Google "lock convoying"