From: nobu.nokada@...
Date: 2003-10-09T14:42:16+09:00
Subject: Re: Standard Queue Implementation and Thread Safety

Hi,

At Thu, 9 Oct 2003 14:03:13 +0900,
Joel VanderWerf wrote:
> >>The implementation is essentially backed by an Array.  Queue provides
> >>synchronized push and pop methods (using Thread.critical), but methods
> >>such as clear, length, and empty? are not synchronized.  What happens
> >>if one thread is in the middle of clearing the queue and then another
> >>comes along to add/remove something from the queue?  Why don't these
> >>other methods need synchronization?  Is Array thread-safe by default?
> > 
> > 
> > Yes.  Builtin methods (more accurately, not written in Ruby)
> > are thread-safe, unless GC runs.  Since Array#empty? and
> > Array#length don't modify anything, and Array#clear just
> > releases but requires no memory, GC doesn't get run.
> 
> So Array#push is not thread-safe, but Queue#push is, right?

Array#push is thread-safe too.

> I understand that Array#push could provoke a GC, but why would GC cause 
> a context-switch?

Though, finalizers could interrupt.  Since they run in same
thread and critical section so context won't switch, but they
may try to modify the object.

$ ulimit -v 10240
$ ruby
  array = Array.new(100, nil)
  while array.size > 10
    ObjectSpace.define_finalizer(Array.new(100000, nil)) {array.clear}
    p array.push(nil).size
  end
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
1

-- 
Nobu Nakada