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24 Jan 2011


      Robert Ash
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On 24 Jan 2011, at 16:25, macruby-devel-request@lists.macosforge.org wrote:
...
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Today's Topics:
1. Re: Thread safety in apply example? (Matt Massicotte)
  2. Re: Thread safety in apply example? (Alan Skipp)
  3. Re: Thread safety in apply example? (Matt Massicotte)
----------------------------------------------------------------------
Message: 1
Date: Mon, 24 Jan 2011 07:22:44 -0800
From: Matt Massicotte <massicotte@apple.com>
To: "MacRuby development discussions."
   <macruby-devel@lists.macosforge.org>
Subject: Re: [MacRuby-devel] Thread safety in apply example?
Message-ID: <48DBC4A8-59D1-42F0-8DD3-2291668D64D2@apple.com>
Content-Type: text/plain; charset="us-ascii"
What's the advantage of this over just iterating over the array serially using each?
Matt
On Jan 24, 2011, at 3:40 AM, Alan Skipp wrote:
...
Warning, horrible hack alert!
I'm sure there must be a far better implementation, but the idea is to have an array subclass which diverts all method calls through a serial dispatch queue, which should ensure thread safe access.
Anyway, here's the horrible hack. By the way, it manages to break Enumerable, other than that it's perfect!
class ThreadSafeArray < Array  
instance_methods.each do |meth|
  eval "
  def #{meth}(*args, &block)
    val = []
    @serial_queue.async(@group) do      
      val[0] = super
    end
    @group.wait
    val[0]
  end"
end
def initialize(size=0, obj=nil)
  @serial_queue = Dispatch::Queue.new("array.serial_queue")
  @group = Dispatch::Group.new
  super
end
end
gcdq = Dispatch::Queue.new('doc')
@result = ThreadSafeArray.new
gcdq.apply(5) {|i| @result[i] = i*i }
p @result  #=> [0, 1, 4, 9, 16, 25]
Having to create an array for every method call to copy the return value into is terrible, is there a better way of getting a reference to the return value from a dispatch queue?
On 22 Jan 2011, at 14:52, Ernest N. Prabhakar, Ph.D. wrote:
...
Hi Charles,
On Jan 21, 2011, at 9:57 PM, Charles Oliver Nutter wrote:
...
I'm curious about this example in Queue#apply's rdoc:
*     gcdq = Dispatch::Queue.new('doc')
*     @result = []
*     gcdq.apply(5) {|i| @result[i] = i*i }
*     p @result  #=> [0, 1, 4, 9, 16, 25]
apply is said to issue the jobs in parallel, so this would be making
concurrent updates to the @result array. Are simple arrays in MacRuby
thread-safe?
My assumption was that doing a parallel assign:
result[i] = i*i
would be safe, since it always accessed a unique portion of memory, but doing a serial append:
result << i*i
would not.  But that may have been a mistake on my part, since the size (at least) needs to be updated. Anyone know better?
-- Ernie P.
...
- Charlie
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Robert Ash

Alan Skipp

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