The post was admittedly vague, but was more of an idea about a general purpose
thread safe array class (in principle, this could apply to any mutable class).
Perhaps implemented as a module that would redirect all method calls through a
serial dispatch queue. This would mean that the array (or other mutable
collection) could be mutated from multiple threads/queues without fear of bad
things happening.
On 24 Jan 2011, at 15:22, Matt Massicotte wrote:
> 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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>>>
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