Threadpool item mailboxes design problem

[Charles Hixson]

On 04/15/2013 10:14 AM, Charles Hixson wrote:
> On 04/14/2013 07:32 PM, Chris Rebert wrote:
>>
>> On Apr 14, 2013 4:27 PM, "Charles Hixson" <charleshixsn at earthlink.net 
>> <mailto:charleshixsn at earthlink.net>> wrote:
>> >
>> > What is the best approach to implementing actors that accept and 
>> post messages (and have no other external contacts).
>>
>> You might look at how some of the existing Python actor libraries are 
>> implemented (perhaps one of these might even save you from 
>> reinventing the wheel):
>>
>> http://www.pykka.org/en/latest/
>> http://www.kamaelia.org/Docs/Axon/Axon.html
>> https://pypi.python.org/pypi/pulsar
>>
>> Kinda old:
>> http://candygram.sourceforge.net/contents.html
>> http://osl.cs.uiuc.edu/parley/
>>
> Candygram looks interesting.  I'd forgotten about it.  The others look 
> either a bit limited (in different ways), or overly general, with the 
> costs that that brings.  I'll need to study Candygram a bit more.  
> However, even Candygram seems to have a RAM centric model that I'd 
> need to work around.  (Well, the mailbox synchronization must clearly 
> be RAM centric, but the storage shouldn't be.)
>>
>> > So far what I've come up with is something like:
>> > actors     = {}
>> > mailboxs = {}
>> >
>> > Stuff actors with actor instances, mailboxes with 
>> multiprocessing.queue instances.   (Actors and mailboxes will have 
>> identical keys, which are id#, but it's got to be a dict rather than 
>> a list, because too many are rolled out to disk.)  And I'm planning 
>> of having the actors running simultaneously and continually in a 
>> threadpool that just loops through the actors that are assigned to 
>> each thread of the pool.
>> <snip>
>> > It would, however, be better if the mailbox could be specific to 
>> the threadpool instance, so less space would be wasted.  Or if the 
>> queues could dynamically resize.  Or if there was a threadsafe dict. 
>>  Or...  But I don't know that any of these are feasible.  (I mean, 
>> yes, I could write all the mail to a database, but is that a better 
>> answer, or even a good one?)
>>
>> My recollection is that the built-in collection types are threadsafe 
>> at least to the limited extent that the operations exposed by their 
>> APIs (e.g. dict.setdefault) are atomic.
>> Perhaps someone will be able to chime in with more details.
>>
> If list operations were threadsafe, why would multiprocessing.queue 
> have been created?  I don't recall any claim that they were.  Still, 
> I've found an assertion on StackOverflow that they are...at least for 
> simple assignment and reading.  And the same would appear to be true 
> of dicts from that post.  This *does* require that either the index be 
> constant, and the stored value be constant, or that the code section 
> be locked during the access.  Fortunately I'm intending to have id#s 
> be unchangable, and the messages to be tuples, and thus constant.
>
> OTOH, to use this approach I'll need to find some way to guarantee 
> that removing messages and posting messages don't occur at the same 
> time.  So that still means I'll need to lock each access.  The answer 
> that seems best is for each thread to have a mailbox that cells within 
> the thread post and read messages from.  This will automatically deal 
> with internal to thread synchronization.  Then I'll need a mailman 
> thread that...
>
> This seems a promising approach, that avoids the problem of fixed 
> length queues, but I'll still need to do a lot of synchronization.  
> Still, it's a lot less, and each thread would be locked for shorter 
> amounts of time.
> -- 
> Charles Hixson
>
Currently it looks as if Pyro is the best option.  It appears that 
Python threads are very contentious, so I'll need to run in processes 
rather than in threads, but I'll still need to transfer messages back 
and forth.  I'll probably use UnixSockets rather than IP, but this could 
change...and using Pyro would make changing it easy.  Still, pickle is 
used in code transmission, and that makes IP a questionable choice.

-- 
Charles Hixson

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