Multiprocessing taking too much time

[John Nagle]

On 7/29/2010 11:08 AM, Shailendra wrote:
> Hi All,
> I have a following situation.
> ==================PSUDO CODE START==================
> class holds_big_array:
>      big_array  #has a big array
>
>      def get_some_element(self, cond) # return some data from the array
> from the big array
> ==================PSUDO CODE END====================
> I wanted to use multiprocessing module to parallelise calling
> "get_some_element". I used following kind of code
>
> ==================PSUDO CODE START==================
> pool = Pool(processes=2)
> holder =holds_big_array() #class instantiation
> def callback_f(result):
>           do something with result
> loop many times
>     pool.apply_async(holder.get_some_element,args,callback=callback_f)
> pool.close()
> pool.join()
> ==================PSUDO CODE END====================
> Note: Had to do something to enable instance method being pickled...
>
> I tested this with less than realistic size of big_array . My parallel
> version works much slower than than the normal serial version (10-20
> sec vs 7-8 min). I was wonder what could be the possible reason.

     It's hard to tell from your "PSUDO CODE", but it looks like each
access to the "big array" involves calling another process.

     Calling a function in another process is done by creating an
object to contain the request, running it through "pickle" to convert
it to a stream of bytes, sending the stream of bytes through a socket or
pipe to the other process, running the byte stream through "unpickle" to
create an object like the original one, but in a different process, and
calling a function on the newly created object in the receiving process. 
  This entire sequence has to be done again in reverse
to get a reply back.

     This is hundreds of times slower than a call to a local function.

     The "multiprocessing module" is not a replacement for thread-level
parallelism.  It looks like it is, but it isn't.  It's only useful for
big tasks which require large amounts of computation and little
interprocess communication.  Appropriately-sized tasks to send out
to another process are things like "parse large web page" or
"compress video file", not "access element of array".

				John Nagle