NSOperationQueue dispatching threads slowly?!

Question

I'm writing my first multithreaded iPhone apps using NSOperationQueue because I've heard it's loads better and potentially faster than managing my own thread dispatching.

I'm calculating the outcome of a Game of Life board by splitting the board into seperate pieces and having seperate threads calculate each board and then splicing them back together, to me this seems like a faster way even with the tremendous overhead of splitting and splicing. I'm creating a NSInvocationOperation object for each board and then sending them to the OperationQueue. After I've sent all the pieces of the board I sit and wait for them all to finish calculating with the waitUntilAllOperationsAreFinished call to the OperationQueue.

This seems like it should work, and it does it works just fine BUT the threads get called out very slooooowwwlllyyyyyy and so it actually ends up taking longer for the multithreaded version to calculate than the single threaded version! OH NOES! I monitored the creation and termination of the NSOperations sent to the NSOperationQueue and found that some just sit in the Operation Queue do-diddly-daddlin for awhile before they get called much later on. At first I thought "Hey maybe the queue can only process so many threads at a time" and then bumped the Queues maxConcurrentOperationCount up to some arbitrary high number (well above the amount of board-pieces) but I experienced the same thing!

I was wondering if maybe someone can tell me how to kick NSOperationQueue into "overdrive" so to say so that it dispatches its queues as quickly as possible, either that or tell me whats going on!

Answer 1

The queue is still fundamentally limited by the processing power of the phone. If the phone can only run two processes simultaneously, you will get (at most) close to a two-fold increase in speed by splitting up the task. Anything more than that, and you are just adding overhead for no gain.

This is especially true if you are running a processor and memory intensive routine like your board calculation. NSOperationQueue makes sense if you have several operations that need to wait for extended periods of time. User interface loops and network downloads would be excellent examples. In those cases, other operations can complete while the inactive ones are waiting for input.

For something like your board, the operation for each piece of the grid never has any wait condition. It is always churning away at full speed until done.

See also: iPhone Maximum thread limit? and concurrency application design

Answer 2

Threads do not magically make your processor run faster.

On a single-processor machine, if your algorithm takes a million instructions to execute, splitting it up into 10 chunks of 100,000 instructions each and running it on 10 threads is still going to take just as long. Actually, it will take longer, because you've added the overhead of splitting, merging, and context switching among the threads.