Objective-C: Calling and copying the same block from multiple threads

Question

I'm dealing with neural networks here, but it's safe to ignore that, as the real question has to deal with blocks in objective-c. Here is my issue. I found a way to convert a neural network into a big block that can be executed all at once. However, it goes really, really slow, relative to activating the network. This seems a bit counterintuitive.

If I gave you a group of nested functions like

CGFloat answer = sin(cos(gaussian(1.5*x + 2.5*y)) + (.3*d + bias))
//or in block notation
^(CGFloat x, CGFloat y, CGFloat d, CGFloat bias) {
 return sin(cos(gaussian(1.5*x + 2.5*y)) + (.3*d + bias));
};

In theory, running that function multiple times should be easier/quicker than looping through a bunch of connections, and setting nodes active/inactive, etc, all of which essentially calculate this same function in the end.

However, when I create a block (see thread: how to create function at runtime) and run this code, it is slow as all hell for any moderately sized network.

Now, what I don't quite understand is:

1. When you copy a block, what exactly are you copying?
2. Let's say, I copy a block twice, copy1 and copy2. If I call copy1 and copy2 on the same thread, is the same function called? I don't understand exactly what the docs mean for block copies: Apple Block Docs
3. Now if I make that copy again, copy1 and copy2, but instead, I call the copies on separate threads, now how do the functions behave? Will this cause some sort of slowdown, as each thread attempts to access the same block?

Answer 1

When you copy a block, what exactly are you copying?

You are copying any state the block has captured. If that block captures no state -- which that block appears not to -- then the copy should be "free" in that the block will be a constant (similar to how @"" works).

Let's say, I copy a block twice, copy1 and copy2. If I call copy1 and copy2 on the same thread, is the same function called? I don't understand exactly what the docs mean for block copies: Apple Block Docs

When a block is copied, the code of the block is never copied. Only the captured state. So, yes, you'll be executing the exact same set of instructions.

Now if I make that copy again, copy1 and copy2, but instead, I call the copies on separate threads, now how do the functions behave? Will this cause some sort of slowdown, as each thread attempts to access the same block?

The data captured within a block is not protected from multi-threaded access in any way so, no, there would be no slowdown (but there will be all the concurrency synchronization fun you might imagine).

Have you tried sampling the app to see what is consuming the CPU cycles? Also, given where you are going with this, you might want to become acquainted with your friendly local disassembler (otool -TtVv binary/or/.o/file) as it can be quite helpful in determining how costly a block copy really is.

---

If you are sampling and seeing lots of time in the block itself, then that is just your computation consuming lots of CPU time. If the block were to consume CPU during the copy, you would see the consumption in a copy helper.

Try creating a source file that contains a bunch of different kinds of blocks; with parameters, without, with captured state, without, with captured blocks with/without captured state, etc.. and a function that calls Block_copy() on each.

Disassemble that and you'll gain a deep understanding on what happens when blocks are copied. Personally, I find x86_64 assembly to be easier to read than ARM. (This all sounds like good blog fodder -- I should write it up).