iOS Threads Wait for Action

Question

I have a processing thread that I use to fill a data buffer. Elsewhere a piece of hardware triggers a callback which reads from this data buffer. The processing thread then kicks in and refills the buffer.

When the buffer fills up I am currently telling the thread to wait by:

while( [self FreeWriteSpace] < mProcessBufferSize && InActive) {
    [NSThread sleepForTimeInterval:.0001];
}

However when I profile I am getting a lot of CPU time spent in sleep. Is there a better way to wait? Do I even care if the profiles says time is spent in sleep?

Answer 1

Time spent in sleep is effectively free. In Instruments, look at "running samples" rather than "all samples." But this still isn't an ideal solution.

First, your sleep interval is crazy. Do you really need .1µs granularity? The system almost certainly isn't giving you because the processor isn't that fast. I have to believe you could up this to .1 or .01. But that's still busy-waiting which is not ideal if you can help it.

The better solution is to use an NSCondition. In this thread, wait on the condition, and in your processing thread, trigger the condition when there's room to write.

Do be careful with your naming. Do not name methods with leading caps (that indicates that it's a class name). And avoid accessing ivars directly (InActive) like this. "InActive" is also a very confusing name. Does it mean the system is active (In Active) or not active (inactive). Naming in Objective-C is extremely important. The compiler will not protect you the way it does in C# and C++. Good naming is how you keep your programs working, and many parts of ObjC rely on it.

You may also want to investigate Grand Central Dispatch, which is particularly designed for these kinds of problems. Look at dispatch_async() to run things when new data comes in.

Answer 2

However when I profile I am getting a lot of CPU time spent in sleep. Is there a better way to wait? Do I even care if the profiles says time is spent in sleep?

Yes -- never, never poll. Polling eats CPU, makes your app less responsive, eats battery, and is an all around waste.

Notify instead.

The easiest way is to use one of the variants of "perform selector on main thread" (see NSThread's documentation). Or dispatch to a queue (including something like dispatch_async(dispatch_get_main_queue(), ^{ ... yo, data be ready ...});).