Default value of maxConcurrentOperationCount for NSOperationQueue

Question

As the title suggests, what is the default value of the maxConcurrentOperationCount for NSOperationQueue?

Is it set to a value of 1?

Answer 1

From the documentation,

The maximum number of concurrent operations set explicitly on the receiver using the setMaxConcurrentOperationCount: method. If no value has been explicitly set, this method returns NSOperationQueueDefaultMaxConcurrentOperationCount by default.

So it is NSOperationQueueDefaultMaxConcurrentOperationCount. If this is set, it will choose an appropriate value based on the number of available processors and other relevant factors.

This is how it is defined:

enum {
  NSOperationQueueDefaultMaxConcurrentOperationCount = -1
};

NSOperationQueueDefaultMaxConcurrentOperationCount: The default maximum number of operations is determined dynamically by the NSOperationQueue object based on current system conditions.

Answer 2

In one of my apps, I add about 35k instances of NSOperation to an NSOperationQueue at once. If I set maxConcurrentOperationCount to 64, I gain about 20x of performance compared to the default value. The CPU load rises from ~120% to 400% which seems to indicate that the default value is so high that the CPU is mostly busy working on the op queue overhead.

Conclusion: if you have many short lived NSOperation it might be worth to play with the maxConcurrentOperationCount.