As the title suggests, what is the default value of the maxConcurrentOperationCount for NSOperationQueue?
Is it set to a value of 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.
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
.
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