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I have a question about custom DispatchQueue.

I created a queue and I use it as a queue for captureOutput: method. Here's a code snippet:

//At the file header
private let videoQueue = DispatchQueue(label: "videoQueue")

//setting queue to AVCaptureVideoDataOutput
dataOutput.setSampleBufferDelegate(self, queue: videoQueue)

After I get the frame, I'm doing an expensive performance with it and I'm doing it for each frame.

When I launch the app, my expensive performance takes to 17 ms to compute and thankfully to that I have around 46-47 fps. So far so good.

But after some time (around 10-15 seconds), this expensive performance starts taking more and more time and in 1-2 minute I end up with 35-36 fps, and instead of 17 ms I have around 20-25 ms.

Unfortunately, I can't provide the code of expensive performance because there's a lot of it and at least XCode tells that I do not have any memory leaks.

I know that manually created DispatchQueue doesn't really work in its own because all tasks I put there eventually end up in iOS default thread pool (I'm talking about BACKGROUND, UTILITY, USER_INTERACTIVE, etc). And for me it looks like videoQueue looses a priority with some period of time.

If my guess is right - is there any way to influence that? The performance of my DispatchQueue is very crucial and I want to give it the highest priority all the time.

If I'm not right, I would very much appreciate if someone can give me a direction I should investigate. Thanks in advance!

Eugene Alexeev
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1 Answers1

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First, I would suspect other parts of your code, and probably some kind of memory accumulation. I would expect you're either reprocessing some portion of the same data over and over again, or you're doing a lot of memory allocation/deallocation (which can lead to memory fragmentation). Lots of memory issues don't show up as "leaks" (because they're not leaks). The way to explore this is with Instruments.

That said, you probably don't want to run this at the default QoS. You shouldn't think of QoS as "priority." It's more complicated than that (which is why it's called "quality-of-service," not "priority"). You should assign work to a queue whose QoS matches the how it impacts the user. In this case, it looks like you are updating the UI in real-time. That matches the .userInteractive QoS:

private let videoQueue = DispatchQueue(label: "videoQueue", qos: .userInteractive)

This may improve things, but I suspect other problems in your code that Instruments will help you reveal.

Rob Napier
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  • Hello and thank you for your response! I will definitely use your tip and will get back with the feedback – Eugene Alexeev Sep 03 '19 at 15:15
  • Hello again! I must say that your advice to set QoS to video queue was certainly useful. Once I did that things got better for sure. But I'm still experiencing this thread "bleeding" and I think that you might be right - it can happen due to frequent memory allocation/deallocation – Eugene Alexeev Sep 06 '19 at 10:41
  • Memory allocation and deallocation are extremely expensive operations that should strongly avoided in high-performance code. You want to allocate all your buffers at the start and then reuse them. – Rob Napier Sep 06 '19 at 13:03