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Understanding Xcode GPU Capture "Group by API Call" timings

I see a lot of timings like this in Xcode GPU Frame Capture. Specifically what I want to call attention to is that the sum of the parts of the command buffer time does not add up to the total that is displayed. I notice that for GPU frame time, Xcode adds up all the command buffer times. But if I only add up the shader times, I get a much lower time (1.5-2 ms lower). I am trying to understand what is going on, I mean where is the GPU time going if it isn't going to the shaders? Notice how the total time is listed as 0.211 ms, but the sum of the parts is only 0.047 ms. I just want to better understand what is going on. Thanks.
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by jwilde.
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How can I profile iOS GPU clock speed?

I spend the majority of my time these days working on and optimizing shaders on iOS. Xcode GPU Profiler has been very helpful for the most part, however one problem that still plagues me is that profiling is very inconsistent. When I first start up an app I generally get a pretty good boost in GPU performance, then it slows down after 30 seconds or so. I imagine this is due to increased workload at the beginning. This means that I will get lower GPU timings for shaders during the first ~30 seconds of the app, but then if I let it sit for longer until I profiler, I get about 10% worse performance. I believe viewing the GPU clock speed would help me better profile this. However, I have not found any way to view this metric. Is there any way to view current GPU clock speed on iOS devices? Even if just through private methods for debugging purposes. Thanks!
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by jwilde.
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