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I am working on an iOS app that uses iPhone’s CPU to do some complex image processing. I noticed when running on its battery, the iPhone 5 CPU is very sluggish right after you press the power switch to wake it up out of hibernation. In many cases, this “warm up” period can be as long as 5 minutes. Normally, my app can process over 28 frames per second and the CPU load is lower than 80%. But in the warm up period, it could only process 19 frames per second and the CPU load is over 95%. The only explanation I can think of is the CPU’s frequency is throttled in this warm-up period. However, the CPU is never throttled if I run my app from XCode. Besides, I never see such a throttling on other iOS devices. I’ve not tried iPad mini or iPad 4 though.

I wonder if anyone-else has seen the same CPU behavior. What can I do to make sure iPhone 5’s A6 processor is always working on this full capability?

Sean
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  • Is it possible that in this "warm up period" that the iPhone is doing more work and hence there are less computing resources available for your program? – Peter M Nov 18 '12 at 14:48
  • It possible. But the up to 5 minutes looks too long. Interesting thing is, if I wake the iPhone 5 up by pressing home button, the throttling is less likely happen. – Sean Nov 18 '12 at 18:46

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Try installing your app on the device in a different scheme and then run Instruments to see whats really going on.

I have not experienced an issue like this with the iPhone 4 or 5, where there is some kind of a throttle period after waking up

raz0r
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  • If I run Instruments from Mac while the app is running,the throttling does not happen. It only happens when it is disconnected from Mac. I turned on Developer logger on the iPhone 5 before I started my app. After the session, I offloaded the log file to Mac, check it with Instruments and found the CPU load was abnormally high. The difficult part is, the local log file of iPhone doesn't give much details on the percentage of CPU usage of each process. – Sean Nov 18 '12 at 18:43