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On an older phone originally designed to run Android 1.5 or 1.6, what are the pros and cons of using a newer Android feature like the JIT compiler?

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As an example, I have an HTC Dream running CyanogenMod 6 and within the options you are able to choose whether to use the JIT or not. Elsewhere I have seen conflicting information on whether the JIT compiler will help or hurt the performance of an older phone. I tried to keep this question from being too phone-specific, as I think an answer to this could help clarify the issue on any older model phone, not just the Dream. – Mingo Feb 15 '11 at 7:46
Great question. The only thing I know about JIT is from my experience with Java/C#. Mostly that it waits to compile some code until run time and the JIT "smartly" caches code that it sees that repeats itself. Wiki might help explain better: – Bryan Denny Feb 15 '11 at 14:31
up vote 7 down vote accepted

JIT typically causes an app to use more RAM due to the JIT cache, although Android's JIT was specifically designed so the additional memory usage was minimal (compared to a typical Java JIT). On phones that are already very RAM-starved to begin with, JIT may result in a decrease in performance.

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This makes sense to me - my Dream is so limited when it comes to RAM that turning the JIT on really killed performance. However, I wonder about other older phones that aren't so short on RAM. – Mingo Feb 16 '11 at 0:34

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