How to disable/enable JIT dynamically

[Tomei]

We found that one app's start-up time is reduced significantly when
the JIT is disable -- we tried disabling the JIT for the entire
platform, and the app's start-up time is much better. (Sorry I cannot
share the exact statistics).

Is there a way to temporarily disable the JIT during the app launch,
and enable it again after the app is launched?

It seems like we can add a JNI method to muck with these two fields:

gDvmJit.threshold
gDvmJit.codeCacheSize

Is there a better way? If not, what's the recommended way of mucking
with them? :-)

Thanks

[Ben Cheng]

Hi Tomei,

We do have the desired delay mechanism implemented in dalvik/vm/compiler/Compiler.c

            :

            dvmLockMutex(&gDvmJit.compilerLock);

            /*

             * TUNING: experiment with the delay & perhaps make it

             * target-specific

             */

            dvmRelativeCondWait(&gDvmJit.compilerQueueActivity,

                                 &gDvmJit.compilerLock, 3000, 0);

            dvmUnlockMutex(&gDvmJit.compilerLock);

            :

Currently the delay is set to be 3 seconds. As the comment and your experiment suggested a target specific value is probably needed here to improve the app startup time.

If you look at ./base/core/java/android/app/ActivityThread.java, you will see a bunch of ensureJitEnabled() calls to enable the JIT if the app is believed to be in the post-startup phase. If specifying a larger startup delay doesn't help the app in question, you probably want to check these ensureJitEnabled() calls to see if any of them jumps the gun.

-Ben

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