Question

I have a security library in C and try to import that into my Android project using NDK. The library depends on three other libraries: pbc, gmp, and openssl. I’ve built the first two libraries as static libraries and the last one as shared libraries.

I timed the encrypt() and decrypt() functions in my library.

On my laptop, it is:

• encrypt() 30ms
• decrypt() 160ms

On my Android device(Droid 2.2.3), it is:

• encrypt() 190ms
• decrypt() 1300ms

The time is only for calling those two functions from the C library. The JNI overhead is very small.

Is this expected?

Update:

Both encrypt() and decrypt() do not have any IO operation, mainly float point operation. And I compiled the code for armeabi-v7a.

Answer 1

First of all I would say that you are lucky – your port works only 8 times slower than laptop version. It is quite a good result for ARM-based platforms.

There are several reasons to be slower on ARM:

• ARM processors simply have less computational power than Intel/AMD desktop CPUs
• Slower memory and lower bandwidth
• Different architectural restrictions (for example floating point unit is separated from integer CPU core in ARM)

You can try the following things to improve the performance of you code:

• If your phone has modern FPU then you can try to recompile all the libraries with additional compiler flag -mfpu=vfpv3 (or -mfpu=neon). It can slightly improve the speed of floating-point calculations because of doubled number of FPU registers.
• Try to build your code with newer compiler. Latest Crystax NDK which is modified version of Google NDK includes gcc 4.6 toolchain. Sometimes newer compiler can produce more effective code.
• Profile your code and optimize bottlenecks. You can use a number of arm specific optimization tricks (here is a bit outdated but good guide) or vertorize code with NEON SIMD.