CODEWITHSUNDEEP
javaandroidc++performanceandroid-ndk
HypnoToad
HypnoToad

Reputation: 76

Why is my native C++ code running so much slower than Java on Android?

I ported some portions of my Java code to C++ to speed up calculations on Android (this was a physics subroutine). What I found was that the native code runs several times slower than the Java code. I thought maybe something was wrong with the configuration of my project, or maybe with the array handling, so I put a simple loop in the HelloAndroidJni project to test raw speed difference, and got similar results.

Java Code:

@Override
protected void onCreate(Bundle savedInstanceState) {
    /* ...generic boilerplate code... */

    TextView tv = (TextView) findViewById(R.id.sample_text);
    int loopCount = 100000;

    //time the native method
    long ticks = System.nanoTime();
    int result = nativeTest(100000);
    long nativeTime = (System.nanoTime() - ticks) / 100000;

    //time the Java method
    ticks = System.nanoTime();
    result = javaTest(100000);
    long javaTime = (System.nanoTime() - ticks) / 100000;

    //present results
    tv.setText("Native=" + nativeTime + "; Java=" + javaTime);
}

The loop in Java:

int javaTest(int count) {
    int result = 0;
    for (int i = 0; i < count; i++) {
        for (int j = 0; j < 100; j++) {
        result += 34432; result++;
        result -= 34431; result--;
    } }
    return result;
}

And the C++ code:

JNIEXPORT jint JNICALL
Java_com_fringecode_helloandroidjni_MainActivity_nativeTest(
        JNIEnv *env, jobject jThis, jint count) {
    int result = 0;
    for (int i = 0; i < count; i++) {
        for (int j = 0; j < 100; j++) {
            result += 34432; result++;
            result -= 34431; result--;
        } }
    return result;
}

The rest of the project is identical to the HelloAndroidJni sample project. The result of a typical run is Native=2580 ms, Java=195 ms. What could be making the native code run so much slower than the Java?

EDIT: Incidentally, the native code runs much faster than Java on the emulator, but on my phone (LG V20 / Snapdragon 820) the native is much slower.

Upvotes: 4

Views: 1598

Answers (1)

Alex Cohn
Alex Cohn

Reputation: 57183

Java on the fly optimization may make your loop be as fast as native. On the other hand, without APP_OPTIM=release the C++ compiler will generate debug unoptimized code.

The takeaway is that actually number crunching in Java may be quite efficient, if coded in disciplined manner. But after all, coding same efficiently in C also requires discipline.

Upvotes: 2

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