faster Math.exp() via JNI?

Question

I need to calculate Math.exp() from java very frequently, is it possible to get a native version to run faster than java\'s Math.exp()

Answer 1

Since the Java code will get compiled to native code with the just-in-time (JIT) compiler, there's really no reason to use JNI to call native code.

Also, you shouldn't cache the results of a method where the input parameters are floating-point real numbers. The gains obtained in time will be very much lost in amount of space used.

Answer 2

Commons Math3 ships with an optimized version: FastMath.exp(double x). It did speed up my code significantly.

Fabien ran some tests and found out that it was almost twice as fast as Math.exp():

 0.75s for Math.exp     sum=1.7182816693332244E7
 0.40s for FastMath.exp sum=1.7182816693332244E7

Here is the javadoc:

Computes exp(x), function result is nearly rounded. It will be correctly rounded to the theoretical value for 99.9% of input values, otherwise it will have a 1 UPL error.

Method:

    Lookup intVal = exp(int(x))
    Lookup fracVal = exp(int(x-int(x) / 1024.0) * 1024.0 );
    Compute z as the exponential of the remaining bits by a polynomial minus one
    exp(x) = intVal * fracVal * (1 + z)

Accuracy: Calculation is done with 63 bits of precision, so result should be correctly rounded for 99.9% of input values, with less than 1 ULP error otherwise.

Answer 3

You might be able to get it to run faster if you do them in batches. Making a JNI call adds overhead, so you don't want to do it for each exp() you need to calculate. I'd try passing an array of 100 values and getting the results to see if it helps performance.

Answer 4

The problem with using JNI is the overhead involved in making the call to JNI. The Java virtual machine is pretty optimized these days, and calls to the built-in Math.exp() are automatically optimized to call straight through to the C exp() function, and they might even be optimized into straight x87 floating-point assembly instructions.

Answer 5

Use Java's.

Also, cache results of the exp and then you can look up the answer faster than calculating them again.

Answer 6

There is a cost associated with calling across the JNI boundary.

If you could move the loop that calls exp() into the native code as well, so that there is just one native call, then you might get better results, but I doubt it will be significantly faster than the pure Java solution.

I don't know the details of your application, but if you have a fairly limited set of possible arguments for the call, you could use a pre-computed look-up table to make your Java code faster.