mmap（）与Java MappedByteBuffer的性能？

Question

I have been developing a C++ project from existing Java code. I have the following C++ code and Java code reading from the same test file, which consists of millions of integers.

C++:

    int * arr = new int[len]; //len is larger than the largest int from the data
    fill_n(arr, len, -1);  //fill with -1
    long loadFromIndex = 0;
    struct stat sizeResults;
    long size;
    if (stat(fileSrc, &sizeResults) == 0) {
        size = sizeResults.st_size; //here size would be ~551950000 for 552M test file
    }
    mmapFile = (char *)mmap(NULL, size, PROT_READ, MAP_SHARED, fd, pageNum*pageSize);
    long offset = loadFromIndex % pageSize;
    while (offset < size) {
        int i = htonl(*((int *)(mmapFile + offset)));
        offset += sizeof(int);
        int j = htonl(*((int *)(mmapFile + offset)));
        offset += sizeof(int);
        swapElem(i, j, arr);
    }
    return arr;

Java:

    IntBuffer bb = srcFile.getChannel()
                    .map(MapMode.READ_ONLY, loadFromIndex, size)
                    .asIntBuffer().asReadOnlyBuffer();
    while (bb.hasRemaining()) {
        int i = bb.get();
        int j = bb.get();
        swapElem(i, j, arr); //arr is an int[] of the same size as the arr in C++ version, filled with -1
    }
    return arr;

void swapElem(arr) in C++ and Java are identical. It compares and modifies values in the array, but the original code is kind of long to post here. For testing purpose, I replaced it with the following function so the loop won't be dead code:

void swapElem(int i, int j, int * arr){   // int[] in Java
    arr[i] = j;
}

I assumed the C++ version should outperform the java version, but the test gives the opposite result -- Java code is almost two times faster than the C++ code. Is there any way to improve the C++ code?

I feel maybe the mmapFile+offset in C++ is repeated too many times so it is O(n) additions for that and O(n) additions for offset+=sizeof(int) , where n is number of integers to read. For Java's IntBuffer.get() , it just directly reads from a buffer's index so no addition operation is needed except O(n) increments of the buffer index by 1. Therefore, including the increments of buffer index, C++ takes O(2n) additions while Java takes O(n) additions. When it comes to millions of data, it might cause significant performance difference.

Following this idea, I modified the C++ code as follows:

    mmapBin = (char *)mmap(NULL, size, PROT_READ, MAP_SHARED, fd, pageNum*pageSize);
    int len = size - loadFromIndex % pageSize;
    char * offset = loadFromIndex % pageSize + mmapBin;
    int index = 0;
    while (index < len) {
        int i = htonl(*((int *)(offset)));
        offset += sizeof(int);
        int j = htonl(*((int *)(offset)));
        offset += sizeof(int);
        index+=2*sizeof(int);
    }

I assumed there will be a slight performance gain, but there isn't.

Can anyone explain why the C++ code works slower than the Java code does? Thanks.

Update:

I have to apologize that when I said -O2 does not work, there was a problem at my end. I messed up Makefile so the C++ code did not recompile using -O2. I've updated the performance and the C++ version using -O2 has outperformed the Java version. This can seal the question, but if anyone would like to share how to improve the C++ code, I will follow. Generally I would expect it to be 2 times faster than the Java code, but currently it is not. Thank you all for your input.

Compiler: g++

Flags: -Wall -c -O2

Java Version: 1.8.0_05

Size of File: 552MB, all 4 byte integers

Processor: 2.53 GHz Intel Core 2 Duo

Memory 4GB 1067 MHz DDR3

Updated Benchmark:

Version Time(ms)

C++: ~1100

Java: ~1400

C++(without the while loop): ~35

Java(without the while loop): ~40

I have something before these code that causes the ~35ms performance(mostly filling the array with -1), but that is not important here.

Answer 1

I have some doubts that the benchmark method is correct. Both codes are "dead" codes. You don't actually use i and j anywhere so the gcc compiler or Java JIT might decide to actually remove the loop seeing that it has no effect on the future code flow.

Anyway, I would change the C++ code to:

mmapFile = (char *)mmap(NULL, size, PROT_READ, MAP_SHARED, fd, pageNum*pageSize);
long offset = loadFromIndex % pageSize;
int i, j;
int szInc = 2 * sizeof(int);
while (offset < size) {
    scanf(mmapFile, "%d", &i);
    scanf(mmapFile, "%d", &j);
    offset += szInc; // offset += 8;
}

This would be the equivalent to Java code. In addition I would continue using -O2 as compilation flags. Keep in mind that htonl is an extra conversion that Java code does not seem to do it.