Cache optimization of C loop: Why doesn't this work?

Question

I'm trying to duplicate the first piece of code on this article

http://www.drdobbs.com/parallel/cache-friendly-code-solving-manycores-ne/240012736

Namely:

static volatile int array[Size];
static void test_function(void)
{
    for (int i = 0; i < Iterations; i++)
        for (int x = 0; x < Size; x++)
          array[x]++;
}

I'm running on OS X with an Ivy Bridge processor, and therefore have 64KiB of L1 cache. However, no matter how much I change around the array size, it takes the same amount of time. Here's my code:

#define ARRAY_SIZE 16 * 1024
#define NUM_ITERATIONS 200000

volatile int array[ARRAY_SIZE];

int main(int argc, const char * argv[])
{
    for (int i = 0; i < NUM_ITERATIONS; i++)
        for (int x = 0; x < ARRAY_SIZE; x++)
            array[x]++;
    return 0;
}

Now, according to the logic suggested by the article, array should be 64KiB and utilize all my L1 cache. However, I've tried this with many difference combinations of ARRAY_SIZE (up to 160 * 1024), setting NUM_ITERATIONS accordingly, but every combination about takes the same amount of time.

I'm using gcc -o cachetest cachetest.c to compile, with no other options. Is there some kind of optimization going on that I don't know about, even though volatile is used? Or are there so many parallel processes and context switching that I can't even tell? What's going on here? I'm so confused.

Thanks SO!

What if... it was running in optimal time to begin with!? tin foil hat — StoryTeller - Unslander Monica
– StoryTeller - Unslander Monica, Commented Nov 24, 2013 at 10:37
A quick look at the assembler would tell you whether the compiler had, for instance, optimised this code away to nothing. — Oliver Charlesworth
– Oliver Charlesworth, Commented Nov 24, 2013 at 10:55
Probably the processor preloads the data? We are accessing it perfectly sequentially. Don't take my word on this, though. — Guido
– Guido, Commented Dec 21, 2013 at 3:51

dragon135 · Accepted Answer · 2013-12-21 03:31:40Z

1

There are 2 things:

Compiler may do some default optimization to your code
Your code does not use array in any other code/functions, it only increment the array value inside loop, so compiler may optimize it more by changing your program to do nothing (just return 0), which is still correct.

I recommend to:

Add more code inside the loop so the compiler will not eliminate your code, for example: printf the array value, or add the array value to a sum variable then print the sum variable at the end of the loop.
Turn off all compiler optimization when compiling by using -O0 option.
Check the assembly file of the code generated by compiler by using -S option

edited Dec 21, 2013 at 3:31

answered Dec 21, 2013 at 3:25

dragon135

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