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Date: Tue, 31 Dec 2013 15:34:19 -0500
From: Bill Cox <>
Subject: Re: [PHC] optimizing memory access speed

I have a cache mystery that's driving me nuts.  I thought that loading 16KB
into L1 cache would allow me to randomly hash 64-bit words in L1 cache with
no cache penalties.  For some reason, if I read through "sourceBlock"
linearly, rather than jumping around, the code runs 2X faster.  Alexander,
you're insights into cache performance seem top notch.  Can you spot the
issue?  The following code has one line commented out in the inner loop.
 Depending on which is commented in, it runs 2X faster or slower,
independent of the optimization level.  I'm compiling on a Core i7 with a
recent version of gcc, using:

        gcc -Wall -m64 -O0 memorycpy.c -o memorycpy

When I compare assembly code, only one instruction changes, so I'm pretty
sure this is a caching issue of some sort.  I just can't figure out what it
is.  Here's the code.  Thanks in advance!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MEM_SIZE (1LL << 31)
#define MEM_LENGTH (MEM_SIZE/sizeof(unsigned long long))
#define BLOCK_SIZE (1LL << 14)
#define BLOCK_LENGTH (BLOCK_SIZE/sizeof(unsigned long long))

int main() {
    unsigned long long *mem = (unsigned long long *)malloc(MEM_SIZE);
    unsigned long long i, j;
    unsigned long long hash = 0x12345;
    unsigned long long hashBlock;
    unsigned long long *sourceBlock;
    unsigned long long *destBlock;
    printf("blockLen: %llu, memLen:%llu\n", BLOCK_LENGTH, MEM_LENGTH);
    // Initialize 1st page
    for(i = 0; i < BLOCK_LENGTH; i++) {
        mem[i] = i;
    for(i = 1; i < MEM_LENGTH/BLOCK_LENGTH; i++) {
        hashBlock = hash % i;
        sourceBlock = mem + hashBlock*BLOCK_LENGTH;
        destBlock = mem + i*BLOCK_LENGTH, BLOCK_SIZE;
        for(j = 0; j < BLOCK_LENGTH; j++) {
            hash ^= sourceBlock[hash & BLOCK_MASK] * 0xdeadbeef + 12345;
            //hash ^= sourceBlock[j & BLOCK_MASK] * 0xdeadbeef + 12345;
            destBlock[j] = hash;
    // Force the optimizer to keep the memory
    printf("%u\n", ((unsigned int *)mem)[rand() % MEM_LENGTH]);
    return 0;

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