I read an article (1.5 years old http://www.drdobbs.com/parallel/cache-friendly-code-solving-manycores-ne/240012736) which talks about cache performance and size of data. They show the following code which they say they ran on an i7 (sandy bridge)
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]++;
}
They make the claim that if they keep Size*Iterations constant, increasing Size, when the size in memory of array increases beyond the L2 cache size they observe a huge spike in time taken to execute (10x).
As an exercise for myself I wanted to try this to see if I could reproduce their results for my machine . (i7 3770k, win7, visual c++ 2012 compiler, Win32 debug mode, no optimizations enabled). To my surprise though, I am not able to see an increase in time taken to execute (even beyond the L3 cache size) which made me think the compiler was somehow optimizing this code. But I dont see any optimizations either. The only change in speed i see is that below the word size of my machine it takes slightly longer. Below are my timings, code listing, and pertinent disassembly.
Does anyone know why:
1) Why the time taken does not increase at all regardless of the size of my array? Or how I could find out?
2) Why does the time taken start high and then decrease until the cache line size is reached, shouldn't more iterations be processed without reading from cache if the data is less than the line size?
Timings:
Size=1,Iterations=1073741824, Time=3829
Size=2,Iterations=536870912, Time=2625
Size=4,Iterations=268435456, Time=2563
Size=16,Iterations=67108864, Time=2906
Size=32,Iterations=33554432, Time=3469
Size=64,Iterations=16777216, Time=3250
Size=256,Iterations=4194304, Time=3140
Size=1024,Iterations=1048576, Time=3110
Size=2048,Iterations=524288, Time=3187
Size=4096,Iterations=262144, Time=3078
Size=8192,Iterations=131072, Time=3125
Size=16384,Iterations=65536, Time=3109
Size=32768,Iterations=32768, Time=3078
Size=65536,Iterations=16384, Time=3078
Size=262144,Iterations=4096, Time=3172
Size=524288,Iterations=2048, Time=3109
Size=1048576,Iterations=1024, Time=3094
Size=2097152,Iterations=512, Time=3313
Size=4194304,Iterations=256, Time=3391
Size=8388608,Iterations=128, Time=3312
Size=33554432,Iterations=32, Time=3109
Size=134217728,Iterations=8, Time=3515
Size=536870912,Iterations=2, Time=3532
code:
#include <string>
#include <cassert>
#include <windows.h>
template <unsigned int SIZE, unsigned int ITERATIONS>
static void test_body(volatile char* array)
{
for (unsigned int i = 0; i < ITERATIONS; i++)
{
for (unsigned int x = 0; x < SIZE; x++)
{
array[x]++;
}
}
}
template <unsigned int SIZE, unsigned int ITERATIONS>
static void test_function()
{
assert(SIZE*ITERATIONS == 1024*1024*1024);
static volatile char array[SIZE];
test_body<SIZE, 1>(array); //warmup
DWORD beginTime = GetTickCount();
test_body<SIZE, ITERATIONS>(array);
DWORD endTime= GetTickCount();
printf("Size=%u,Iterations=%u, Time=%d\n", SIZE,ITERATIONS, endTime-beginTime);
}
int main()
{
enum { eIterations= 1024*1024*1024};
test_function<1, eIterations>();
test_function<2, eIterations/2>();
test_function<4, eIterations/4>();
test_function<16, eIterations/16>();
test_function<32, eIterations/ 32>();
test_function<64, eIterations/ 64>();
test_function<256, eIterations/ 256>();
test_function<1024, eIterations/ 1024>();
test_function<2048, eIterations/ 2048>();
test_function<4096, eIterations/ 4096>();
test_function<8192, eIterations/ 8192>();
test_function<16384, eIterations/ 16384>();
test_function<32768, eIterations/ 32768>();
test_function<65536, eIterations/ 65536>();
test_function<262144, eIterations/ 262144>();
test_function<524288, eIterations/ 524288>();
test_function<1048576, eIterations/ 1048576>();
test_function<2097152, eIterations/ 2097152>();
test_function<4194304, eIterations/ 4194304>();
test_function<8388608, eIterations/ 8388608>();
test_function<33554432, eIterations/ 33554432>();
test_function<134217728, eIterations/ 134217728>();
test_function<536870912, eIterations/ 536870912>();
}
Disassembly
for (unsigned int i = 0; i < ITERATIONS; i++)
00281A59 mov dword ptr [ebp-4],0
00281A60 jmp test_body<536870912,2>+1Bh (0281A6Bh)
00281A62 mov eax,dword ptr [ebp-4]
00281A65 add eax,1
00281A68 mov dword ptr [ebp-4],eax
00281A6B cmp dword ptr [ebp-4],2
00281A6F jae test_body<536870912,2>+53h (0281AA3h)
{
for (unsigned int x = 0; x < SIZE; x++)
00281A71 mov dword ptr [ebp-8],0
00281A78 jmp test_body<536870912,2>+33h (0281A83h)
00281A7A mov eax,dword ptr [ebp-8]
{
for (unsigned int x = 0; x < SIZE; x++)
00281A7D add eax,1
00281A80 mov dword ptr [ebp-8],eax
00281A83 cmp dword ptr [ebp-8],20000000h
00281A8A jae test_body<536870912,2>+51h (0281AA1h)
{
array[x]++;
00281A8C mov eax,dword ptr [array]
00281A8F add eax,dword ptr [ebp-8]
00281A92 mov cl,byte ptr [eax]
00281A94 add cl,1
00281A97 mov edx,dword ptr [array]
00281A9A add edx,dword ptr [ebp-8]
00281A9D mov byte ptr [edx],cl
}
00281A9F jmp test_body<536870912,2>+2Ah (0281A7Ah)
}
00281AA1 jmp test_body<536870912,2>+12h (0281A62h)
