Is there a way not to use cache in c++ - c++

Question

This question might be a bit weird but I wonder if there is a way NOT to use cache in c++.
I'm doing some tests, in this test I'm loading 2 GB (512*4 MB matrices) to memory, then do some correlations among them and calculate performance.
When I run the code for the 1st run, the running time is t1+x second, in the 2nd run, total time is t2+x seconds where t1 and t2 are loading time of 2 GB matrices and t1 > t2. (approx. t1=20, t2=5 sec). My assumption is it is because in the 2nd run, cache is used. (I don't know if there can be any other reason that decreases loading time like that.)
My problem with this is since there is no standards in loading times, the results are deceptive in some cases. So I want a standard in IO time. The only thing comes to my mind is not to use cache if there is a way.
Is there a way to standardize my IO time?
I'm using Windows 7 x64 and working on visual studio 2010, my RAM is 32 GB.
TEST RESULTS: I've compared average loading times of 4MB binary file in 5 options.
The options are 1st run with my original code, 2nd run with original code, using FILE_FLAG_NO_BUFFER, 1st run using cache and 2nd run as Roy Longbottom suggested.
1st run : 39.1 ms
2nd run : 10.4 ms
no_buffer : 127.8 ms
cache_1st run : 27.4 ms
cache_2nd run : 19.2 ms
My original read code is as follows:
void readNoise(string fpath,Mat& data){
FILE* fp = fopen(fpath.c_str(),"rb");
if (!fp)perror("fopen");
float* buffer= new float[size];
for(int i=0;i<size;++i) {
fread(buffer,sizeof(float),size,fp);
for(int j=0;j<size;++j){
data.at<float>(i,j)=buffer[j];
}
}
fclose(fp);
free(buffer);
}
I noticed a mistake in my code which is to do dynamic allocation, when I change dynamic allocation to static allocation, running time of readNoise method becomes same as cache used version of Roy Longbottom.
The difference of two run decreased but the question remains the same: "How to standardize running time of both first and second run"?

Answer 1

Benchmarking, specifically micro-benchmarking is a quite complex scenario and there are many ways you may inadvertently gather false performance data. You should look into micro benchmarking libraries such as google/benchmark and use one of them to perform your tests.
As you can see from your example, external factors such as the file system cache may cause the timing of individual runs to vary greatly.

Answer 2

Following is the code I use for my Windows drivespeed32 benchmark (free stuff - Google for drivespeed32), followed by results for 2000 MB files via Windows 7 and cached speeds for a smaller file. Code for Linux version also shown.
if (useCache)
{
hFile = CreateFile(testFile, GENERIC_READ,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN, NULL);
}
else
{
hFile = CreateFile(testFile, GENERIC_READ,
FILE_SHARE_READ | FILE_SHARE_WRITE,
NULL, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN
| FILE_FLAG_NO_BUFFERING, NULL);
}
if (hFile == INVALID_HANDLE_VALUE)
{
SetCurrentDirectory(currentDir);
printf (" Cannot open data file for reading\n\n");
fprintf (outfile, " Cannot open data file for reading\n\n");
fclose(outfile);
printf(" Press Enter\n");
g = getchar();
return 0;
}
Intended for smaller files like 8, 16, 32 MB, so times out after 1 set.
2000 MB File 1 2 3 4 5
Writing MB/sec 85.51 85.40 85.64 83.79 83.19
Reading MB/sec 84.34 85.77 85.60 85.88 85.15
Running Time Too Long At 246 Seconds - No More File Sizes
---------------------------------------------------------------------
8 MB Cached File 1 2 3 4 5
Writing MB/sec 1650.43 1432.86 1536.61 1504.16 1481.58
Reading MB/sec 2225.53 2361.99 2271.81 2235.04 2316.13
Linux Version
if (useCache)
{
handle = open(testFile, O_RDONLY);
}
else
{
handle = open(testFile, O_RDONLY | O_DIRECT);
}
if (handle == -1)
{
printf (" Cannot open data file for reading\n\n");
fprintf (outfile, " Cannot open data file for reading\n\n");
fclose(outfile);
printf(" Press Enter\n");
g = getchar();
return 0;
}