Fast File Loading (Pt. 1)

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Reading data efficiently from Hard Disk and DVD units is vital for video games and one of the more important problems to solve in the next generation of games. While we are getting 20x performance in processing power and memory size, we are only getting 4x performance improvement in data devices (dvd for consoles).

I describe in this article how to efficiently read raw data from disk (hdd, dvd) oriented towards streaming files in a realtime application (although the concepts are useful in other areas). The platform used is Win32, but all the topics covered could be easily ported to other platforms.

I have included a project for Visual Studio 2005 with all the code described here and all the framework to test the different techniques. You can download it here. The machine where I have done the tests is a 3.2GHz Pentium 4. The devices used for testing are:

• A 7200rpm Hard Disk with an average read performance of 46.8 MB/s (measured using Hd Tach)

• A DVD-Rom unit (using a DVD+RW media) with a peak performance of 2.40Mb (measured using Nero CD-DVD Speed)

Windows (and all the Operating Systems in general) uses part of the physical memory (not being used by processes) for file caching (you can view how much memory is being used for the File System Cache in the Task Manager). To avoid Windows caching my test files I implemented a function that flushes the cache reading big files before measuring times. The tests were executed 10 times, lasting each one several minutes.

So let’s start the travel… Objective: having a 100MB file in the CPU memory as fast as possible.

1. The Standard and Portable way

The first option is using portable code from the standard C library. All we know the benefits of portability. So we allocate a buffer and fread() the file.

FILE *fp = fopen(FileName, “rb”);
fread(&g_buffer[0], 1, FileSize, fp);
fclose(fp);

2. The Win32 Native way

Trying to improve the native approach we go for the Win32 native file functions: CreateFile and ReadFile

HANDLE hFile = CreateFile(FileName, GENERIC_READ, 0, 0, OPEN_EXISTING,FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN, 0);

DWORD dwNumberOfBytesRead = 0;
ReadFile(hFile, &g_buffer[0], FileSize, &dwNumberOfBytesRead, 0);

CloseHandle(hFile);

Nearly the same performance. FILE_FLAG_SEQUENTIAL_SCAN is used to direct the Cache Manager to access the file sequentially. It is recommended to use it when reading large files with sequential access. I though that the FILE_FLAG_SEQUENTIAL_SCAN hint would give a better improvement than this but obviously the fread implementation in Win32 is doing a good job.

3. File Memory Mapping

Our next approach is trying memory mapped files where the system reads the data from disk on demand using the same mechanism that is used in Virtual Memory.

HANDLE hFile = CreateFile(FileName, GENERIC_READ, 0, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL | FILE_FLAG_SEQUENTIAL_SCAN, 0);

HANDLE hFileMapping = CreateFileMapping(hFile, 0, PAGE_READONLY, 0, FileSize, 0);

int iPos = 0;
const unsigned int BlockSize = 128 * 1024;

while(iPos < FileSize)
{
   int iLeft = FileSize - iPos;
   int iBytesToRead = iLeft > BlockSize ? BlockSize: iLeft;

   void *rawBuffer = MapViewOfFile(hFileMapping, FILE_MAP_READ, 0, iPos, iBytesToRead);
   memcpy(&g_buffer[iPos], rawBuffer, iBytesToRead);
   UnmapViewOfFile(rawBuffer);

   iPos += iBytesToRead;
}

CloseHandle(hFileMapping);
CloseHandle(hFile);

We are getting a significant improvement when reading from the DVD. Reading from Hard Disk is nearly the same performance.

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