Why C++ fread is so different from Windows ReadFile?

Question

Greetings,
I write a piece of code to simulate system I/O perf. However the result is so different between fread (IOPS is 5000+) and Windows ReadFile (IOPS 200 ~ 300) for a 7200RPM HDD (it's 180 ~ 190 Random read IOPS by IOMeter). Could you give me a hint where's the problem?

Thanks.

Nai Yan.

Code - Windows ReadFile (with cache)

DWORD WINAPI FileReadThreadEntry(LPVOID lpThreadParameter)
{
	//File access with cache enabled
	input * in = (input*) lpThreadParameter; 

	LPCTSTR path = (LPCTSTR) in->path;

	HANDLE fp = CreateFile(
		path,
		GENERIC_READ,
		FILE_SHARE_READ,
		NULL,
		OPEN_EXISTING,
		FILE_ATTRIBUTE_NORMAL,//|FILE_FLAG_NO_BUFFERING,
		NULL);


	unsigned int sPos = in->starting;

	int * result = in->r;

	if(fp != NULL)
	{
		unsigned long pos;
		bool bRead;

		DWORD bNum;

		for (int i=0; i<length/(threadCount*interval);i++)
		{
			pos = i * interval;

			if ((SetFilePointer(fp,(sPos+pos),NULL,FILE_BEGIN)!=0))
			{
				if ((bRead = ReadFile(fp,&result[sPos/interval + i],512,&bNum,NULL))== true)
				{
					InterlockedIncrement(&completeIOs);
				}
				else
				{
					printf("file read err...\n");
					exit(-1);
				}
			}
		}			
		CloseHandle(fp);
		fp = NULL;
	}

	else
	{
		printf("File open err... \n");
		exit(-1);
	}
}

C++ fread code

DWORD WINAPI FileReadThreadEntry(LPVOID lpThreadParameter)
{
	input * in = (input*) lpThreadParameter; 

	LPCTSTR path = (LPCTSTR) in->path;

	FILE * fp = fopen(path,"rb");

	int sPos = in->starting;

	int * result = in->r;

	if(fp != NULL)
	{
		fpos_t pos;
		for (int i=0; i<length/(threadCount*interval);i++)
		{
			pos = i * interval;
			fsetpos(fp,&pos);
			if (fread(&result[sPos/interval + i],512,1,fp) ==1)
			{
				InterlockedIncrement(&completeIOs);
			}
			else
			{
				printf("file read err...\n");
				exit(-1);
			}
		}

		fclose(fp);
		fp = NULL;
		}

	else
	{
		printf("File open err... \n");
		exit(-1);
	}
}

Answer 1

Hello,

This forum is for answering questions related to C++ AMP, PPL, ConcRT and other C++ related threading questions.

Since the ask is specifically around the performance of ReadFile, your best to get a response is to post here: http://answers.microsoft.com/en-us/windows/forum

Thanks!

---

Rahul V. Patil