Please help- Why windows multi-threading data fetching IOPS too faster (beyond HW max benchmark)?


  • Greetings, 

          Can anybody help me a little out of my difficulty? 

          I have a SSD and I am trying to use it to simulate my program I/O performance, however, IOPS calculated from my program is much much faster than IOMeter. 

          My SSD is PLEXTOR PX-128M3S, by IOMeter, its max 512B random read IOPS is around 94k (queue depth is 32). 
          However my program (32 windows threads) can reach around 500k 512B IOPS, around 5 times of IOMeter!!! I did data validation but didn't find any error in data fetching. It's because my data fetching in order?

          I paste my code belwo (it mainly fetch 512B from file and release it; I did use 4bytes (an int) to validate program logic and didn't find problem), can anybody help me figure out where I am wrong? 

          Thanks so much in advance!!

    Nai Yan. 

    #include <stdio.h>
    #include <Windows.h>
    **  Purpose: Verify file random read IOPS in comparison with IOMeter    
    **  Author:  Nai Yan
    **  Date:    Feb. 9th, 2012
    //Global variables
    long completeIOs = 0; 
    long completeBytes = 0;
    int  threadCount = 32;
    unsigned long long length = 1073741824;                  //4G test file
    int interval = 1024;
    int resultArrayLen = 320000;
    int *result = new int[resultArrayLen];
    //Method declarison
    double GetSecs(void);					           //Calculate out duration
    int InitPool(long long,char*,int);		     		  //Initialize test data for testing, if successful, return 1; otherwise, return a non 1 value. 
    int * FileRead(char * path);
    unsigned int DataVerification(int*, int sampleItem);		                 //Verify data fetched from pool
    int main()
    	int sampleItem = 0x1;
    	char * fPath = "G:\\workspace\\4G.bin";
    	unsigned int invalidIO = 0;
    	if (InitPool(length,fPath,sampleItem)!= 1)
    	   printf("File write err... \n");
    	//start do random I/Os from initialized file
    	double start = GetSecs();
    	int * fetchResult = FileRead(fPath);
    	double end = GetSecs();
    	printf("File read IOPS is %.4f per second.. \n",completeIOs/(end - start));
    	//start data validation, for 4 bytes fetch only
    //	invalidIO = DataVerification(fetchResult,sampleItem);
    //	if (invalidIO !=0)
    //	{
    //		printf("Total invalid data fetch IOs are %d", invalidIO);
    //	}
    	return 0;
    int InitPool(long long length, char* path, int sample)
    	printf("Start initializing test data ... \n");
    	FILE * fp = fopen(path,"wb");
    	if (fp == NULL)
    		printf("file open err... \n");
    		exit (-1);
    	else									//initialize file for testing
    		for (int i=0; i<length; i++)
    		fp = NULL;
    		printf("Data initialization is complete...\n");
    		return 1;
    double GetSecs(void)
        LARGE_INTEGER frequency;
        LARGE_INTEGER start;
        if(! QueryPerformanceFrequency(&frequency)) 
            printf("QueryPerformanceFrequency Failed\n");
        if(! QueryPerformanceCounter(&start))
            printf("QueryPerformanceCounter Failed\n");
    	return ((double)start.QuadPart/(double)frequency.QuadPart);
    class input
    	char *path;
    	int starting;
    	input (int st, char * filePath):starting(st),path(filePath){}
    DWORD WINAPI FileReadThreadEntry(LPVOID lpThreadParameter)
    	input * in = (input*) lpThreadParameter; 
    	char* path = 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<resultArrayLen/threadCount;i++)
    			pos = i * interval;
    			//For 512 bytes fetch each time
    			unsigned char *c =new unsigned char [512];
    			if (fread(c,512,1,fp) ==1)
    				delete c;
    			//For 4 bytes fetch each time
    			/*if (fread(&result[sPos + i],sizeof(int),1,fp) ==1)
    				printf("file read err...\n");
    		fp = NULL;
    		printf("File open err... \n");
    int * FileRead(char * p)
    	printf("Starting reading file ... \n");
    	HANDLE mWorkThread[256];                      //max 256 threads
    	completeIOs = 0;
    	int slice = int (resultArrayLen/threadCount);
    	for(int i = 0; i < threadCount; i++)
    		mWorkThread[i] = CreateThread(
    					(LPVOID)(new input(i*slice,p)),
       WaitForMultipleObjects(threadCount, mWorkThread, TRUE, INFINITE);
       printf("File read complete... \n");
       return result;
    unsigned int DataVerification(int* result, int sampleItem)
    	unsigned int invalid = 0;
    	for (int i=0; i< resultArrayLen/interval;i++)
    		if (result[i]!=sampleItem)
    			invalid ++;
    	return invalid;

    • Přesunutý Rahul V. Patil 23. května 2012 20:30 Related to where the other thread is (From:Parallel Computing in C++ and Native Code)
    10. února 2012 7:03

Všechny reakce

  • You're doing cached I/O and thus you are effectively only measuring the system overhead.

    If you want to actually measure the underlying hardware you need to do uncached I/O using CreateFile/ReadFile with FILE_FLAG_NO_BUFFERING (also see


    17. února 2012 12:38
  • Thank you, Stefan. I just tried with your suggestion. It seems that Windows ReadFile is close to reality (IOMeter). However I am still looking for the root cause why Windows ReadFile is so slower than C++ fread - I am using another thread ( to follow up this question. Sorry, I forgot to update you for this.

    If you have further comments on this question, please help!

    Nai Yan.

    2. března 2012 8:46