shouldn't disk writes be slower than reads

jwstric2 · 08-28-2004, 11:01 PM

Before I start, I understand the logic behind buffer cache and what not with writes and reads. Whats I was doing was benchmarking the writing and reading of a file with x process doing some x read operatioins and looking at performance hits. The machine Im working with has 512 MB memory so I use fairly large files to assure none of it stays in the buffer (2-3 times the memory size).

What I'm find is no matter what the load on the system the write factor seems to stay constant + or - 3-4 seconds. The read factor has a linear like quality until a certain load then an expodential increase as I suspected would happen.

The issue Im trying to solve is why my writes are not producing the results that I expect. I have played with sync, fsync on the file, ect.. but still no change. Any ideas would be awesome.

Code:

#include <stdio.h>
#include <errno.h>
#include <sys/time.h>

#define BLOCK_SIZE_WRITE 4096
#define FILE_NAME "/tmp/WRITE_TEST_OUT"



double currentTime(void)
{
     struct timeval tval;
     gettimeofday(&tval, NULL);
     return (tval.tv_sec + tval.tv_usec/1000000.0);
}

main(int argc, char **argv){
  FILE *fd_w, *fd_r;
  unsigned long num_writes = 0;
  char buffer[BLOCK_SIZE_WRITE];
  char file_name[255];
  int returnVal = 0;
  unsigned long i,j;
  double start_time, end_time, write_time, read_time;
  unsigned long FILE_SIZE; 

  if(argc < 2){
    fprintf(stderr, "BAD NUM ARGS:  %s [file size] \n", argv[0]);
    exit(1);
  } 
    
  FILE_SIZE = atol(argv[1]);

  fprintf(stderr, "FILE SIZE IS %u\n", FILE_SIZE);
  
  fprintf(stderr, "STARTING WRITE TEST\n");
  start_time = currentTime();
  for(i=0; i<1; i++){
    fd_w=fopen(FILE_NAME, "w+b");
    if(!fd_w){
      perror("ERROR CREATING FILE");
      exit(1);
    }
  

    num_writes = FILE_SIZE/(unsigned long)BLOCK_SIZE_WRITE;

    
    for(j=0; j<num_writes; j++){
      returnVal = fwrite(buffer, BLOCK_SIZE_WRITE, 1, fd_w);
      if(returnVal != 1){
        perror("ERROR WRITING TO FILE");
        break;
      }      
    }
    sync();    
    close(fd_w);
  }

  end_time = currentTime();    
  write_time = end_time-start_time;
  fprintf(stderr, "ENDING WRITE TEST\n");

  fprintf(stderr, "STARTING READ TEST\n");
  start_time = currentTime();
  for(i=0; i<1; i++){
    fd_r=fopen(FILE_NAME, "r");
    if(!fd_r){
      perror("ERROR OPENING FILE\n");
      exit(1);
    }
  

    num_writes = FILE_SIZE/BLOCK_SIZE_WRITE;

    
    for(j=0; j<num_writes-1; j++){
      returnVal = fread(buffer, BLOCK_SIZE_WRITE, 1, fd_r);
      if(returnVal != 1){
        printf("J IS %lu\n", j*BLOCK_SIZE_WRITE);
        perror("ERROR READING FROM FILE");
        break;
      }      
    }    
    close(fd_r);
  }
  end_time = currentTime();    
  read_time = end_time-start_time;
  fprintf(stderr, "EDNING READ TEST\n");


  printf("%g %g\n", write_time, read_time);

  
  remove(FILE_NAME);
}