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Howto test performance of data alignment

This article introduces howto write a program that tests which data alignment is most suitable for better memory I/O operations in a target architecture.

Data alignment

Data alignment is an important issue for all programmers who directly use memory. Data alignment affects how well your software performs, and even if your software runs at all Read more on data alignment from IBM ...

The performance of various alignments depends on three main factors :

Whether the processor has hardware support for unaligned access
The memory access granularity
Buffer size used to access memory

Program for testing data alignment performance

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>
#include <unistd.h>
 
#define DATASIZE 1000000000
uint32_t        stampstart();
uint32_t        stampstop(uint32_t start);
void            Munge32(void *data, uint32_t size);
void            Munge64(void *data, uint32_t size);
 
#ifndef MUNGE64
#define MUNGE Munge32
#else
#define MUNGE Munge64
#endif
 
int
main(int argc, char **argv)
{
 
	uint32_t        align;
	uint8_t        *data;
	uint32_t        start;
	uint8_t        *unaligned_data __attribute__ ((aligned(2)));
	uint8_t        *aligned_data8 __attribute__ ((aligned(8)));
	uint8_t        *aligned_data16 __attribute__ ((aligned(16)));
	uint8_t        *aligned_data32 __attribute__ ((aligned(32)));
 
	if (argc != 2)
	  {
		  printf("usage : %s alignment\n", argv[0]);
		  exit(-1);
	  }
 
	align = atoi(argv[1]);
 
	switch (align)
	  {
	  case 8:
		  printf("\nData aligned at 8 bits\n");
		  start = stampstart();
		  aligned_data8 = malloc(DATASIZE);
		  MUNGE(aligned_data8, DATASIZE);
		  free(aligned_data8);
		  stampstop(start);
		  break;
 
	  case 16:
		  printf("\nData aligned at 16 bits\n");
		  start = stampstart();
		  aligned_data16 = malloc(DATASIZE);
		  MUNGE(aligned_data16, DATASIZE);
		  free(aligned_data16);
		  stampstop(start);
		  break;
 
	  case 32:
		  printf("\nData aligned at 32 bits\n");
		  start = stampstart();
		  aligned_data32 = malloc(DATASIZE);
		  MUNGE(aligned_data32, DATASIZE);
		  free(aligned_data32);
		  stampstop(start);
		  break;
 
	  default:		/* Unalign data */
		  printf("\nData unaligned \n");
		  start = stampstart();
		  unaligned_data = malloc(DATASIZE);
		  MUNGE(unaligned_data, DATASIZE);
		  free(unaligned_data);
		  stampstop(start);
	  }
 
	return 0;
 
}
 
void
Munge64(void *data, uint32_t size)
{
 
	double         *data64 = (double *) data;
	double         *data64End = data64 + (size >> 3);	/* Divide size by 8. */
	uint8_t        *data8 = (uint8_t *) data64End;
	uint8_t        *data8End = data8 + (size & 0x00000007);	/* Strip upper 29 bits. */
 
	printf("Using 64bit blocks\n");
 
	while (data64 != data64End)
	  {
		  *data64++ = -*data64;
	  }
	while (data8 != data8End)
	  {
		  *data8++ = -*data8;
	  }
}
 
void
Munge32(void *data, uint32_t size)
{
 
	uint32_t       *data32 = (uint32_t *) data;
	uint32_t       *data32End = data32 + (size >> 2);	/* Divide size by 4. */
	uint8_t        *data8 = (uint8_t *) data32End;
	uint8_t        *data8End = data8 + (size & 0x00000003);	/* Strip upper 30 bits. */
 
	printf("Using 32bit blocks\n");
 
	while (data32 != data32End)
	  {
		  *data32++ = -*data32;
	  }
	while (data8 != data8End)
	  {
		  *data8++ = -*data8;
	  }
}
 
uint32_t
stampstart()
{
	struct timeval  tv;
	struct timezone tz;
	struct tm      *tm;
	uint32_t        start;
 
	gettimeofday(&tv, &tz);
	tm = localtime(&tv.tv_sec);
 
	printf("TIMESTAMP-START\t  %d:%02d:%02d:%d (~%d ms)\n", tm->tm_hour,
	       tm->tm_min, tm->tm_sec, tv.tv_usec,
	       tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
	       tm->tm_sec * 1000 + tv.tv_usec / 1000);
 
	start = tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
		tm->tm_sec * 1000 + tv.tv_usec / 1000;
 
	return (start);
 
}
 
uint32_t
stampstop(uint32_t start)
{
 
	struct timeval  tv;
	struct timezone tz;
	struct tm      *tm;
	uint32_t        stop;
 
	gettimeofday(&tv, &tz);
	tm = localtime(&tv.tv_sec);
 
	stop = tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
		tm->tm_sec * 1000 + tv.tv_usec / 1000;
 
	printf("TIMESTAMP-END\t  %d:%02d:%02d:%d (~%d ms) \n", tm->tm_hour,
	       tm->tm_min, tm->tm_sec, tv.tv_usec,
	       tm->tm_hour * 3600 * 1000 + tm->tm_min * 60 * 1000 +
	       tm->tm_sec * 1000 + tv.tv_usec / 1000);
 
	printf("ELAPSED\t  %d ms\n", stop - start);
 
	return (stop);
 
}