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Full Discussion: Use of alloca function
Top Forums Programming Use of alloca function Post 302366529 by achenle on Thursday 29th of October 2009 07:33:34 PM
Old 10-29-2009
Why use alloca()? Oh, how about a few orders of magnitude faster performance under multithreaded heap contention?

This

Code:
malloc() took 103497088262 ns
alloca() took 135368983 ns

came from this code:

Code:
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <alloca.h>

#define BUFSIZE 8192
#define NUM_THR 8

static volatile int flag = 0;

typedef void ( *func_ptr )( void );

void test_malloc( void )
{
    void *ptr;
    ptr = malloc( BUFSIZE );
    free( ptr );
    return;
}

void test_alloca( void )
{
    void *ptr;
    ptr = alloca( BUFSIZE );
    return;
}

void *run( void *arg )
{
    func_ptr f;
    int ii;

    if ( flag )
        f = test_malloc;
    else
        f = test_alloca;
    for ( ii = 0; ii < ( 10 * 1024 * 1024 ); ii++ )
        f();
    return( NULL );
}

int main( int argc, char **argv )
{
    pthread_t tids[ NUM_THR ];
    hrtime_t start;
    hrtime_t end;
    void *status;
    int ii;

    flag = 1;
    start = gethrtime();
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_create( &tids[ ii ], NULL, run, NULL );
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_join( tids[ ii ], &status );
    end = gethrtime();
    printf( "malloc() took %lld ns\n", end - start );

    flag = 0;
    start = gethrtime();
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_create( &tids[ ii ], NULL, run, NULL );
    for ( ii = 0; ii < NUM_THR; ii++ )
        pthread_join( tids[ ii ], &status );
    end = gethrtime();
    printf( "alloca() took %lld ns\n", end - start );
    return( 0 );
}

run under Solaris 10 running on an IBM e326m with 2 dual-core 2.4 GHz Opterons:
 

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LEARN ABOUT PHP

gethrtime

gethrtime(3C)						   Standard C Library Functions 					     gethrtime(3C)

NAME

       gethrtime, gethrvtime - get high resolution time

SYNOPSIS

       #include <sys/time.h>

       hrtime_t gethrtime(void);

       hrtime_t gethrvtime(void);

DESCRIPTION

       The  gethrtime()  function returns the current high-resolution real time. Time is expressed as nanoseconds since some arbitrary time in the
       past; it is not correlated in any way to the time of day, and thus is not subject to resetting or drifting by way of adjtime(2) or settime-
       ofday(3C). The hi-res timer is ideally suited to performance measurement tasks, where cheap, accurate interval timing is required.

       The gethrvtime() function returns the current high-resolution LWP virtual time, expressed as total nanoseconds of execution time.

       The gethrtime() and gethrvtime() functions both return an hrtime_t, which is a 64-bit (long long) signed integer.

EXAMPLES

       The following code fragment measures the average cost of getpid(2):

       hrtime_t start, end;
       int i, iters = 100;

       start = gethrtime();
       for (i = 0; i < iters; i++)
	       getpid();
       end = gethrtime();

       printf("Avg getpid() time = %lld nsec
", (end - start) / iters);

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |MT-Safe			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), adjtime(2), gettimeofday(3C), settimeofday(3C), attributes(5)

NOTES

       Although  the  units of hi-res time are always the same (nanoseconds), the actual resolution is hardware dependent.  Hi-res time is guaran-
       teed to be monotonic (it won't go backward, it won't periodically wrap) and linear (it won't occasionally speed up or slow down for adjust-
       ment, like the time of day can), but not necessarily unique: two sufficiently proximate calls may return the same value.

SunOS 5.10							    7 Sep 2004							     gethrtime(3C)
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