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Reply-To: moss AT cs DOT umass DOT edu
Subject: Re: calloc speed difference
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From: Eliot Moss <moss AT cs DOT umass DOT edu>
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Date: Fri, 12 Jan 2018 17:00:24 -0500
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On 1/12/2018 9:06 AM, Christian Franke wrote:

> This variant of the above code adds one write access to each 4KiB page (guarded by "volatile" to 
> prevent dead assignment optimization):
> 
> #include <stdio.h>
> #include <stdlib.h>
> #define ALLOCATION_SIZE (100 * 1024 * 1024)
> int main (int argc, char *argv[]) {
>      for (int i = 0; i < 1000; i++) {
>          void *temp = calloc(ALLOCATION_SIZE, 1);
>          if ( temp == NULL ) {
>             printf("drat! calloc returned NULL\n");
>             return 1;
>          }
>          for (int j = 0; j < ALLOCATION_SIZE; j += 4096)
>            ((volatile char *)temp)[j] = (char)i;
>          free(temp);
>      }
>      return 0;
> }
> 
> Results:
> 
> Cygwin: ~310s
> MinGW: ~210s

Good analysis!  There remains a lot of room for improvement, but this
shows good reasons to dig deeper to understand what goes on with large
allocations.

EM

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