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POHow completely unused memory can be fragmented?
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copied!<p>Following C code example (taken from <a href="http://bugs.python.org/issue19246" rel="nofollow">http://bugs.python.org/issue19246</a>) executed on Windows 7 64-bit, while compiled in 32-bit mode</p> <pre><code>#include <stdio.h> #include <stdlib.h> #include <assert.h> int create_huge_linked_list(void **last_item) { int i; void *prev_item = NULL; for(i = sizeof(void *); i < 1000000; i++) { void *new_item = malloc(i); if(new_item == NULL) { break; } *(void **)new_item = prev_item; prev_item = new_item; } *last_item = prev_item; return i; } void free_linked_list(void *last_item) { while(last_item != NULL) { void *prev_item = *(void **)last_item; free(last_item); last_item = prev_item; } } int stress_heap() { void *last_item; int amount = create_huge_linked_list(&last_item); free_linked_list(last_item); return amount; } void stress_twice(void) { int first = stress_heap(); int second = stress_heap(); printf("%i %i %f%%\n", first, second, 100.0 * second / first); } void stress_and_alloc_1_mb() { void *ptr; ptr = malloc(1000000); if(ptr != NULL) { printf("Successfully allocated 1 MB before stress\n"); free(ptr); stress_heap(); ptr = malloc(1000000); if(ptr != NULL) { printf("Successfully allocated 1 MB after stress\n"); free(ptr); } else { printf("Failed to allocate 1 MB after stress\n"); } } else { printf("Failed to allocate 1 MB before stress\n"); } } int main() { stress_and_alloc_1_mb(); stress_twice(); return 0; } </code></pre> <p>Outputs:</p> <pre><code>Successfully allocated 1 MB before stress Failed to allocate 1 MB after stress 64855 64857 100.003084% </code></pre> <p>Result may be interpreted as: after whole memory allocation and then freeing it, memory of the process is fragmented so bad, that there is no chunk with length of 1 MB. However, the stress procedure can be repeated continuously without memory errors.</p> <p>The questions are:</p> <ol> <li>How memory can be fragmented if it is completely unused?</li> <li>How memory fragmentation can be fixed for that process?</li> </ol>

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