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I have the following task to demonstrate false sharing and wrote a simple program: #include <sys/times.h> #include <time.h> #include <stdio.h> #include <pthread.h> long long int tmsBegin1,tmsEnd1,tmsBegin2,tmsEnd2,tmsBegin3,tmsEnd3; int array[100]; void *heavy_loop(void *param) { int index = *((int*)param); int i; for (i = 0; i < 100000000; i++) array[index]+=3; } int main(int argc, char *argv[]) { int first_elem = 0; int bad_elem = 1; int good_elem = 32; long long time1; long long time2; long long time3; pthread_t thread_1; pthread_t thread_2; tmsBegin3 = clock(); heavy_loop((void*)&first_elem); heavy_loop((void*)&bad_elem); tmsEnd3 = clock(); tmsBegin1 = clock(); pthread_create(&thread_1, NULL, heavy_loop, (void*)&first_elem); pthread_create(&thread_2, NULL, heavy_loop, (void*)&bad_elem); pthread_join(thread_1, NULL); pthread_join(thread_2, NULL); tmsEnd1 = clock(); tmsBegin2 = clock(); pthread_create(&thread_1, NULL, heavy_loop, (void*)&first_elem); pthread_create(&thread_2, NULL, heavy_loop, (void*)&good_elem); pthread_join(thread_1, NULL); pthread_join(thread_2, NULL); tmsEnd2 = clock(); printf("%d %d %d\n", array[first_elem],array[bad_elem],array[good_elem]); time1 = (tmsEnd1-tmsBegin1)*1000/CLOCKS_PER_SEC; time2 = (tmsEnd2-tmsBegin2)*1000/CLOCKS_PER_SEC; time3 = (tmsEnd3-tmsBegin3)*1000/CLOCKS_PER_SEC; printf("%lld ms\n", time1); printf("%lld ms\n", time2); printf("%lld ms\n", time3); return 0; } I was very surprised when I saw the results (I run it on my i5-430M processor). With false sharing, it was 1020 ms. Without false sharing, it was 710 ms, only 30% faster instead of 300% (it was written on some sites that it would be faster than 300-400%). Without using pthreads, it was 580 ms.

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