Why is multithreading slower than sequential programming in my case? - c

Question

I'm new to multithreading and try to learn it through a simple program, which adds 1 to n and return the sum. In the sequential case, the main call the sumFrom1 function twice for n = 1e5 and 2e5; in the multithreaded cases, two threads are created using pthread_create and two sums are calculated in separate thread. The multithreadting version is much slower than the sequential version (see results below). I run this on a 12-CPU platform and there are no communication between threads.
Multithreaded:
Thread 1 returns: 0
Thread 2 returns: 0
sum of 1..10000: 50005000
sum of 1..20000: 200010000
time: 156 seconds
Sequential:
sum of 1..10000: 50005000
sum of 1..20000: 200010000
time: 56 seconds
When I add -O2 in compilation, the time of multithreaded version (9s) is less than that of sequential version (11s), but not much as I expect. I can always have the -O2 flag on but I'm curious about the low speed of multithreading in the unoptimized case. Should it be slower than sequential version? If not, what can I do to make it faster?
The code:
#include <stdio.h>
#include <pthread.h>
#include <time.h>
typedef struct my_struct
{
int n;
int sum;
}my_struct_t;
void *sumFrom1(void* sit)
{
my_struct_t* local_sit = (my_struct_t*) sit;
int i;
int nsim = 500000; // Loops for consuming time
int j;
for(j = 0; j < nsim; j++)
{
local_sit->sum = 0;
for(i = 0; i <= local_sit->n; i++)
local_sit->sum += i;
}
}
int main(int argc, char *argv[])
{
pthread_t thread1;
pthread_t thread2;
my_struct_t si1;
my_struct_t si2;
int iret1;
int iret2;
time_t t1;
time_t t2;
si1.n = 10000;
si2.n = 20000;
if(argc == 2 && atoi(argv[1]) == 1) // Use "./prog 1" to test the time of multithreaded version
{
t1 = time(0);
iret1 = pthread_create(&thread1, NULL, sumFrom1, (void*)&si1);
iret2 = pthread_create(&thread2, NULL, sumFrom1, (void*)&si2);
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
t2 = time(0);
printf("Thread 1 returns: %d\n",iret1);
printf("Thread 2 returns: %d\n",iret2);
printf("sum of 1..%d: %d\n", si1.n, si1.sum);
printf("sum of 1..%d: %d\n", si2.n, si2.sum);
printf("time: %d seconds", t2 - t1);
}
else // Use "./prog" to test the time of sequential version
{
t1 = time(0);
sumFrom1((void*)&si1);
sumFrom1((void*)&si2);
t2 = time(0);
printf("sum of 1..%d: %d\n", si1.n, si1.sum);
printf("sum of 1..%d: %d\n", si2.n, si2.sum);
printf("time: %d seconds", t2 - t1);
}
return 0;
}
UPDATE1:
After a little googling on "false sharing" (Thanks, #Martin James!), I think it is the main cause. There are (at least) two ways to fix it:
The first way is inserting a buffer zone between the two structs (Thanks, #dasblinkenlight):
my_struct_t si1;
char memHolder[4096];
my_struct_t si2;
Without -O2, the time consuming decreases from ~156s to ~38s.
The second way is avoiding frequently updating sit->sum, which can be realized using a temp variable in sumFrom1 (as #Jens Gustedt replied):
for(int sum = 0, j = 0; j < nsim; j++)
{
sum = 0;
for(i = 0; i <= local_sit->n; i++)
sum += i;
}
local_sit->sum = sum;
Without -O2, the time consuming decreases from ~156s to ~35s or ~109s (It has two peaks! I don't know why.). With -O2, the time consuming stays ~8s.

Answer 1

By modifying your code to
typedef struct my_struct
{
size_t n;
size_t sum;
}my_struct_t;
void *sumFrom1(void* sit)
{
my_struct_t* local_sit = sit;
size_t nsim = 500000; // Loops for consuming time
size_t n = local_sit->n;
size_t sum = 0;
for(size_t j = 0; j < nsim; j++)
{
for(size_t i = 0; i <= n; i++)
sum += i;
}
local_sit->sum = sum;
return 0;
}
the phenomenon disappears. The problems you had:
using int as a datatype is completely wrong for such a test. Your
figures where such that the sum overflowed. Overflow of signed types is undefined behavior. You are lucky that it didn't eat your lunch.
having bounds and summation variables with indirection buys you
additional loads and stores, that in case of -O0 are really done as
such, with all the implications of false sharing and stuff like that.
Your code also observed other errors:
a missing include for atoi
superflouous cast to and from void*
printing of time_t as int
Please compile your code with -Wall before posting.