C multithread performance issue - c

Question

I am writing a multi-threaded program to traverse an n x n matrix, where the elements in the main diagonal are processed in a parallel manner, as shown in the code below:
int main(int argc, char * argv[] )
{
/* VARIABLES INITIALIZATION HERE */
gettimeofday(&start_t, NULL); //start timing
for (int slice = 0; slice < 2 * n - 1; ++slice)
{
z = slice < n ? 0 : slice - n + 1;
int L = 0;
pthread_t threads[slice-z-z+1];
struct thread_data td[slice-z-z+1];
for (int j=z; j<=slice-z; ++j)
{
td[L].index= L;
printf("create:%d\n", L );
pthread_create(&threads[L],NULL,mult_thread,(void *)&td[L]);
L++;
}
for (int j=0; j<L; j++)
{
pthread_join(threads[j],NULL);
}
}
gettimeofday(&end_t, NULL);
printf("Total time taken by CPU: %ld \n", ( (end_t.tv_sec - start_t.tv_sec)*1000000 + end_t.tv_usec - start_t.tv_usec));
return (0);
}
void *mult_thread(void *t)
{
struct thread_data *my_data= (struct thread_data*) t;
/* SOME ADDITIONAL CODE LINES HERE */
printf("ThreadFunction:%d\n", (*my_data).index );
return (NULL);
}
The problem is that this multithreaded implementation gave me a very bad performance compared with the serial (naive) implementation.
Are there some adjustments that could be done to improve the performance of the multithreaded version ??

Answer 1

a thread pool may make it better.
define a new struct type as follow.
typedef struct {
struct thread_data * data;
int status; // 0: ready
// 1: adding data
// 2: data handling, 3: done
int next_free;
} thread_node;
init :
size_t thread_size = 8;
thread_node * nodes = (thread_node *)malloc(thread_size * sizeof(thread_node));
for(int i = 0 ; i < thread_size - 1 ; i++ ) {
nodes[i].next_free = i + 1;
nodes[i].status = 0 ;
}
nodes[thread_size - 1].next_free = -1;
int current_free_node = 0 ;
pthread_mutex_t mutex;
get thread :
int alloc() {
pthread_mutex_lock(&mutex);
int rt = current_free_node;
if(current_free_node != -1) {
current_free_node = nodes[current_free_node].next_free;
nodes[rt].status = 1;
}
pthread_mutex_unlock(&mutex);
return rt;
}
return thread :
void back(int idx) {
pthread_mutex_lock(&mutex);
nodes[idx].next_free = current_free_node;
current_free_node = idx;
nodes[idx].status = 0;
pthread_mutex_unlock(&mutex);
}
create the threads first, and use alloc() to try to get a idle thread, update the pointer.
don't use join to judge the status.
modify your mult_thread as a loop and after the job finished , just change your status to 3
for each loop in the thread , you may give it more work
I wish it will give you some help.
------------ UPDATED Apr. 23, 2015 -------------------
here is a example.
compile & run with command
$ g++ thread_pool.cc -o tp -pthread --std=c++
yu:thread_pool yu$ g++ tp.cc -o tp -pthread --std=c++11 && ./tp
1227135.147 1227176.546 1227217.944 1227259.340...
time cost 1 : 1068.339091 ms
1227135.147 1227176.546 1227217.944 1227259.340...
time cost 2 : 548.221607 ms
you may also remove timer and it can also compiled as a std c99 file.
In current , the thread size has been limited to 2. You may also adjust the parameter thread_size, and recompile & run again. More threads may give your some more advantage(in my pc, if I change the thread size to 4, the task will finish in 280ms), while too much thread number may not help you too much if you have no enough cpu thread.