a question about pthread performance Post: 302267180

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Top Forums Programming a question about pthread performance Post 302267180 by mgig on Thursday 11th of December 2008 07:25:55 PM

12-11-2008

Registered User

a question about pthread performance

Hello,

I run my pthread code on Linux with 4 processors. However, the speed up is only 2 times.

The code is about solving equation (G+s(i)C)z(i)=B*us(i), i=1,...,n. Here G,C are m*m matrix, B*us(i) is a m*1 vector and s(i) are n different numbers. I need to solve the equation n times to get z(1)...z(n). Here, I use multithread to solve the n equations.
eg. Now I have 4 thread and n=12. Thread(1) solves 4 equations (G+s(1,2,3,4)C)X(1,2,3,4)=B. Thread(2) solves (G+s(5,6,7,8)C)X(5,6,7,8)=B....

I use pthread_creat() and pthread_join(). However, for 4thread, it is not as 4 times faster as 1 thread, only 2 times instead. For 2 thread, it is about 1.5 times as 1 thread. What I have observed that, to solve one equation under 4 thread is much slower than solving one equation using 1 thread.

Can anybody tell me what is the reason? Thanks a lot.

Below is the code. Thanks,

Code:

typedef struct{
  int thread_no;
  int allthread;
  mat *G,*C,*B;
  mat *us, *Z;
  vec *samples;
  double *Control, *Info;
} parm;

 
 void sampleLU(int thread_no, int allthread, mat *G, mat *C, mat *B, mat &us, mat &Z, vec &samples, double Control[], double Info[])
{
  Real_Timer lu_symbolic_init, lu_symbolic_free, lu_numerical, lu_solve_time;
  Real_Timer sCpG_run_time;
  int np = samples.size();
  int nDim = B->m;
  int start, stop;
 
  start = thread_no * (int)(np/allthread);
  stop = start + (int)(np/allthread) - 1;
  if( thread_no == allthread-1 ) stop = np-1; 
  for (int i = start; i<=stop; i++ ){
    cs *A;
    if(thread_no == 0) {
      sCpG_run_time.start();
      A = G+sample(i)*C;
      sCpG_run_time.stop();
    }
    else{
      A = G+sample(i)*C;
    }
 
 /* LU decomposition */
...
 if(i == start){
   if(thread_no == 0) {
     lu_symbolic_init.start();
     A = LU;
     lu_symbolic_init.stop();
   }
   else {
    A = LU
   }
 }

 /* solve Az = b  */
 double* z = new double[nDim];
 vec b(nDim);
 b.zeros();
 if(thread_no == 0) lu_solve_time.start();
 LUz=b
 vec zz(z, nDim);
 delete [] z;
 Z.set_col(i, zz); 
 ...
  }
  if(thread_no == 0){
    std::cout << "Thread" << thread_no << " sC+G \t:" << sCpG_run_time.get_time() << std::endl;
    std::cout << "Thread" << thread_no << " symbolic initial time: \t"<<lu_symbolic_init.get_time()<<std::endl;
    std::cout << "Thread" << thread_no << " LU decomposition time \t:" << lu_numerical.get_time() << std::endl;
    std::cout << "Thread" << thread_no << " symbolic free time: \t"<<lu_symbolic_init.get_time()<<std::endl;
    std::cout << "Thread" << thread_no << " LU solve time   \t: " << lu_solve_time.get_time() <<std::endl;
    
  }
}
void * psampleLU(void *arg)
{
  parm *p = (parm *)arg;
  sampleLU(p->thread_no, p->allthread, p->G, p->C, p->B, *(p->us), *(p->Z), *(p->samples), p->Control, p->Info);
  return NULL;
}

Last edited by otheus; 12-15-2008 at 09:24 AM.. Reason: Added code tags

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LEARN ABOUT SUNOS

pthread.h

pthread.h(3HEAD)						      Headers							  pthread.h(3HEAD)

NAME

       pthread.h, pthread - threads

SYNOPSIS

       #include <pthread.h>

DESCRIPTION

       The <pthread.h> header defines the following symbols:

       PTHREAD_BARRIER_SERIAL_THREAD
       PTHREAD_CANCEL_ASYNCHRONOUS
       PTHREAD_CANCEL_ENABLE
       PTHREAD_CANCEL_DEFERRED
       PTHREAD_CANCEL_DISABLE
       PTHREAD_CANCELED
       PTHREAD_COND_INITIALIZER
       PTHREAD_CREATE_DETACHED
       PTHREAD_CREATE_JOINABLE
       PTHREAD_EXPLICIT_SCHED
       PTHREAD_INHERIT_SCHED
       PTHREAD_MUTEX_DEFAULT
       PTHREAD_MUTEX_ERRORCHECK
       PTHREAD_MUTEX_INITIALIZER
       PTHREAD_MUTEX_NORMAL
       PTHREAD_MUTEX_RECURSIVE
       PTHREAD_ONCE_INIT
       PTHREAD_PRIO_INHERIT
       PTHREAD_PRIO_NONE
       PTHREAD_PRIO_PROTECT
       PTHREAD_PROCESS_SHARED
       PTHREAD_PROCESS_PRIVATE
       PTHREAD_SCOPE_PROCESS
       PTHREAD_SCOPE_SYSTEM

       The types listed below are defined as described in <sys/types.h>. See types.h(3HEAD).

       pthread_attr_t
       pthread_barrier_t
       pthread_barrierattr_t
       pthread_cond_t
       pthread_condattr_t
       pthread_key_t
       pthread_mutex_t
       pthread_mutexattr_t
       pthread_once_t
       pthread_rwlock_t
       pthread_rwlockattr_t
       pthread_spinlock_t
       pthread_t

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       sched.h(3HEAD),	  time.h(3HEAD),    types.h(3HEAD),   pthread_attr_getguardsize(3C),   pthread_attr_init(3C),	pthread_attr_setscope(3C),
       pthread_cancel(3C),   pthread_cleanup_pop(3C),	pthread_cond_init(3C),	 pthread_cond_signal(3C),   pthread_cond_wait(3C),    pthread_con-
       dattr_init(3C),	pthread_create(3C), pthread_detach(3C), pthread_equal(3C), pthread_exit(3C), pthread_getconcurrency(3C), pthread_getsched-
       param(3C),    pthread_join(3C),	  pthread_key_create(3C),    pthread_key_delete(3C),	pthread_mutex_init(3C),    pthread_mutex_lock(3C),
       pthread_mutex_setprioceiling(3C),   pthread_mutexattr_init(3C),	 pthread_mutexattr_init(3C),   pthread_mutexattr_init(3C),  pthread_mutex-
       attr_gettype(3C),     pthread_mutexattr_getprotocol(3C),     pthread_once(3C),	  pthread_rwlock_init(3C),	pthread_rwlock_rdlock(3C),
       pthread_rwlock_unlock(3C),  pthread_rwlock_wrlock(3C),  pthread_rwlockattr_getpshared(3C),  pthread_rwlockattr_init(3C),  pthread_self(3C),
       pthread_setcancelstate(3C), pthread_setspecific(3C), attributes(5), standards(5)

SunOS 5.10							    9 Sep 2004							  pthread.h(3HEAD)