File: thread_pool.c

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/*
**      PJL C++ Library
**      thread_pool.c
**
**      Copyright (C) 1998  Paul J. Lucas
**
**      This program is free software; you can redistribute it and/or modify
**      it under the terms of the GNU General Public License as published by
**      the Free Software Foundation; either version 2 of the License, or
**      (at your option) any later version.
**
**      This program is distributed in the hope that it will be useful,
**      but WITHOUT ANY WARRANTY; without even the implied warranty of
**      MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
**      GNU General Public License for more details.
**
**      You should have received a copy of the GNU General Public License
**      along with this program; if not, write to the Free Software
**      Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

#ifdef  MULTI_THREADED

// standard
#include <cstdlib>                      /* for exit(2) */
#include <time.h>
#ifdef  DEBUG_threads
#include <iostream>
#endif

// local
#include "platform.h"
#include "thread_pool.h"
#include "util.h"

extern char const   *me;

using namespace std;

namespace PJL {

pthread_key_t       thread_pool::thread::thread_obj_key_;

//
// Macros to wrap critical sections.
//
#define DEFER_CANCEL() \
    { int cancel_type; \
    ::pthread_setcanceltype( PTHREAD_CANCEL_DEFERRED, &cancel_type )

#define RESTORE_CANCEL() \
    ::pthread_setcanceltype( cancel_type, 0 ); \
    ::pthread_testcancel(); }

//
// Define a more robust mutex locking mechanism by ensuring that the mutex will
// be unlocked even if the thread is cancelled.  Also roll in optional critical
// section protection since we're doing pthread_setcanceltype() anyway.
//
// See also: Bradford Nichols, Dick Buttlar, and Jacqueline Proulx Farrell.
// "Pthreads Programming," O'Reilly & Associates, Sebastopol, CA, 1996.
// pp. 141-142.
//
#define MUTEX_LOCK(M,D) { \
    int cancel_type; bool const defer_cancel = (D); \
    ::pthread_setcanceltype( PTHREAD_CANCEL_DEFERRED, &cancel_type ); \
    pthread_cleanup_push( (void (*)(void*))::pthread_mutex_unlock, (M) ); \
    ::pthread_mutex_lock( (M) ); \
    if ( D ) ; else { \
        ::pthread_setcanceltype( cancel_type, 0 ); \
        ::pthread_testcancel(); \
    }

#define MUTEX_UNLOCK() \
    pthread_cleanup_pop( 1 ); \
    if ( defer_cancel ) { \
        ::pthread_setcanceltype( cancel_type, 0 ); \
        ::pthread_testcancel(); \
    } \
}

//*****************************************************************************
//
// SYNOPSIS
//
        void thread_pool_decrement_busy( void *p )
//
// DESCRIPTION
//
//      This is a clean-up function that is called to decrement the number of
//      threads that are "busy" performing a task.  It gets called either when
//      (1) thread::main() completes its task or (2) if the thread gets killed
//      while performing its task.  A clean-up function is used to guarantee
//      that this happends for case (2).
//
// NOTE
//
//      This function is declared extern "C" since it is called via the C
//      library function pthread_cleanup_push() and, because it's a C function,
//      it expects C linkage.
//
// PARAMETERS
//
//      p   Pointer to an instance of a thread_pool::thread.
//
//*****************************************************************************
{
    thread_pool::thread *const t = static_cast<thread_pool::thread*>( p );
    MUTEX_LOCK( &t->pool_.t_busy_lock_, true );
    --t->pool_.t_busy_;
    MUTEX_UNLOCK();
}

//*****************************************************************************
//
// SYNOPSIS
//
        extern "C" void thread_pool_thread_data_cleanup( void *p )
//
// DESCRIPTION
//
//      This is a thread-specific data "destructor" (in the POSIX thread sense,
//      not in the C++ sense).  Its job is to destroy this POSIX thread's
//      associated thread object.
//
// PARAMETERS
//
//      p   Pointer to an instance of a thread_pool::thread.
//
//*****************************************************************************
{
    thread_pool::thread *const t = static_cast<thread_pool::thread*>( p );
#   ifdef DEBUG_threads
    cerr << "thread_pool_thread_data_cleanup(" << (unsigned long)t << ')'
         << endl;
#   endif
    if ( t ) {
        //
        // The thread object's destructor hasn't been run, i.e., this thread
        // has been exited/killed directly somehow.  Therefore, destroy our
        // associated thread object.  But set a flag in the thread object so
        // its destructor won't try to exit/kill the thread again.
        //
        t->in_cleanup_ = true;
        delete t;
    }
}

//*****************************************************************************
//
// SYNOPSIS
//
        void* thread_pool_thread_main( void *p )
//
// DESCRIPTION
//
//      This is the starting point of execution for a POSIX thread.  It waits
//      for a task to appear in its thread pool's task queue and performs the
//      task.  After completion, it waits for the next task.
//
//      If more threads currently exist than the minimum requested (because
//      more were created as a result of a high request load), then only wait a
//      finite amount of time for a task to appear.  If no task appears in that
//      amount of time, then the request load must've lessened so we can
//      destroy this POSIX thread.
//
// NOTE
//
//      This function is declared extern "C" since it is called via the C
//      library function pthread_create() and, because it's a C function, it
//      expects C linkage.
//
// PARAMETERS
//
//      p   Pointer to an instance of a thread_pool::thread.
//
// RETURN VALUE
//
//      This function never returns.  It goes away only when the thread is
//      destroyed.  The "return 0" at the end is just to make the C++ compiler
//      happy.
//
//*****************************************************************************
{
    int result = ::pthread_detach( ::pthread_self() );
    if ( result ) {
        error() << "could not detach thread" << error_string( result );
        ::exit( Exit_No_Detach_Thread );
    }
    register thread_pool::thread *const t =
        static_cast<thread_pool::thread*>( p );

    //
    // Put a copy of the pointer to the thread object into thread-specific data
    // so it can serve as a flag whether to run the clean-up function
    // thread_pool_thread_data_cleanup().
    //
    static pthread_once_t thread_once = PTHREAD_ONCE_INIT;
    ::pthread_once( &thread_once, thread_pool_thread_once );
    ::pthread_setspecific( thread_pool::thread::thread_obj_key_, p );

    //
    // We need to wait for the "run" mutex to become unlocked before continuing
    // to run the main() function to ensure that the thread pool object to
    // which we belong has been fully constructed before we access its data
    // members.  (It becomes unlocked when the run() member function is
    // called.)
    //
    MUTEX_LOCK( &t->run_lock_, false );
    MUTEX_UNLOCK();
    //
    // It served its only purpose so destroy it.
    //
    ::pthread_mutex_destroy( &t->run_lock_ );

    while ( true ) {

#       ifdef DEBUG_threads
        cerr << "thread_pool_thread_main(): waiting for task" << endl;
#       endif

        thread_pool::thread::argument_type arg;
        result = 0;

        MUTEX_LOCK( &t->pool_.q_lock_, false );
        while ( t->pool_.queue_.empty() ) {
            bool no;
            MUTEX_LOCK( &t->pool_.t_lock_, false );
            no = t->pool_.threads_.size() <= t->pool_.min_threads_;
            ::pthread_cond_signal( &t->pool_.t_idle_ );
            MUTEX_UNLOCK();
            if ( no ) {
                //
                // There are no threads beyond those originally created: signal
                // that we're idle and wait indefinitely for a task.
                //
                ::pthread_cond_wait(
                    &t->pool_.q_not_empty_,
                    &t->pool_.q_lock_
                );
                result = 0;             // ignore possible prior timeout
                continue;
            }
            //
            // More threads exist than the minimum.  Check to see if we timed
            // out waiting for a task on the last loop.
            //
            if ( result == ETIMEDOUT ) {
                //
                // No task became available: commit suicide by deleting the
                // instance of the thread: it will exit the POSIX thread so the
                // "delete" below will never return.
                //
                delete t;
                internal_error
                    << "thread_pool_thread_main(): thread exists after "
                       "alleged destruction"
                    << report_error;
            }
            //
            // Wait only a finite amount of time for a task to become
            // available.
            //
            struct timespec future;
            future.tv_sec = ::time( 0 ) + t->pool_.timeout_;
            future.tv_nsec = 0;
            result = ::pthread_cond_timedwait(
                &t->pool_.q_not_empty_, &t->pool_.q_lock_, &future
            );
            //
            // Loop around again to retest the condition that there are still
            // more threads than the minimum.  We want to be absolutely sure
            // before we commit suicide.
            //
        }

#       ifdef DEBUG_threads
        cerr << "thread_pool_thread_main(): got task" << endl;
#       endif

        DEFER_CANCEL();
        arg = t->pool_.queue_.front();
        t->pool_.queue_.pop();
        RESTORE_CANCEL();
        MUTEX_UNLOCK();                 // t->pool_.q_lock_

        MUTEX_LOCK( &t->pool_.t_busy_lock_, true );
        ++t->pool_.t_busy_;
        MUTEX_UNLOCK();
        pthread_cleanup_push( thread_pool_decrement_busy, t );

#       ifdef DEBUG_threads
        cerr << "thread_pool_thread_main(): performing task" << endl;
#       endif

        t->main( arg );                 // do the real work

#       ifdef DEBUG_threads
        cerr << "thread_pool_thread_main(): completed task" << endl;
#       endif

        pthread_cleanup_pop( true );
    }

    //
    // We never get here due to the infinite loop above, but put a return
    // statement here just to make the compiler happy.
    //
    return 0;
}

//*****************************************************************************
//
// SYNOPSIS
//
        void thread_pool_thread_once()
//
// DESCRIPTION
//
//      Perform initialzation for all threads exactly once.  Currently, create
//      a thread-specific data key.
//
// NOTE
//
//      This function is declared extern "C" since it is called via the C
//      library function pthread_create() and, because it's a C function, it
//      expects C linkage.
//
//*****************************************************************************
{
    int const result = ::pthread_key_create(
        &thread_pool::thread::thread_obj_key_, thread_pool_thread_data_cleanup
    );
    if ( result ) {
        error() << "could not create thread key" << error_string( result )
                << endl;
        ::exit( Exit_No_Create_Thread_Key );
    }
}

//*****************************************************************************
//
// SYNOPSIS
//
        thread_pool::thread::thread(
            thread_pool &p, thread_start_function_type start_func
        )
//
// DESCRIPTION
//
//      Construct (initialize) a thread by creating a POSIX thread and passing
//      it a pointer to ourselves.
//
// PARAMETERS
//
//      p           The thread_pool to which this thread belongs.
//
//      start_func  The function that is called upon thread creation.
//
//*****************************************************************************
    : in_cleanup_( false ), pool_( p )
{
#   ifdef DEBUG_threads
    cerr << "thread::thread(" << (unsigned long)this << ')' << endl;
#   endif

    //
    // Create a locked "run" mutex that the soon-to-be-created thread will wait
    // for before running the code in the start function that accesses our data
    // members.  The only reason this is necessary is because the prototype
    // thread that is passed to the thread_pool constructor is created before
    // the thread_pool is fully constructed.
    //
    if ( ::pthread_mutex_init( &run_lock_, 0 ) ) {
        error() << "could not init thread mutex" << endl;
        ::exit( Exit_No_Init_Thread_Mutex );
    }
    ::pthread_mutex_lock( &run_lock_ );

    int const result = ::pthread_create( &thread_, 0, start_func, this );
    if ( result ) {
        error() << "could not create thread" << error_string( result );
        ::exit( Exit_No_Create_Thread );
    }
}

//*****************************************************************************
//
// SYNOPSIS
//
        thread_pool::thread::~thread()
//
// DESCRIPTION
//
//      Destroy a thread.
//
//*****************************************************************************
{
#   ifdef DEBUG_threads
    cerr << "thread::~thread(" << (unsigned long)this << ')' << endl;
#   endif

    DEFER_CANCEL();
    if ( !pool_.destructing_ ) {
        //
        // We are committing suicide.  But first, we have to delete the pointer
        // to us in our thread pool's set of threads.
        //
        MUTEX_LOCK( &pool_.t_lock_, false );
        pool_.threads_.erase( this );
        MUTEX_UNLOCK();
    } else {
        //
        // The thread pool to which we belong has had its destructor called and
        // is in the process of destroying itself and us.  Therefore, we don't
        // have to do anything.
        //
    }

    if ( !in_cleanup_ ) {
        //
        // This destructor is not being called via the thread-specific data
        // destructor thread_pool_thread_data_cleanup(): null-out the
        // thread-specific data so thread_pool_thread_data_cleanup() won't be
        // called when this thread terminates.
        //
        ::pthread_setspecific( thread_obj_key_, 0 );
        //
        // Since we're not being called via the thread-specific data destructor
        // thread_pool_thread_data_cleanup(), kill our associated POSIX thread.
        //
        if ( ::pthread_equal( thread_, ::pthread_self() ) ) {
            //
            // This thread is committing suicide because there are more than
            // the minimum number of threads and it timed out waiting for a
            // task: call pthread_exit() in this case since it's cleaner than
            // pthread_cancel().
            //
            ::pthread_exit( 0 );
        } else {
            //
            // This destructor is actually running in a thread that is
            // different from the one being terminated.
            //
            ::pthread_cancel( thread_ );
        }
    }
    RESTORE_CANCEL();
}

//*****************************************************************************
//
// SYNOPSIS
//
        thread_pool::thread_pool(
            thread *prototype, int min_threads, int max_threads, int timeout
        )
//
// DESCRIPTION
//
//      Construct (initialize) a thread_pool creating as many threads as
//      requested.
//
// PARAMETERS
//
//      prototype       A pointer to an instance of a class derived from
//                      "thread" used to create new instances of itself.
//
//      min_threads     The minimum number of threads to keep available.
//
//      max_threads     The maximum number of threads to allow.
//
//      timeout         The number of seconds for a thread to wait for a task
//                      before committing suicide, but only if more than
//                      min_threads presently exist.
//
//*****************************************************************************
    : min_threads_( min_threads ), max_threads_( max_threads ), t_busy_( 0 ),
      destructing_( false ), timeout_( timeout )
{
    if ( ::pthread_mutex_init( &t_busy_lock_, 0 ) ||
         ::pthread_mutex_init( &q_lock_, 0 ) ||
         ::pthread_mutex_init( &t_lock_, 0 )
    ) {
        error() << "could not init thread mutex" << endl;
        ::exit( Exit_No_Init_Thread_Mutex );
    }
    if ( ::pthread_cond_init( &q_not_empty_, 0 ) ||
         ::pthread_cond_init( &t_idle_, 0 )
    ) {
        error() << "could not init thread condition" << endl;
        ::exit( Exit_No_Init_Thread_Condition );
    }

    MUTEX_LOCK( &t_lock_, true );
    threads_.insert( prototype );
    prototype->run();
    for ( int i = 1; i < min_threads_; ++i )
        threads_.insert( prototype->create_and_run() );
    MUTEX_UNLOCK();
}

//*****************************************************************************
//
// SYNOPSIS
//
        thread_pool::~thread_pool()
//
// DESCRIPTION
//
//      Destroy a thread_pool by destroying all the threads in it.
//
//*****************************************************************************
{
    DEFER_CANCEL();
    //
    // Set the destructing_ flag to prevent the thread destructor from removing
    // itself from our set since we're deleting the whole set anyway.
    //
    destructing_ = true;

    MUTEX_LOCK( &t_lock_, false );
    for ( thread_set::iterator t = threads_.begin(); t != threads_.end(); ++t )
        delete *t;
    MUTEX_UNLOCK();

    ::pthread_cond_destroy( &t_idle_ );
    ::pthread_cond_destroy( &q_not_empty_ );
    ::pthread_mutex_destroy( &t_lock_ );
    ::pthread_mutex_destroy( &t_busy_lock_ );
    ::pthread_mutex_destroy( &q_lock_ );
    RESTORE_CANCEL();
}

//*****************************************************************************
//
// SYNOPSIS
//
        bool thread_pool::new_task( thread::argument_type arg, bool block )
//
// DESCRIPTION
//
//      Add a new task to the task queue for threads to work on.
//
// PARAMETERS
//
//      arg     The argument to pass to thread_pool_thread_main().
//
//      block   If all the threads are busy and no more can be created and
//              block = true, then the calling thread will block until a thread
//              becomes available.
//
// RETURN VALUE
//
//      Returns false only if block = false and the task can noe be queued
//      because all the threads are busy.
//
//*****************************************************************************
{
#   ifdef DEBUG_threads
    cerr << "thread_pool::new_task()" << endl;
#   endif

    bool all_are_busy, queue_task = true;

    MUTEX_LOCK( &t_lock_, false );
    MUTEX_LOCK( &t_busy_lock_, false );
    all_are_busy = t_busy_ == threads_.size();
    MUTEX_UNLOCK();

    if ( all_are_busy ) {
        if ( threads_.size() < max_threads_ ) {
            //
            // We haven't maxed-out the number of threads we can make, so
            // create another one to handle the request by using the first
            // thread in the pool as a prototype.
            //
#           ifdef DEBUG_threads
            cerr << "creating a new thread" << endl;
#           endif
            thread *const prototype = *threads_.begin();
            DEFER_CANCEL();
            threads_.insert( prototype->create_and_run() );
            RESTORE_CANCEL();
        } else if ( block ) {
            //
            // We've maxed out the number of threads we can make, so just wait
            // until one becomes idle.
            //
#           ifdef DEBUG_threads
            cerr << "waiting for idle thread" << endl;
#           endif
            ::pthread_cond_wait( &t_idle_, &t_lock_ );
        } else
            queue_task = false;
    }
    MUTEX_UNLOCK();                     // t_lock_

    if ( queue_task ) {
        MUTEX_LOCK( &q_lock_, true );
        queue_.push( arg );
        ::pthread_cond_signal( &q_not_empty_ );
        MUTEX_UNLOCK();
    }

    return queue_task;
}

} // namespace PJL

#endif  /* MULTI_THREADED */
/* vim:set et sw=4 ts=4: */