tpool(n) tpool(n) 







tpool - Part of the Tcl threading extension implementing pools of
    worker threads.








package require Tcl 8.4


package require Thread ?2.6?


tpool::create ?options?


tpool::names


tpool::post ?-detached? ?-nowait? tpool
    script


tpool::wait tpool joblist ?varname?


tpool::cancel tpool joblist ?varname?


tpool::get tpool job


tpool::preserve tpool


tpool::release tpool












This package creates and manages pools of worker threads. It
    allows you to post jobs to worker threads and wait for their completion. The
    threadpool implementation is Tcl event-loop aware. That means that any time
    a caller is forced to wait for an event (job being completed or a worker
    thread becoming idle or initialized), the implementation will enter the
    event loop and allow for servicing of other pending file or timer (or any
    other supported) events.









  

  
This command creates new threadpool. It accepts several options as
      key-value pairs. Options are used to tune some threadpool parameters. The
      command returns the ID of the newly created threadpool.
    
Following options are supported:

  







  

  
Minimum number of worker threads needed for this threadpool instance.
      During threadpool creation, the implementation will create somany worker
      threads upfront and will keep at least number of them alive during the
      lifetime of the threadpool instance. Default value of this parameter is 0
      (zero). which means that a newly threadpool will have no worker threads
      initialy. All worker threads will be started on demand by callers running
      tpool::post command and posting jobs to the job queue.

  

  
Maximum number of worker threads allowed for this threadpool instance. If
      a new job is pending and there are no idle worker threads available, the
      implementation will try to create new worker thread. If the number of
      available worker threads is lower than the given number, new worker thread
      will start. The caller will automatically enter the event loop and wait
      until the worker thread has initialized. If. however, the number of
      available worker threads is equal to the given number, the caller will
      enter the event loop and wait for the first worker thread to get idle,
      thus ready to run the job. Default value of this parameter is 4 (four),
      which means that the threadpool instance will allow maximum of 4 worker
      threads running jobs or being idle waiting for new jobs to get posted to
      the job queue.

  

  
Time in seconds an idle worker thread waits for the job to get posted to
      the job queue. If no job arrives during this interval and the time
      expires, the worker thread will check the number of currently available
      worker threads and if the number is higher than the number set by the
      minthreads option, it will exit. If an exitscript has been
      defined, the exiting worker thread will first run the script and then
      exit. Errors from the exit script, if any, are ignored.
    
The idle worker thread is not servicing the event loop. If
        you, however, put the worker thread into the event loop, by evaluating
        the vwait or other related Tcl commands, the worker thread will
        not be in the idle state, hence the idle timer will not be taken into
        account. Default value for this option is unspecified, hence, the Tcl
        interpreter of the worker thread will contain just the initial set of
        Tcl commands.

  

  

  
Sets a Tcl script used to initialize new worker thread. This is usually
      used to load packages and commands in the worker, set default variables,
      create namespaces, and such. If the passed script runs into a Tcl error,
      the worker will not be created and the initiating command (either the
      tpool::create or tpool::post) will throw error. Default
      value for this option is unspecified, hence, the Tcl interpreter of the
      worker thread will contain just the initial set of Tcl commands.

  

  
Sets a Tcl script run when the idle worker thread exits. This is normaly
      used to cleanup the state of the worker thread, release reserved
      resources, cleanup memory and such. Default value for this option is
      unspecified, thus no Tcl script will run on the worker thread exit.









  

  
This command returns a list of IDs of threadpools created with the
      tpool::create command. If no threadpools were found, the command
      will return empty list.

  

  
This command sends a script to the target tpool threadpool
      for execution. The script will be executed in the first available idle
      worker thread. If there are no idle worker threads available, the command
      will create new one, enter the event loop and service events until the
      newly created thread is initialized. If the current number of worker
      threads is equal to the maximum number of worker threads, as defined
      during the threadpool creation, the command will enter the event loop and
      service events while waiting for one of the worker threads to become idle.
      If the optional ?-nowait? argument is given, the command will not wait for
      one idle worker. It will just place the job in the pool's job queue and
      return immediately.
    
The command returns the ID of the posted job. This ID is used
        for subsequent tpool::wait, tpool::get and
        tpool::cancel commands to wait for and retrieve result of the
        posted script, or cancel the posted job respectively. If the optional
        ?-detached? argument is specified, the command will post a detached job.
        A detached job can not be cancelled or waited upon and is not identified
        by the job ID.

    
If the threadpool tpool is not found in the list of
        active thread pools, the command will throw error. The error will also
        be triggered if the newly created worker thread fails to initialize.

  

  

  
This command waits for one or many jobs, whose job IDs are given in the
      joblist to get processed by the worker thread(s). If none of the
      specified jobs are ready, the command will enter the event loop, service
      events and wait for the first job to get ready.
    
The command returns the list of completed job IDs. If the
        optional variable ?varname? is given, it will be set to the list of jobs
        in the joblist which are still pending. If the threadpool
        tpool is not found in the list of active thread pools, the
        command will throw error.

  

  

  
This command cancels the previously posted jobs given by the
      joblist to the pool tpool. Job cancellation succeeds only
      for job still waiting to be processed. If the job is already being
      executed by one of the worker threads, the job will not be cancelled. The
      command returns the list of cancelled job IDs. If the optional variable
      ?varname? is given, it will be set to the list of jobs in the
      joblist which were not cancelled. If the threadpool tpool is
      not found in the list of active thread pools, the command will throw
      error.

  

  
This command retrieves the result of the previously posted job.
      Only results of jobs waited upon with the tpool::wait command can
      be retrieved. If the execution of the script resulted in error, the
      command will throw the error and update the errorInfo and
      errorCode variables correspondingly. If the pool tpool is
      not found in the list of threadpools, the command will throw error. If the
      job job is not ready for retrieval, because it is currently being
      executed by the worker thread, the command will throw error.

  

  
Each call to this command increments the reference counter of the
      threadpool tpool by one (1). Command returns the value of the
      reference counter after the increment. By incrementing the reference
      counter, the caller signalizes that he/she wishes to use the resource for
      a longer period of time.

  

  
Each call to this command decrements the reference counter of the
      threadpool tpool by one (1).Command returns the value of the
      reference counter after the decrement. When the reference counter reaches
      zero (0), the threadpool tpool is marked for termination. You
      should not reference the threadpool after the tpool::release
      command returns zero. The tpool handle goes out of scope and should
      not be used any more. Any following reference to the same threadpool
      handle will result in Tcl error.










Threadpool is one of the most common threading paradigm when it
    comes to server applications handling a large number of relatively small
    tasks. A very simplistic model for building a server application would be to
    create a new thread each time a request arrives and service the request in
    the new thread. One of the disadvantages of this approach is that the
    overhead of creating a new thread for each request is significant; a server
    that created a new thread for each request would spend more time and consume
    more system resources in creating and destroying threads than in processing
    actual user requests. In addition to the overhead of creating and destroying
    threads, active threads consume system resources. Creating too many threads
    can cause the system to run out of memory or trash due to excessive memory
    consumption.


A thread pool offers a solution to both the problem of thread
    life-cycle overhead and the problem of resource trashing. By reusing threads
    for multiple tasks, the thread-creation overhead is spread over many tasks.
    As a bonus, because the thread already exists when a request arrives, the
    delay introduced by thread creation is eliminated. Thus, the request can be
    serviced immediately. Furthermore, by properly tuning the number of threads
    in the thread pool, resource thrashing may also be eliminated by forcing any
    request to wait until a thread is available to process it.








thread, tsv, ttrace








thread, threadpool







  

    2.6
    Tcl Threading