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| <body><div class="doctools"> |
| <h1 class="doctools_title">tpool(n) 2.8 "Tcl Threading"</h1> |
| <div id="name" class="doctools_section"><h2><a name="name">Name</a></h2> |
| <p>tpool - Part of the Tcl threading extension implementing pools of worker threads.</p> |
| </div> |
| <div id="toc" class="doctools_section"><h2><a name="toc">Table Of Contents</a></h2> |
| <ul class="doctools_toc"> |
| <li class="doctools_section"><a href="#toc">Table Of Contents</a></li> |
| <li class="doctools_section"><a href="#synopsis">Synopsis</a></li> |
| <li class="doctools_section"><a href="#section1">Description</a></li> |
| <li class="doctools_section"><a href="#section2">COMMANDS</a></li> |
| <li class="doctools_section"><a href="#section3">DISCUSSION</a></li> |
| <li class="doctools_section"><a href="#see-also">See Also</a></li> |
| <li class="doctools_section"><a href="#keywords">Keywords</a></li> |
| </ul> |
| </div> |
| <div id="synopsis" class="doctools_section"><h2><a name="synopsis">Synopsis</a></h2> |
| <div class="doctools_synopsis"> |
| <ul class="doctools_requirements"> |
| <li>package require <b class="pkgname">Tcl 8.4</b></li> |
| <li>package require <b class="pkgname">Thread <span class="opt">?2.8?</span></b></li> |
| </ul> |
| <ul class="doctools_syntax"> |
| <li><a href="#1"><b class="cmd">tpool::create</b> <span class="opt">?options?</span></a></li> |
| <li><a href="#2"><b class="cmd">tpool::names</b></a></li> |
| <li><a href="#3"><b class="cmd">tpool::post</b> <span class="opt">?-detached?</span> <span class="opt">?-nowait?</span> <i class="arg">tpool</i> <i class="arg">script</i></a></li> |
| <li><a href="#4"><b class="cmd">tpool::wait</b> <i class="arg">tpool</i> <i class="arg">joblist</i> <span class="opt">?varname?</span></a></li> |
| <li><a href="#5"><b class="cmd">tpool::cancel</b> <i class="arg">tpool</i> <i class="arg">joblist</i> <span class="opt">?varname?</span></a></li> |
| <li><a href="#6"><b class="cmd">tpool::get</b> <i class="arg">tpool</i> <i class="arg">job</i></a></li> |
| <li><a href="#7"><b class="cmd">tpool::preserve</b> <i class="arg">tpool</i></a></li> |
| <li><a href="#8"><b class="cmd">tpool::release</b> <i class="arg">tpool</i></a></li> |
| <li><a href="#9"><b class="cmd">tpool::suspend</b> <i class="arg">tpool</i></a></li> |
| <li><a href="#10"><b class="cmd">tpool::resume</b> <i class="arg">tpool</i></a></li> |
| </ul> |
| </div> |
| </div> |
| <div id="section1" class="doctools_section"><h2><a name="section1">Description</a></h2> |
| <p>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.</p> |
| </div> |
| <div id="section2" class="doctools_section"><h2><a name="section2">COMMANDS</a></h2> |
| <dl class="doctools_definitions"> |
| <dt><a name="1"><b class="cmd">tpool::create</b> <span class="opt">?options?</span></a></dt> |
| <dd><p>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.</p> |
| <p>Following options are supported:</p> |
| <dl class="doctools_options"> |
| <dt><b class="option">-minworkers</b> <i class="arg">number</i></dt> |
| <dd><p>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 <b class="cmd">tpool::post</b> command |
| and posting jobs to the job queue.</p></dd> |
| <dt><b class="option">-maxworkers</b> <i class="arg">number</i></dt> |
| <dd><p>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.</p></dd> |
| <dt><b class="option">-idletime</b> <i class="arg">seconds</i></dt> |
| <dd><p>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 |
| <b class="option">minthreads</b> option, it will exit. |
| If an <b class="option">exitscript</b> has been defined, the exiting worker thread |
| will first run the script and then exit. Errors from the exit script, |
| if any, are ignored.</p> |
| <p>The idle worker thread is not servicing the event loop. If you, however, |
| put the worker thread into the event loop, by evaluating the |
| <b class="cmd">vwait</b> 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.</p></dd> |
| <dt><b class="option">-initcmd</b> <i class="arg">script</i></dt> |
| <dd><p>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 |
| <b class="cmd">tpool::create</b> or <b class="cmd">tpool::post</b>) 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.</p></dd> |
| <dt><b class="option">-exitcmd</b> <i class="arg">script</i></dt> |
| <dd><p>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.</p></dd> |
| </dl></dd> |
| <dt><a name="2"><b class="cmd">tpool::names</b></a></dt> |
| <dd><p>This command returns a list of IDs of threadpools created with the |
| <b class="cmd">tpool::create</b> command. If no threadpools were found, the |
| command will return empty list.</p></dd> |
| <dt><a name="3"><b class="cmd">tpool::post</b> <span class="opt">?-detached?</span> <span class="opt">?-nowait?</span> <i class="arg">tpool</i> <i class="arg">script</i></a></dt> |
| <dd><p>This command sends a <i class="arg">script</i> to the target <i class="arg">tpool</i> 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 <span class="opt">?-nowait?</span> 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.</p> |
| <p>The command returns the ID of the posted job. This ID is used for subsequent |
| <b class="cmd">tpool::wait</b>, <b class="cmd">tpool::get</b> and <b class="cmd">tpool::cancel</b> commands to wait |
| for and retrieve result of the posted script, or cancel the posted job |
| respectively. If the optional <span class="opt">?-detached?</span> 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.</p> |
| <p>If the threadpool <i class="arg">tpool</i> 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.</p></dd> |
| <dt><a name="4"><b class="cmd">tpool::wait</b> <i class="arg">tpool</i> <i class="arg">joblist</i> <span class="opt">?varname?</span></a></dt> |
| <dd><p>This command waits for one or many jobs, whose job IDs are given in the |
| <i class="arg">joblist</i> 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.</p> |
| <p>The command returns the list of completed job IDs. If the optional variable |
| <span class="opt">?varname?</span> is given, it will be set to the list of jobs in the |
| <i class="arg">joblist</i> which are still pending. If the threadpool <i class="arg">tpool</i> |
| is not found in the list of active thread pools, the command will throw error.</p></dd> |
| <dt><a name="5"><b class="cmd">tpool::cancel</b> <i class="arg">tpool</i> <i class="arg">joblist</i> <span class="opt">?varname?</span></a></dt> |
| <dd><p>This command cancels the previously posted jobs given by the <i class="arg">joblist</i> |
| to the pool <i class="arg">tpool</i>. 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 |
| <span class="opt">?varname?</span> is given, it will be set to the list of jobs in the |
| <i class="arg">joblist</i> which were not cancelled. If the threadpool <i class="arg">tpool</i> |
| is not found in the list of active thread pools, the command will throw error.</p></dd> |
| <dt><a name="6"><b class="cmd">tpool::get</b> <i class="arg">tpool</i> <i class="arg">job</i></a></dt> |
| <dd><p>This command retrieves the result of the previously posted <i class="arg">job</i>. |
| Only results of jobs waited upon with the <b class="cmd">tpool::wait</b> command |
| can be retrieved. If the execution of the script resulted in error, |
| the command will throw the error and update the <b class="variable">errorInfo</b> and |
| <b class="variable">errorCode</b> variables correspondingly. If the pool <i class="arg">tpool</i> |
| is not found in the list of threadpools, the command will throw error. |
| If the job <i class="arg">job</i> is not ready for retrieval, because it is currently |
| being executed by the worker thread, the command will throw error.</p></dd> |
| <dt><a name="7"><b class="cmd">tpool::preserve</b> <i class="arg">tpool</i></a></dt> |
| <dd><p>Each call to this command increments the reference counter of the |
| threadpool <i class="arg">tpool</i> 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.</p></dd> |
| <dt><a name="8"><b class="cmd">tpool::release</b> <i class="arg">tpool</i></a></dt> |
| <dd><p>Each call to this command decrements the reference counter of the |
| threadpool <i class="arg">tpool</i> by one (1).Command returns the value of the |
| reference counter after the decrement. |
| When the reference counter reaches zero (0), the threadpool <i class="arg">tpool</i> |
| is marked for termination. You should not reference the threadpool |
| after the <b class="cmd">tpool::release</b> command returns zero. The <i class="arg">tpool</i> |
| 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.</p></dd> |
| <dt><a name="9"><b class="cmd">tpool::suspend</b> <i class="arg">tpool</i></a></dt> |
| <dd><p>Suspends processing work on this queue. All pool workers are paused |
| but additional work can be added to the pool. Note that adding the |
| additional work will not increase the number of workers dynamically |
| as the pool processing is suspended. Number of workers is maintained |
| to the count that was found prior suspending worker activity. |
| If you need to assure certain number of worker threads, use the |
| <b class="option">minworkers</b> option of the <b class="cmd">tpool::create</b> command.</p></dd> |
| <dt><a name="10"><b class="cmd">tpool::resume</b> <i class="arg">tpool</i></a></dt> |
| <dd><p>Resume processing work on this queue. All paused (suspended) |
| workers are free to get work from the pool. Note that resuming pool |
| operation will just let already created workers to proceed. |
| It will not create additional worker threads to handle the work |
| posted to the pool's work queue.</p></dd> |
| </dl> |
| </div> |
| <div id="section3" class="doctools_section"><h2><a name="section3">DISCUSSION</a></h2> |
| <p>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.</p> |
| <p>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.</p> |
| </div> |
| <div id="see-also" class="doctools_section"><h2><a name="see-also">See Also</a></h2> |
| <p>thread, tsv, ttrace</p> |
| </div> |
| <div id="keywords" class="doctools_section"><h2><a name="keywords">Keywords</a></h2> |
| <p>thread, threadpool</p> |
| </div> |
| </div></body></html> |
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