<manpage cat="package" id="thread" title="Tcl Threading" version="2.5">
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     Copyright (c) 2002 Zoran Vasiljevic 
     
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<namesection>
<name>tpool</name>
<desc>Part of the Tcl threading extension implementing pools of worker threads.</desc>
</namesection>

<synopsis>
<syntax>
<p>
<cmd>package require Thread ?2.5?</cmd></p>
<cmd>tpool::create</cmd> ?<m>options</m>?
<cmd>tpool::post</cmd> <m>tpoolId</m> <m>script</m>
<cmd>tpool::wait</cmd> <m>tpoolId</m> <m>jobIdList</m> ?<m>varName</m>?
<cmd>tpool::get</cmd> <m>tpoolId</m> <m>jobId</m>
<cmd>tpool::names</cmd>
<cmd>tpool::preserve</cmd> <m>tpoolId</m>
<cmd>tpool::release</cmd> <m>tpoolId</m>
</syntax>
</synopsis>

<section>
<title>DESCRIPTION</title>
<p>
</p>
</section>

<section>
<title>COMMANDS</title>
<commandlist>

<commanddef>
<command><cmd>tpool::create </cmd> ?<m>options</m>?</command>
<desc>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>
Following <m>options</m> are supported:
</p>

<commandlist>

<commanddef>
<command><m>-minthreads number</m></command>
<desc>Minimum <m>number</m> of threads needed for this threadpool instance.
During threadpool creation, the implementation will create somany
worker threads upfront and will keep at least <m>number</m> of them alive
during the lifetime of the threadpool instance.<br/>
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 <cmd>tpool::post</cmd> command and 
posting jobs to the job queue.
</desc>
</commanddef>

<commanddef>
<command><m>-maxthreads number</m></command>
<desc>Maximum <m>number</m> of 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 <m>number</m>,
new worker thread will start and the caller will enter event loop and
wait until the worker thread has initialized. If. however, the
number of available worker threads is equal to <m>number</m>, the caller
will enter the event loop and wait for the first worker thread to get idle, 
thus ready to run the job.<br/>
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.</desc>
</commanddef>

<commanddef>
<command><m>-idletime seconds</m></command>
<desc>Time in <m>seconds</m> 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. If the number is higher than the 
number set by the <m>minthreads</m> option, it will exit. 
If an <m>exitscript</m> has been defined, the exiting worker thread 
will first run the script and then exit. Errors from the exit script, 
if any, are ignored.
<br/>
The idle worker thread is not servicing the event loop. If you, however,
put the worker thread into the event loop, by evaluating the
<cmd>vwait</cmd> 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.<br/>
Default value for this option is zero (0), meaning that no idle time
is set. Worker threads will thus, once started, never exit.
</desc>
</commanddef>


<commanddef>
<command><m>-initcmd script</m></command>
<desc>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 <m>-initcmd script</m> runs in Tcl error, 
the worker will not be created and the initiating command (either the 
<cmd>tpool::create</cmd> or <cmd>tpool::post</cmd>) will throw the
error.
<br/>
Default value for this option is unspecified, hence, the Tcl interpreter of 
the worker thread will contain just the initial set of Tcl commands.</desc>
</commanddef>

<commanddef>
<command><m>-exitcmd script</m></command>
<desc>Tcl script run when the idle worker thread exits. This is normaly used
to cleanup the state of the worker thread, release reserved resources and 
cleanup memory. <br/>
Default value for this option is unspecified, thus no Tcl script will run on 
the worker thread exit.</desc>
</commanddef>

</commandlist>
</desc>
</commanddef>

<commanddef>
<command><cmd>tpool::post</cmd> <m>tpoolId</m> <m>script</m></command>
<desc>This command sends a <m>script</m> to the target <m>tpoolId</m> 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
with the <cmd>tpool::create</cmd> command, the command will enter the event loop
and service events while waiting for one of the worker threads to become idle.
<br/>
The command returns the ID of the posted job. This ID is used for subsequent
<cmd>tpool::wait</cmd> and <cmd>tpool::get</cmd> commands to wait for and 
retrieve result of the posted script respectively. If the threadpool <m>tpoolId</m> 
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.
</desc>
</commanddef>

<commanddef>
<command><cmd>tpool::wait</cmd> <m>tpoolId</m> <m>jobIdList</m> ?<m>varName</m>?</command>
<desc>This command waits for one or many jobs, whose job IDs are given in the
<m>jobIdList</m> 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.
<br/>
The command returns the list of completed job IDs. If the optional variable
<m>varName</m> is given, it will be set to the list of jobs in the <m>jobIdList</m> 
which are still pending. If the threadpool <m>tpoolId</m> is not found in the 
list of active thread pools, the command will throw error.
</desc>
</commanddef>

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

<commanddef>
<command><cmd>tpool::names</cmd></command>
<desc>This command returns a list of IDs of threadpools created with the 
<cmd>tpool::create</cmd> command. If no threadpools were found, the
command will return empty list.</desc>
</commanddef>

<commanddef>
<command><cmd>tpool::preserve</cmd> <m>tpoolId</m></command>
<desc>Each call to this command increments the reference counter of the
threadpool <m>tpoolId</m> by one (1). Command returns the value of the 
reference counter after the increment.
<br/>
With reference counting, one can implement controlled access to a shared
threadpool resource. By incrementing the reference counter, the caller 
signalizes that he/she wishes to use the resource for a longer period of time.
By decrementing the counter, using the  <cmd>tpool::release</cmd> command, 
caller signalizes that he/she has finished using the resource.</desc>
</commanddef>

<commanddef>
<command><cmd>tpool::release</cmd> <m>tpoolId</m></command>
<desc>Each call to this command decrements the reference counter of the 
threadpool <m>tpoolId</m> by one (1).Command returns the value of the 
reference counter after the decrement. When the reference counter reaches 
zero (0), the threadpool <m>tpoolId</m> is marked for termination.
You should not reference the threadpool after the <cmd>tpool::release</cmd>
command returns zero. The <m>tpoolId</m> 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.</desc>
</commanddef>

</commandlist>
</section>

<section>
<title>DISCUSSION</title>
<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.
<br/>
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>
</section>

<seealso>
<ref href="http://www.tcl.tk/doc/howto/thread_model.html">Guide to the Tcl threading model
</ref>
</seealso>

<keywords>
<keyword>threads</keyword>
<keyword>pool</keyword>
</keywords>
</manpage>