[comment {-*- tcl -*- doctools manpage}]
[manpage_begin tpool n 2.6]
[moddesc {Tcl Threading}]
[titledesc {
    Part of the Tcl threading extension implementing pools of worker threads.
}]
[require Tcl 8.3]
[require Thread [opt 2.6]]

[description]
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.

[section COMMANDS]

[list_begin definitions]

[call [cmd tpool::create] [opt options]]

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.
[nl]
Following options are supported:

[list_begin definitions]

[lst_item {-minthreads number}]
Minimum number of 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 [cmd tpool::post] command
and posting jobs to the job queue.

[lst_item {-maxthreads number}]
Maximum number 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 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.


[lst_item {-idletime seconds}]
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
[option minthreads] option, it will exit. 
If an [option 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.
[nl]
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] 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.

[lst_item {-initcmd script}]

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
[cmd tpool::create] or [cmd 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.

[lst_item {-exitcmd script}]

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.

[list_end]

[call [cmd tpool::names]]

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

[call [cmd tpool::post] [opt -detached] [opt -nowait] [arg tpool] [arg script]]

This command sends a [arg script] to the target [arg 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 [opt -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.
[nl]
The command returns the ID of the posted job. This ID is used for subsequent
[cmd tpool::wait], [cmd tpool::get] and [cmd tpool::cancel] commands to wait
for and retrieve result of the posted script, or cancel the posted job
respectively. If the optional [opt -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.
[nl]
If the threadpool [arg 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.

[call [cmd tpool::wait] [arg tpool] [arg joblist] [opt varname]]

This command waits for one or many jobs, whose job IDs are given in the
[arg 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.
[nl]
The command returns the list of completed job IDs. If the optional variable
[opt varname] is given, it will be set to the list of jobs in the 
[arg joblist] which are still pending. If the threadpool [arg tpool] 
is not found in the list of active thread pools, the command will throw error.

[call [cmd tpool::cancel] [arg tpool] [arg joblist] [opt varname]]

This command cancels the previously posted jobs given by the [arg joblist]
to the pool [arg 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
[opt varname] is given, it will be set to the list of jobs in the 
[arg joblist] which were not cancelled. If the threadpool [arg tpool] 
is not found in the list of active thread pools, the command will throw error.

[call [cmd tpool::get] [arg tpool] [arg job]]

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

[call [cmd tpool::preserve] [arg tpool]]

Each call to this command increments the reference counter of the
threadpool [arg 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.

[call [cmd tpool::release] [arg tpool]]

Each call to this command decrements the reference counter of the 
threadpool [arg tpool] by one (1).Command returns the value of the 
reference counter after the decrement. 
When the reference counter reaches zero (0), the threadpool [arg tpool]
is marked for termination. You should not reference the threadpool 
after the [cmd tpool::release] command returns zero. The [arg 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.

[list_end]


[section DISCUSSION]

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.
[para]
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.

[see_also tsv ttrace thread]

[keywords thread threadpool]

[manpage_end]
