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

NAME

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

SYNOPSIS

       package require Tcl  8.4

       package require Thread  ?2.8?

       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

       tpool::suspend tpool

       tpool::resume tpool

________________________________________________________________________________________________________________

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.

COMMANDS

tpool::create ?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.

Following options are supported:

-minworkers number
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.

-maxworkers number
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.

-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 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.

-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 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.

-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.

tpool::names
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.

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

tpool::wait tpool joblist ?varname?
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.

tpool::cancel tpool joblist ?varname?
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.

tpool::get tpool job
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.

tpool::preserve tpool
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.

tpool::release tpool
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.

tpool::suspend tpool
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 minworkers option of the tpool::create command.

tpool::resume tpool
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.

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.

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.

KEYWORDS