tpool(n)tpool(n)______________________________________________________________________________NAMEtpool - Part of the Tcl threading extension implementing pools of
worker threads.
SYNOPSIS
package require Tcl 8.4
package require Thread ?2.7?
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_________________________________________________________________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 thread‐
pool instance. During threadpool creation, the implemen‐
tation 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 com‐
mand and posting jobs to the job queue.
-maxworkers number
Maximum number of worker threads allowed for this thread‐
pool 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 avail‐
able 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 thread‐
pool instance will allow maximum of 4 worker threads run‐
ning 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 com‐
mands, 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 con‐
tain 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 avail‐
able, 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 cre‐
ation, 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 com‐
mands 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 ini‐
tialize.
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 thread‐
pool 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 can‐
celled. 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 refer‐
ence to the same threadpool handle will result in Tcl error.
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 signifi‐
cant; 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 tun‐
ing 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
thread, tsv, ttrace
KEYWORDS
thread, threadpool
Tcl Threading 2.7 tpool(n)
