Another simple, easy to use, effective but fully featured thread pool API for C language

This API implements a task parallelization mechanism based on the thread pool pattern.

Usage

The thread pool pattern allows to run parallel tasks easily without the burden of thread management.

The user:

1. declares a thread pool and chooses the maximum number of parallel workers in charge of the execution of tasks (threadpool_create_and_start ()) ;
2. submits tasks to the thread pool (threadpool_add_task ()) that will be passed to one of available workers and will be executed asynchronously ;
3. waits for all the tasks to be completed (threadpool_wait_and_destroy ()).

Files

Interface	Implementation
wqm.h	wqm.c

Libraries

Create a static and dynamic library using :

$ make libs
cc -O -fPIC   -c -o wqm.o wqm.c
ar rcs libwqm.a wqm.o
libwqm.a:wqm.o:0000000000000200 R ALL_TASKS
libwqm.a:wqm.o:00000000000001f0 R LAST_TASK
libwqm.a:wqm.o:0000000000000210 R NB_CPU
libwqm.a:wqm.o:00000000000001f8 R NEXT_TASK
libwqm.a:wqm.o:0000000000000208 R SEQUENTIAL
libwqm.a:wqm.o:0000000000000eac T threadpool_add_task
libwqm.a:wqm.o:00000000000022ff T threadpool_cancel_task
libwqm.a:wqm.o:0000000000000baf T threadpool_create_and_start
libwqm.a:wqm.o:00000000000022d6 T threadpool_global_data
libwqm.a:wqm.o:0000000000002773 T threadpool_global_resource
libwqm.a:wqm.o:0000000000000b18 T threadpool_monitor_every_100ms
libwqm.a:wqm.o:00000000000008ad T threadpool_monitor_to_terminal
libwqm.a:wqm.o:000000000000262e T threadpool_set_global_resource_manager
libwqm.a:wqm.o:000000000000253f T threadpool_set_idle_timeout
libwqm.a:wqm.o:00000000000017b5 T threadpool_set_monitor
libwqm.a:wqm.o:000000000000279f T threadpool_set_worker_local_data_manager
libwqm.a:wqm.o:00000000000005c5 T threadpool_task_continuation
libwqm.a:wqm.o:000000000000162b T threadpool_task_continue
libwqm.a:wqm.o:00000000000018d1 T threadpool_wait_and_destroy
libwqm.a:wqm.o:0000000000001b1d T threadpool_worker_local_data
cc -shared -o libwqm.so wqm.o

Static library	Dynamic library
libwqm.a	libwqm.so

Interface

Basic functionalities

Function	Description
threadpool_create_and_start	Creates and starts a new pool of workers
threadpool_add_task	Adds a task to the pool of workers
threadpool_wait_and_destroy	Waits for all the tasks to be done and destroy the pool of workers
threadpool_cancel_task	Cancels all pending tasks, the last or next submitted task, or a specific task
threadpool_set_monitor	Sets a user-defined function to retrieve and display monitoring information

Those features are detailed below.

Data management

Function	Description
threadpool_global_data	Gives access to the user defined shared global data of the pool of workers
threadpool_set_worker_local_data_manager	Defines the workers' local data manager functions
threadpool_worker_local_data	Gives access to the user defined local data of a worker
threadpool_set_global_resource_manager	Defines the resource manager functions
threadpool_global_resource	Gives access to the global resource of the thread pool
threadpool_set_idle_timeout	Modifies the idle time out (default is 0.1 s)

Those features are detailed below.

Virtual tasks

Function	Description
threadpool_task_continuation	Defines a virtual task to be processed after the response of an asynchronous call
threadpool_task_continue	Callback function to be called by the callback function of an asynchronous call to proceed a virtual task

Those features are detailed below.

Unique features

This implementation of a thread pool brings unique features, not found anywhere else at the time of writing.

1. It uses the standard (minimalist) C11 thread library <threads.h>, rather the POSIX threads. It can therefore be ported more easily to systems other than unix-like systems.
2. The data passed to tasks (via threadpool_add_task ()) can be accessed, retrieved and released multi-thread-safely after completion of the task (via the user defined function job_delete ()), allowing collecting data at task termination.
3. Global data can be defined and accessed (via threadpool_global_data ()) by all tasks.
4. Local data can be defined (via threadpool_set_worker_local_data_manager ()) and accessed (via threadpool_worker_local_data ()) for each worker of the thread pool.
5. Global resources can be allocated and deallocated (via threadpool_set_global_resource_manager ()) and accessed (via threadpool_global_resource ()) for the thread pool.
6. Workers will stay alive for a short idle time, ready to process new submitted tasks, even though threadpool_wait_and_destroy () has already been called and no tasks are available, as long as some other tasks are still being processed and could therefore create new tasks dynamically.
7. The activity of the thread pool can be monitored and displayed by a front-end user defined function (via threadpool_set_monitor ()).
8. Pending tasks can be canceled after submission (via threadpool_cancel_task ()).
9. The thread pool can wait for asynchronous calls without blocking workers (via threadpool_task_continuation () and threadpool_task_continue ()).

Those features are detailed below.

Detailed API

The API is defined in file wqm.h.

Create a thread pool

struct threadpool *threadpool_create_and_start (size_t nb_workers,
                                                void *global_data)

A thread pool is declared and started with a call to threadpool_create_and_start ().

The first argument nb_workers is the number of requested workers, that is the maximum number of tasks that should be executed in parallel by the system.

• NB_CPU can be used as first argument to fit to the number of processors currently available in the system (as returned by get_nprocs () with GNU C standard library).
• SEQUENTIAL can be used as first argument to create a sequential thread pool: tasks will be executed asynchronously, one at a time, and in the order there were submitted.

The maximum number of workers can be defined to a higher value than the number of CPUs as workers will be started only when solicited and will be released when unused after an idle time.

Nevertheless, the actual number of workers will be limited by the operating system to a lower value than nb_workers.

Options

• The second argument global_data, if not null, is a pointer to global data. This pointer can next be retrieved by tasks with the function threadpool_global_data ().

Submit a task

size_t threadpool_add_task (struct threadpool *threadpool,
                            int (*work) (struct threadpool * threadpool, void *job),
                            void *job,
                            void (*job_delete) (void *job))

A task is submitted to the thread pool with a call to threadpool_add_task ().

• The first argument threadpool is a thread pool returned by a previous call to threadpool_create_and_start ().
• The second argument work is a user defined function to be executed by a worker of the thread pool on job. This function work should return EXIT_SUCCESS on success, EXIT_FAILURE otherwise. work should be made thread-safe as several work will be running in parallel (that's the whole point of the thread pool). This function receives the thread pool threadpool and the job job as arguments, as they were passed to threadpool_add_task. Therefore, work can itself (multi-thread-safely) call threadpool_add_task if needed.
• The third argument job is a pointer to the data to be used by the task and that will be processed by work. It should be fully initialized before it is passed to threadpool_add_task. It can be allocated automatically, statically or dynamically, as long it survives until the job is done by work.

The function threadpool_add_task returns a unique id of the submitted task, or 0 on error (with errno set to ENOMEM).

Options

• The fourth argument job_delete, if not null, is a user defined function called at termination of the task (after processing or cancellation.) This function receives the job of the task as an argument.

job_delete should be used if the job was allocated dynamically in order to release and destroy the data after work is done and avoid memory leaks.

job_delete is called in a multi-thread-safe manner and can therefore safely aggregate results to those of previous tasks for instance (in a map and reduce pattern for instance). See below.

job_delete could as well be called manually (rather than passed as an argument to threadpool_add_task) at the very end of work (), but it then would not be executed multi-thread-safely, forbidding any aggregation.

If job was allocated with a single malloc () and affiliated functions, free () is a possible choice for job_delete.

Multi-thread safe task post-processing

Besides simply releasing data job, the user-defined job_delete can also be used as a callback function, called after the task is done or canceled, to retrieve the used and possibly modified job and take actions (displaying, computing, aggregating, ...).

For instance, a user-defined type job_t could be declared as a structure containing:

typedef struct {
  struct { ... } input;
  struct { ... } result;
  int done;
} job_t;

A job of type job_t would then be passed to threadpool_add_task.

The task post-processing pattern stands in 3 steps :

1. A job is allocated and initialized before it is passed to threadpool_add_task:
   • input data is set ;
   • done is set to 0 ;
2. In the user-defined function work:
   • the previously set input data is used to compute the result data of the task ;
   • done is set to 1 (and therefore remains to 0 if the task was canceled by threadpool_cancel_task) ;
3. In the user-defined function job_delete:
   • if done is set to 1, an aggregated result of all the tasks can then be multi-thread-safely updated from result ; global_data, passed to threadpool_create_and_start and retrievable by threadpool_global_data, is a possible choice to hold the aggregated result ;
   • any required deallocation of job is done.

See below for the example fuzzy words of such a pattern.

Access to global and local thread data

Global and local data of threads can be retrieved and updated safely in the context of working threads.

The global data of a thread pool (global_data as passed to threadpool_create_and_start):

• should be fully initialized before threadpool_create_and_start is called ;
• should survive after threadpool_wait_and_destroy is called ;
• can be accessed inside user-defined functions make_local, delete_local (as passed to threadpool_create_and_start), work and job_delete (as passed to threadpool_add_task), and deallocator (as passed to threadpool_set_resource_manager) with :

void *threadpool_global_data (void)

threadpool_global_data can not be called in the resource allocator set by threadpool_set_resource_manager.

The local data of a worker (created and returned by make_local and destroyed by delete_local, as passed to threadpool_set_worker_local_data_manager) can be accessed inside user-defined functions work and job_delete (as passed to threadpool_add_task) with :

void threadpool_set_worker_local_data_manager (struct threadpool *threadpool, void *(*make_local) (void), void (*delete_local) (void *local_data))

void *threadpool_worker_local_data (void)

These two functions must be called in the context of a running thread, otherwise they return 0.

Cancel tasks

size_t threadpool_cancel_task (struct threadpool *threadpool, size_t task_id)

Previously submitted and still pending tasks can be canceled. task_id is:

• either a unique id returned by a previous call to threadpool_add_task ;
• or ALL_TASKS to cancel all still pending tasks ;
• or NEXT_TASK to cancel the next still pending submitted task (it can be used several times in a row) ;
• or LAST_TASK to cancel the last still pending submitted task (it can be used several times in a row).

Canceled tasks won't be processed, but job_delete, as optionally passed to threadpool_add_task, will be called though.

The function returns the number of canceled tasks, if any, or 0 if there are not any left pending task to be canceled.

Wait for all submitted tasks to be completed

void threadpool_wait_and_destroy (struct threadpool *threadpool)

The single argument threadpool is a thread pool returned by a previous call to threadpool_create_and_start ().

This function declares that all the tasks have been submitted. It then waits for all the tasks to be completed by workers.

threadpool should not be used after a call to threadpool_wait_and_destroy ().

Monitor the thread pool activity

A monitoring of the thread pool activity can optionally be activated by calling

void threadpool_set_monitor (struct threadpool *threadpool, threadpool_monitor_handler handler, void *arg, threadpool_monitor_filter filter)

A user-defined handler function is passed as second argument, with signature

void (*threadpool_monitor_handler) (struct threadpool_monitor, void *arg)

0 or a user-defined filter function can be passed as fourth argument, with signature

int (*threadpool_monitor_filter) (struct threadpool_monitor d)

The handler will be called to retrieve and display information about the activity of the thread pool. It :

• will be called whenever the state of the thread pool changes and, if filter is not null, whenever filter returns non-zero. filter should be set to 0 to monitor every change of the thread pool state.
• will be passed the argument arg (which can be 0) previously passed to threadpool_set_monitor as thirdargument,
• will be called asynchronously, without interfering with the execution of workers (actually, a sequential dedicated asynchronous thread pool is used for monitoring),
• will be executed multi-thread-safely,
• should not be called after threadpool_wait_and_destroy has been called.

The monitoring data are passed to the handler function in a structure threadpool_monitor which contains:

• struct threadpool *threadpool: the thread pool for which the monitoring handler is called ;
• double time: the elapsed seconds since the creation of the thread pool (threadpool_create_and_start) ;
• int closed : 1 if the thread pool has been closed (by a call to threadpool_wait_and_destroy), 0 otherwise ;
• size_t workers.nb_requested: the requested number of workers, as defined at the creation of the thread pool ;
• size_t workers.nb_max: the maximum number of workers granted by the operating system (<= workers.nb_requested) ;
• size_t workers.nb_alive: the number of alive workers, either running (tasks.nb_processing) or waiting (workers.nb_idle) ;
• size_t workers.nb_idle: the number of idle worker, i.e. waiting (some time) for a task to process ;
• size_t tasks.nb_pending: the number of tasks submitted to the thread pool and not yet processed or being processed ;
• size_t tasks.nb_processing: the number of running workers, i.e. processing a task ;
• size_t tasks.nb_asynchronous: the number of asynchronous (virtual) tasks ;
• size_t tasks.nb_succeeded: the number of already processed and succeeded tasks by the thread pool (a task is considered successful when work, the function passed to threadpool_add_task, returns 0) ;
• size_t tasks.nb_failed: the number of already processed and failed tasks by the thread pool (a task is considered failed when work, the function passed to threadpool_add_task, does not return 0) ;
• size_t tasks.nb_canceled: the number of canceled tasks ;
• size_t tasks.nb_submitted : the number of submitted tasks (either pending, processing, succeeded, failed or canceled).

A handler threadpool_monitor_to_terminal is available for convenience:

• It can be used as the second argument of threadpool_set_monitor.
• It displays monitoring data as text sent to a stream of type FILE *, passed as the third argument of threadpool_set_monitor. stderr will be used by default if this third argument is NULL.

A filter threadpool_monitor_every_100ms is available for convenience:

• It can be used as the fourth argument of threadpool_set_monitor.
• It permits to monitor not more often than every 100 ms.

Manage data

Data used in the context of a thread pool can be managed globally or locally with four different ways, depending on the scope and life-cycle of the data.

Scope	Access	Management
Thread pool	threadpool_global_data	threadpool_create_and_start, see Manage global data
Active thread pool (running workers)	threadpool_global_resource	threadpool_set_global_resource_manager, see Manage global resources
Worker	threadpoolworker_local_data	threadpool_set_worker_local_data_manager, see Manage worker local data
Task	job in work	threadpool_add_task, see Manage task data

Manage global data

A global context data of a thread pool can be passed as second argument when it is created with threadpool_create_and_start. This data should be allocated (statically, automatically or dynamically) before calling threadpool_create_and_start and respectively deallocated after calling threadpool_wait_and_destroy.

This global data of a thread pool can be accessed inside:

• user-defined functions allocator and deallocator passed to threadpool_set_global_resource_manager ;
• user-defined functions make_local and delete_local (as passed to threadpool_set_worker_local_data_manager) ;
• user-defined functions work and job_delete (as passed to threadpool_add_task) with :

void *threadpool_global_data (void)

Manage global resources

In case external resources should be allocated for tasks processing (for instance a shared connection to a database), user-defined functions allocator and deallocator can be set with:

void threadpool_set_global_resource_manager (struct threadpool *threadpool, void *(*allocator) (void *global_data), void (*deallocator) (void *resource))

This function should be called after threadpool_create_and_start and before adding tasks to the thread poll with threadpool_add_task. Otherwise, it would have no effect (as workers are already running) and errno would be set to ECANCELED.

• The user-defined function allocator:
  • should fully initialize and return the global resource of the thread pool ;
  • is passed the global_data of the thread pool as an argument ;
  • will generally be called once per thread pool, before processing the very first task.
• The user-defined function deallocator:
  • should fully release the global resource of the thread pool ;
  • is passed the resource to deallocate, as previously returned by allocator, as an argument ;
  • will generally be called once per thread pool, after all tasks have been processed or canceled.

Moreover, if the thread pool remains idle (waiting for tasks to process) for too long (see below), resources will be deallocated automatically, and will be reallocated automatically when the thread pool gets alive again.

The global resource of a thread pool (as returned by the allocator passed to threadpool_set_global_resource_manager) can be accessed inside user-defined functions make_local, delete_local (as passed to threadpool_set_worker_local_data_manager), work and job_delete (as passed to threadpool_add_task) with :

void *threadpool_global_resource (void)

Timeout delay of idle workers

The timeout delay before thread pool internal and external resources are deallocated when a thread pool is kept idle for too long can be modified with:

void threadpool_set_idle_timeout (struct threadpool *threadpool, double delay)

delay, in seconds, should be a non negative value, otherwise it is ignored and errno is set to EINVAL. It can not exceed 10.000.000 seconds.

This delay should be set to a value larger than the time required to allocate global resources for tasks. It should nevertheless be kept as low as possible to release unused scarce resources when workers are kept idle for a long period.

The default delay is 0.1 seconds when a thread pool is created by threadpool_create_and_start.

The delay is set to 10.000.000 seconds by default after threadpool_set_global_resource_manager is called : resources will not be deallocated by default if the thread pool is idle. threadpool_set_idle_timeout should be called after threadpool_set_global_resource_manager to lower the delay in order to deallocate scarce resources after a specified idle delay.

Manage worker local data

In case resources should be allocated for each worker (for instance a connection to a database), user-defined functions make_local and delete_local can be set with:

void threadpool_set_worker_local_data_manager (struct threadpool *threadpool, void *(*make_local) (void), void (*delete_local) (void *local_data));

This function should be called after threadpool_create_and_start and before adding tasks to the thread poll with threadpool_add_task. Otherwise, it would have no effect (as workers are already running) and errno would be set to ECANCELED.

make_local will be called when a worker is created and delete_local when it is terminated:

• The argument make_local, if not null, is a user defined function that returns a pointer to data for a local usage by a worker. This pointer can next be retrieved by tasks with the function threadpool_worker_local_data (). make_local is called in a multi-thread-safe manner at the initialization of a worker and is passed the pointer to global data. make_local can therefore safely access (and update) the content of global_data if needed.
• The argument delete_local, if not null, is a user defined function that is executed to release and destroys the local data used by each worker (passed as an argument) when a worker stops. delete_local is called in a multi-thread-safe manner at the termination of a worker and is passed the pointer to global data. delete_local can therefore safely access (and update) the content of global_data if needed (to gather results or statistics for instance).

The local data of a worker (as returned by the make_local passed to threadpool_set_worker_local_data_manager) can be accessed inside user-defined functions work and job_delete (as passed to threadpool_add_task) with :

void *threadpoolworker_local_data (void)

Manage task data

The data of a job is passed to a task when it is added to a thread pool with threadpool_add_task.

See Submit a task.

Manage asynchronous calls (virtual tasks)

In case a task would need to use an asynchronous call, the continuation of the task work_continuator can be specified by calling

uint64_t threadpool_task_continuation (int (*work_continuator) (struct threadpool * threadpool, void *data), double seconds)

just before the asynchronous call.

The function work_continuator indicates the function to be called when the asynchronous call notifies its completion. work_continuator should return EXIT_SUCCESS on success, EXIT_FAILURE otherwise.

• threadpool_task_continuation returns 0 and sets errno to EINVAL if work_continuator is null.
• threadpool_task_continuation returns 0 and sets errno to EPERM if it is called outside of:
  • a function work of a task scheduled with threadpool_add_task ;
  • a function work_continuator of a task scheduled with threadpool_task_continuation ;
• otherwise threadpool_task_continuation returns a non-zero unique ID that should be passed to the asynchronous call.

seconds is a timeout delay, in seconds, for the asynchronous response. After this delay, the asynchronous response will be ignored (see below).

In the callback function of the asynchronous call (when the asynchronous call notifies its completion), the function

int threadpool_task_continue (uint64_t uid)

should be called with the ID previously retrieved to continue to process the initial task.

The function work_continuator previously declared by threadpool_task_continuation will then be automatically called:

• If threadpool_task_continue is called before the timeout delay seconds has elapsed, work_continuator will be scheduled by the thread pool and EXIT_SUCCESS will be returned.
• If threadpool_task_continue is called after the timeout delay seconds has elapsed, threadpool_task_continue will have no effect, will return EXIT_FAILURE and errno will be set to ETIMEOUT.

The task does not block any worker between the calls to threadpool_task_continuation and threadpool_task_continue. Workers are available to process any other tasks (either asynchronous or not): the tasks using threadpool_task_continuation and threadpool_task_continue behave like virtual tasks.

This features was inspired from Java virtual threads (see https://openjdk.org/jeps/444).

Examples

Type make to compile and run the examples (in sub-folder examples).

Quick sort in place

An example of the usage of thread pool is given in files qsip_wc.c and qsip_wc_test.c here.

It sorts 2 bunches of 50 lists of 1.000.000 numbers.

Two encapsulated thread pools are used : one to distribute 100 tasks over 7 monitored threads, each task sorting 1000000 numbers distributed over the CPU threads.

• qsip_wc.c is an attempt to implement a parallelized version of the quick sort algorithm (using a thread pool);

  • It uses features such as global data, worker local data, dynamic creation and deletion of jobs.
  • It reveals that a parallelized quick sort is inefficient due to thread management overhead.

• qsip_wc_test.c is an example of a thread pool that sorts several arrays using the above parallelized version of the quick sort algorithm.

  • It uses features such as global data, worker local data, task cancellation, (fake) resource management and monitoring.

Running this example with

$ make qsip_wc_test

yields:

Sorting 1,000,000 elements (multi-threaded quick sort in place), 100 times:
Initializing 100,000,000 random numbers...
7 workers requested and processing...
Allocate resources...
(=) succeeded tasks, (X) failed tasks, (*) processing tasks, (.) pending tasks, (/) canceled tasks, (~) idle thread pool.
[0x5cbba68eb7d0 (7)][    0.0000s][   0] 
[0x5cbba68eb7d0 (7)][    0.0002s][   1] .
Resources allocated.
[0x5cbba68eb7d0 (7)][    2.0006s][   1] .
[0x5cbba68eb7d0 (7)][    2.0010s][   2] ..
Will go to sleep for 16 seconds...
[0x5cbba68eb7d0 (7)][    2.0012s][   3] ...
[0x5cbba68eb7d0 (7)][    2.0014s][   4] ....
...
[0x5cbba68eb7d0 (7)][   13.1516s][  50] =================================================*
[0x5cbba68eb7d0 (7)][   13.2413s][  50] ================================================== ~
[0x5cbba68eb7d0 (7)][   14.1519s][  50] ================================================== ~
Deallocate resources...
[0x5cbba68eb7d0 (7)][   14.2414s][  50] ==================================================
Resources deallocated.
[0x5cbba68eb7d0 (7)][   16.2417s][  50] ==================================================
Stop sleeping after 16 seconds.
Allocate resources...
[0x5cbba68eb7d0 (7)][   18.0050s][  51] ==================================================.
Resources allocated.
[0x5cbba68eb7d0 (7)][   20.0054s][  51] ==================================================.
[0x5cbba68eb7d0 (7)][   20.0056s][  52] ==================================================..
...
[0x5cbba68eb7d0 (7)][   24.6119s][ 100] ===============================================================*////////////////////////////////////
[0x5cbba68eb7d0 (7)][   24.7960s][ 100] ================================================================//////////////////////////////////// ~
Deallocate resources...
Resources deallocated.
[0x5cbba68eb7d0 (7)][   26.7966s][ 100] ================================================================////////////////////////////////////
Done.

Fuzzy words

This example matches a list of french fuzzy words against the french dictionary.

Run it with:

$ make fuzzyword

Two encapsulated thread pools are used : one to distribute the list of words on one monitored single thread (words are processed sequentially), each word being compared to the entries (distributed over the CPU threads) of the dictionary.

It uses job_delete as a callback function for task post-processing and threadpool_set_global_resource_manager for global resource management.

Intensive

This example requests more workers than what the system permits.

Run it with:

$ make intensive

Timers (virtual tasks)

This example uses threadpool_task_continuation and threadpool_task_continue to create asynchronous virtual tasks.

Asynchronous tasks (a pause of one second) can be processed with a thread pool of one worker only: those tasks behave like virtual tasks which do not block the worker (awesome !)

Run it with:

$ make timers

Implementation insights

The API is implemented in C11 (file wqm.c) using the standard C thread library <threads.h>. It is highly inspired from the great book "Programming with POSIX Threads" written by David R. Butenhof, 21st ed., 2008, Addison Wesley.

It has been heavily tested, but bugs are still possible. Please don't hesitate to report them to me.

Management of workers

Workers are started automatically when needed, that is when a new task is submitted whereas all workers are already booked. Workers are running in parallel, asynchronously. Each worker can process one task at a time. A task is processed as soon as a worker is available.

A worker stops when all submitted tasks have been processed or after an idle time. Idle workers are kept ready for new tasks for a short time and are then stopped automatically to release system resources.

For instance, say a task requires 2 seconds to be processed and the maximum idle delay for a worker is half a second:

• if the task is repeatedly submitted every 3 seconds to the thread pool, one worker will be created and activated on submission, and stopped after completion of the task and an idle time (of half a second).
• if the task is submitted every 2.4 seconds to the thread pool, one worker will be created and activated on submission, kept idle and reactivated for the next task.
• if the task is submitted every 2 seconds, one worker will be active to process the tasks.
• if the task is submitted every second, two workers will be active to process the tasks.
• if the rate of submitted tasks is very high, the maximum number of workers (as passed to threadpool_create_and_start ()) will be active.

Therefore, the number for workers automatically adapts to the rate and duration for tasks.

That's it. Have fun and let me know!

Zed is dead, but C is not.