Multi-threading - Do you need it?

What is it?

Info

The information in this section is adapted from the following URL; Multithreaded Application Tutorial, excluding the tutorial.

A multi-threaded application can run multiple tasks simultaneously by creating separate threads. The Main Thread manages user interactions, while other threads handle background tasks.

This setup improves application responsiveness during intensive operations. Multi-threading is beneficial for running tasks in the background, maintaining user interface responsiveness, and managing multiple client requests in server applications.

Do you need multi-threading?

Multi-threading is good for:

• Managing blocking handles like network tasks.
• Using multiple processors at once to process a large dataset.
• Using multiple processors to process different parts of an image simultaneously, improving efficiency and speed.
• Etc.

Before using multiple threads to speed up CPU-bound work:

1. Measure the program first and identify the actual bottleneck.
2. Check that the work can be divided into independent parts.
3. Consider the cost of creating threads, synchronising shared state, and combining the results.

Multi-threaded apps are complex and harder to debug

For simpler tasks, one thread may be enough. In a GUI application, small jobs can sometimes be split into short timer or queued-event steps. Blocking I/O and CPU-intensive work should normally run in a worker TThread, with UI updates synchronised back to the main thread.

Application.ProcessMessages only processes the current thread's event queue; it does not create parallel execution. Calling it from inside a long event handler can also let other handlers run while the application state is only partly updated. It is therefore not a general substitute for a worker thread.

Units needed for a multi-threaded application

Windows

Windows does not require an additional thread-manager unit.

Linux, macOS, and FreeBSD

On Unix-like systems, include cthreads first in the program's uses section, or second when cmem is deliberately listed first. If the application needs wide-string locale support, include cwstring near the beginning too.

cmem is optional: use it only when you intentionally want the C memory manager, not merely because the application is multi-threaded. A minimal cross-platform program can begin as follows.

program AMultiThreadedProgram;

{$mode objfpc}{$H+}{$J-}
uses
{$IFDEF UNIX}
  cthreads,
{$ENDIF}
  Classes;

begin
  WriteLn('Thread support initialized.');
end.

See the Free Pascal documentation for the cthreads startup requirements and the cwstring unit.