Chapter 1. What are threads? Why use them?
In this chapter:
In the early days of computing, all programming was essentially single
threaded. You created your program by punching holes into cards or tape,
submitted your deck of cards to the local computing centre, and after a
few days, you received another deck of cards, containing, if you were lucky,
the required results. All processing was batch, not time critical, first
come first served, and when your program was running, it had exclusive
use of the computer's time.
Things have moved on. The concept of multiple threads of execution first
appeared with time sharing systems, where more than one person could be
logged into a central mainframe computer at once. It was important to ensure
that the processing time of the machine was fairly divided between all
users, and the operating systems of the time made use of the "process"
and "thread" concepts. Desktop computers have seen a similar progression.
Early DOS and Windows systems were single tasking. Your program ran exclusively
on the machine, or not at all. With increasingly sophisticated applications,
and increasing demands on personal computers, especially with respect to
high performance in the graphics and networking areas, multiprocess and
multithread operating systems are now commonplace. Multithreading on PC's
has mainly been driven by the need for better performance and usability
The first concept to define is that of the process. Most Windows
95, 98 and NT users have a good intuitive idea of what a process is. They
see it as a program which runs on the machine, co-existing and sharing
CPU, disk and memory resources with other programs. Programmers know a
process to be an invocation of executable code, such that that code has
a unique existence, and the instructions executed by that process are executed
in an ordered manner. On the whole, processes execute in isolation. The
resources they use (memory, disk, I/O, CPU time) are virtualised, such
that every process has its own set of virtual resources, untouched by other
processes. The operating system provides this virtualisation. Processes
execute modules of code. These may be disjoint; in the sense that,
the executable modules of code comprising Windows Explorer and Microsoft
Word are disjoint. However, they may also be shared, as in the case of
DLL's. The code for a DLL is typically being executed in the context of
many different processes, often simultaneously. The execution of instructions
is on the whole not ordered between processes: Microsoft word does not
stop opening a document just because the print spooler is currently sending
something to the printer! Of course, where different processes interact,
the programmer must impose an ordering, a central problem which will be
Our next concept is that of the thread. Threads were developed
when it became clear that it was desirable to have applications which performed
sets of actions in a more loosely time ordered fashion, possibly performing
several sets of actions at once. In situations where some actions would
cause a considerable delay in one thread of execution (e.g.. waiting for
the user to do something), it was often desirable to have the program still
perform other actions concurrently (e.g. background spell checking, or
processing incoming network messages). However, the overhead of creating
a whole new process for each concurrent action, and then having the processes
communicate was often far too much of an overhead.
If one needs to look for a good example of multithreading, then Windows
Explorer (i.e. the Windows Shell) is an excellent example. Double click
on "My Computer", and click through a few sub folders, creating new windows
as you go. Now invoke a lengthy copy operation on one of those windows.
The progress bar pops up, and that particular window does not respond to
user input. However, all the other windows are perfectly usable. Obviously,
several things are going on at once, but only one copy of explorer.exe
is running. This is the essence of multithreading.
In most systems that support multithreading, there may be many users making
simultaneous requests on the computer system. Normally the number of physical
processors in the system is fewer than the number of threads that might
be run in parallel. Most systems support time slicing, also known
as pre-emptive multitasking, in order to get around this problem.
In a system that is time sliced, threads run for a short while, and are
then pre-empted; that is, a hardware timer fires which causes the operating
system to re-evaluate which threads should be run, potentially stopping
execution on currently running threads, and running other threads which
have not be executed recently. This allows even single processor machines
to run multiple threads. On PC's a timeslice tends to be about about fifty
five milliseconds long.
Why use threads?
Threads should not alter the semantics of a program. They simply change
the timing of operations. As a result, they are almost always used as an
elegant solution to performance related problems. Here are some examples
of situations where you might use threads:
All of these examples have one thing in common: In the program, some operations
incur a potentially large delay or CPU hogging, but this delay or CPU usage
is unacceptable for other operations; they need to be serviced now.
Of course there are other miscellaneous benefits, and here they are:
Doing lengthy processing: When a windows application is calculating it
cannot process any more messages. As a result, the display cannot be updated.
Doing background processing: Some tasks may not be time critical, but need
to execute continuously.
Doing I/O work: I/O to disk or to network can have unpredictable delays.
Threads allow you to ensure that I/O latency does not delay unrelated parts
of your application.
The wise use of threads turns slow, clunky, not very responsive programs
into crisply responsive, efficient, fast programs, and can radically simplify
various performance and usability problems.
Making use of multiprocessor systems: You can't expect one application
with only one thread to make use of two or more processors! Chapter 3 explains
this in more detail.
Efficient time sharing: Using thread and process priorities, you can ensure
that everyone gets a fair allocation of CPU time.
© Martin Harvey