Chapter 3. Basic synchronization.

In this chapter:

What data is shared between threads?

First of all, it is worth know exactly what state is stored on a per process and per thread basis. Each thread has its own program counter and processor state. This means that threads progress independently through the code. Each thread also has its own stack, so local variables are inherently local to each thread, and no synchronization issues exist for these variables. Global data in the program can be freely shared between threads, and thus synchronization problems may exist with these variables. Of course, if a variable is globally accessible, but only one thread uses it, then there is no problem. The same situation holds for memory allocated on the heap (normally objects): in principle, any thread could access a particular object, but if the program is written so that only one thread has a pointer to a particular object, then only one thread can access it, and no concurrency problem exists.

Delphi provides a threadvar keyword. This allows "global" variables to be declared where a copy of the variable is created for each thread. This feature is not used much because it is often more convenient to put such variables inside a TThread class, hence creating one variable instance for each TThread descendant created.

Atomicity when accessing shared data.

In order to understand how to make threads work together, it is necessary to understand the concept of atomicity. An action or sequence of actions is atomic if the action or sequence is indivisible. When a thread performs an atomic action, it means that all other threads view the action as either having not started, or as having completed. It is not possible for one thread to catch another "in the act". If no synchronization is performed between threads, then just about all operations are non-atomic. Let's take a simple example. Consider this fragment of code. What could be simpler? Unfortunately, even this trivial piece of code can cause problems if two separate threads use it to increment the shared variable A. This single pascal statement breaks down into three operations at the assembler level.

Read A from memory into a processor register.
Add 1 to processor register.
Write contents of processor register to A in memory.

Even on a single processor machine, the execution of this code by multiple threads may cause problems. The reason it does so is because of scheduling operations. When only one processor exists, only one thread actually executes at one time, but the Win32 scheduler switches between them at about 18 times per second. The scheduler may stop one thread running and start another thread at any time: the scheduling is pre-emptive. The operating system does not wait for permission before suspending one thread and starting another: the switch may happen at any time. Since the switch can occur between any two processor instructions, it may occur at an inconvenient point in the middle of a function, and even half way through the execution of one particular program statement. Let's imagine that two threads are executing the example code on a uniprocessor machine (X and Y). In a nice case, the program may be running, and the scheduling operations may miss this critical point, giving the expected results: A is incremented by two.
 
Instructions executed by Thread X
Instructions executed by Thread Y
Value of variable A in memory
<Other Instructions>
Thread Suspended
1
Read A from memory into a processor register.
Thread Suspended
1
Add 1 to processor register.
Thread Suspended
1
Write contents of processor register (2) to A in memory.
Thread Suspended
2
<Other Instructions>
Thread Suspended
2
THREAD SWITCH
THREAD SWITCH
2
Thread Suspended
<Other Instructions>
2
Thread Suspended
Read A from memory into a processor register.
2
Thread Suspended
Add 1 to processor register.
2
Thread Suspended
Write contents of processor register to A (3) in memory.
3
Thread Suspended
<Other Instructions>
3

However, this is by no means guaranteed, and is up to blind chance. Murphy's law being what it is, the following situation may occur:
 
Instructions executed by Thread X
Instructions executed by Thread Y
Value of variable A in memory
<Other Instructions>
Thread Suspended
1
Read A from memory into a processor register.
Thread Suspended
1
Add 1 to processor register.
Thread Suspended
1
THREAD SWITCH
THREAD SWITCH
1
Thread Suspended
<Other Instructions>
1
Thread Suspended
Read A from memory into a processor register.
1
Thread Suspended
Add 1 to processor register.
1
Thread Suspended
Write contents of processor register (2)to A in memory.
2
THREAD SWITCH
THREAD SWITCH
2
Write contents of processor register (2) to A in memory.
Thread Suspended
2
<Other Instructions>
Thread Suspended
2

In this case, A is not incremented by two but by only one. Oh dear! If A happens to be the position of a progress bar, then perhaps this isn't such a problem, but if it's anything more important, like a count of the number of items in a list, then one is likely to run into problems. If the shared variable happens to be a pointer then one can expect all sorts of unpleasant results. This is known as a race condition.

Additional VCL problems.

The VCL contains no protection against these conflicts. This means that thread switches may occur when one or more threads are executing VCL code. A lot of the VCL is sufficiently well contained that this is not a problem. Unfortunately, components, and in particular, children of TControl contain various mechanisms which do not take kindly to thread switches. A thread switch at the wrong time can wreak complete havoc, corrupting reference counts for shared handles, destroying not only data, but also links between components.

Even when threads are not executing VCL code, lack of synchronization can still cause further problems: It is not enough to ensure that the main VCL thread is dormant before another thread dives in and modifies something. Some code in the VCL may execute which (for instance) pops up a dialog box, or invokes a disk write, suspending the main thread. If another thread modifies shared data, it may appear to the main thread that some global data has magically changed as a result of the call to display a dialog or write to a file. This is obviously not acceptable and means that either only one thread can execute VCL code, or a mechanism must be found to ensure that separate threads do not interfere with each other.

Fun with multiprocessor machines.

Luckily for the application writer, the problem is not made any more complex for machines with more than one processor. The synchronization methods that Delphi and Windows provide work equally well whatever the number of processors. Implementors of the Windows operating systems have had to write extra code to cope with multiprocessing: Windows NT 4 informs the user at bootup whether it is using the uniprocessor or multiprocessor kernel. However, for the application writer, this is all hidden. You do not need to worry about how many processors the machine has any more than you have to worry about which chipset the motherboard uses.

The Delphi solution: TThread.Synchronize.

Delphi provides a solution which is ideal for beginners to thread writing. It is simple and overcomes all the problems mentioned so far. TThread has a method called Synchronize. This method takes as a parameter another parameterless method which you want to be executed. You are then guaranteed that the code in the parameterless method will be executed as a result of the synchronize call, and will not conflict with the VCL thread. As far as the non-VCL thread that calls synchronize is concerned, it appears that all the code in the parameterless method happens at the moment synchronize is called.

Hmm. Sound confusing? Quite possibly. I'll illustrate this with an example. We will modify our prime number program, so that instead of showing a message box, it indicates whether a number is prime or not by adding some text to a memo in the main form. First of all, we'll add a new memo (ResultsMemo) to our main form, like this. Now we can do the real work. We add another method (UpdateResults) to our thread which displays the results on the memo, and instead of calling ShowMessage, we call Synchronize, passing this method as a parameter. The declaration of the thread and the modified parts now look like this. Note that UpdateResults accesses both the main form, and a result string. From the viewpoint of the main VCL thread, the main form appears to be modified in response to an event. From the viewpoint of the prime calculation thread, the result string is accessed during the call to Synchronize.

How does this work? What does Synchronize do?

Code which is invoked when synchronize is called can perform anything that the main VCL thread might do. In addition, it can also modify data associated with its own thread object, safe in the knowledge that the execution of its own thread is at a particular point (the call to synchronize). What actually happens is rather elegant, and best illustrated by another diagram.
When synchronize is called, the prime calculation thread is suspended. At this point, the main VCL thread may be suspended in the idle state, it may be suspended temporarily on I/O or other operations, or it may be executing. If it is not suspended in a totally idle state (main application message loop), then the prime calculation thread keeps waiting. Once the main thread becomes idle, the parameterless function passed to synchronize executes in the context of the main VCL thread. In our case, the parameterless function is called UpdateResults, and plays around with a memo. This ensures that no conflicts will occur with the main VCL thread, and in essence, the processing of this code is much like the processing of any Delphi code which occurs in response to the application being sent a message. No conflicts occur with the thread that called synchronize because it is suspended at a known safe point (somewhere in the code for TThread.Synchronise).

Once this "processing by proxy" completes, the main VCL thread is free to go about its normal work, and the thread that called synchronize is resumed, and returns from the function call. Thus a call to Synchronize appears to be another message to the main VCL thread, and a function call to the Prime calculation thread. The threads are at known locations, and do not execute concurrently. No race conditions occur. Problem solved.

Synchronizing to non-VCL threads.

My previous example shows how a single thread can be made to interact with the main VCL thread. In effect it borrows time from the main VCL thread to do this. This doesn't work arbitrarily between threads. If you have two non-VCL threads, X and Y, you can't call synchronize in X alone and then modify data stored in Y. It is necessary to call synchronize from both threads when reading or writing the shared data. In effect, this means that the data is modified by the main VCL thread, and all the other threads synchronize to the main VCL thread every time they need to access this data. This is workable, but inefficient, especially if the main thread is busy: every time the two threads need to communicate, they have to wait for a third thread to become idle. Later on, we shall see how to control concurrency between the threads and have them communicate directly.

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© Martin Harvey 2000.