Boost C++ Libraries

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C++ Boost

Boost.Threads

Rationale


Introduction
Rationale for the Creation of Boost.Threads
Rationale for the Low Level Primitives Supported in Boost.Threads
Rationale for the Lock Design
Rationale for NonCopyable Thread Type
Rationale for not providing Event Variables

Introduction

This page explains the rationale behind various design decisions in the Boost.Threads library. Having the rationale documented here should explain how we arrived at the current design as well as prevent future rehashing of discussions and thought processes that have already occurred. It can also give users a lot of insight into the design process required for this library.

Rationale for the Creation of Boost.Threads

Processes often have a degree of "potential parallelism" and it can often be more intuitive to design systems with this in mind. Further, these parallel processes can result in more responsive programs. The benefits for multithreaded programming are quite well known to most modern programmers, yet the C++ language doesn't directly support this concept.

Many platforms support multithreaded programming despite the fact that the language doesn't support it. They do this through external libraries, which are, unfortunately, platform specific. POSIX has tried to address this problem through the standardization of a "pthread" library. However, this is a standard only on POSIX platforms, so its portability is limited.

Another problem with POSIX and other platform specific thread libraries is that they are almost universally C based libraries. This leaves several C++ specific issues unresolved, such as what happens when an exception is thrown in a thread. Further, there are some C++ concepts, such as destructors, that can make usage much easier than what's available in a C library.

What's truly needed is C++ language support for threads. However, the C++ standards committee needs existing practice or a good proposal as a starting point for adding this to the standard.

The Boost.Threads library was developed to provide a C++ developer with a portable interface for writing multithreaded programs on numerous platforms. There's a hope that the library can be the basis for a more detailed proposal for the C++ standards committee to consider for inclusion in the next C++ standard.

Rationale for the Low Level Primitives Supported in Boost.Threads

The Boost.Threads library supplies a set of low level primitives for writing multithreaded programs, such as mutexes and condition variables. In fact, the first release of Boost.Threads supports only these low level primitives. However, computer science research has shown that use of these primitives is difficult since it's difficult to mathematically prove that a usage pattern is correct, meaning it doesn't result in race conditions or deadlocks. There are several algebras (such as CSP, CCS and Join calculus) that have been developed to help write provably correct parallel processes. In order to prove the correctness these processes must be coded using higher level abstractions. So why does Boost.Threads support the lower level concepts?

The reason is simple: the higher level concepts need to be implemented using at least some of the lower level concepts. So having portable lower level concepts makes it easier to develop the higher level concepts and will allow researchers to experiment with various techniques.

Beyond this theoretical application of higher level concepts, however, the fact remains that many multithreaded programs are written using only the lower level concepts, so they are useful in and of themselves, even if it's hard to prove that their usage is correct. Since many users will be familiar with these lower level concepts but be unfamiliar with any of the higher level concepts there's also an argument for accessibility.

Rationale for the Lock Design

Programmers who are used to multithreaded programming issues will quickly note that the Boost.Thread's design for mutex lock concepts is not thread-safe (this is clearly documented as well). At first this may seem like a serious design flaw. Why have a multithreading primitive that's not thread-safe itself?

A lock object is not a synchronization primitive. A lock object's sole responsibility is to ensure that a mutex is both locked and unlocked in a manner that won't result in the common error of locking a mutex and then forgetting to unlock it. This means that instances of a lock object are only going to be created, at least in theory, within block scope and won't be shared between threads. Only the mutex objects will be created outside of block scope and/or shared between threads. Though it's possible to create a lock object outside of block scope and to share it between threads to do so would not be a typical usage (in fact, to do so would likely be an error). Nor are there any cases when such usage would be required.

Lock objects must maintain some state information. In order to allow a program to determine if a try_lock or timed_lock was successful the lock object must retain state indicating the success or failure of the call made in its constructor. If a lock object were to have such state and remain thread-safe it would need to synchronize access to the state information which would result in roughly doubling the time of most operations. Worse, since checking the state can occur only by a call after construction we'd have a race condition if the lock object were shared between threads.

So, to avoid the overhead of synchronizing access to the state information and to avoid the race condition the Boost.Threads library simply does nothing to make lock objects thread-safe. Instead, sharing a lock object between threads results in undefined behavior. Since the only proper usage of lock objects is within block scope this isn't a problem, and so long as the lock object is properly used there's no danger of any multithreading issues.

Rationale for NonCopyable Thread Type

Programmers who are used to C libraries for multithreaded programming are likely to wonder why Boost.Threads uses a noncopyable design for boost::thread. After all, the C thread types are copyable, and you often have a need for copying them within user code. However, careful comparison of C designs to C++ designs shows a flaw in this logic.

All C types are copyable. It is, in fact, not possible to make a noncopyable type in C. For this reason types that represent system resources in C are often designed to behave very similarly to a pointer to dynamic memory. There's an API for acquiring the resource and an API for releasing the resources. For memory we have pointers as the type and alloc/free for the acquisition and release APIs. For files we have FILE* as the type and fopen/fclose for the acquisition and release APIs. You can freely copy instances of the types but must manually manage the lifetime of the actual resource through the acquisition and release APIs.

C++ designs recognize that the acquisition and release APIs are error prone and try to eliminate possible errors by acquiring the resource in the constructor and releasing it in the destructor. The best example of such a design is the std::iostream set of classes which can represent the same resource as the FILE* type in C. A file is opened in the std::fstream's constructor and closed in its destructor. However, if an iostream were copyable it could lead to a file being closed twice, an obvious error, so the std::iostream types are noncopyable by design. This is the same design used by boost::thread, which is a simple and easy to understand design that's consistent with other C++ standard types.

During the design of boost::thread it was pointed out that it would be possible to allow it to be a copyable type if some form of "reference management" were used, such as ref-counting or ref-lists, and many argued for a boost::thread_ref design instead. The reasoning was that copying "thread" objects was a typical need in the C libraries, and so presumably would be in the C++ libraries as well. It was also thought that implementations could provide more efficient reference management than wrappers (such as boost::shared_ptr) around a noncopyable thread concept. Analysis of whether or not these arguments would hold true doesn't appear to bear them out. To illustrate the analysis we'll first provide pseudo-code illustrating the six typical usage patterns of a thread object.

1. Simple creation of a thread.

void foo()
{
   create_thread(&bar);
}

2. Creation of a thread that's later joined.

Void foo()
{
   thread = create_thread(&bar);
   join(thread);
}

3. Simple creation of several threads in a loop.

Void foo()
{
   for (int i=0; i<NUM_THREADS; ++i)
      create_thread(&bar);
}

4. Creation of several threads in a loop which are later joined.

Void foo()
{
   for (int i=0; i<NUM_THREADS; ++i)
      threads[i] = create_thread(&bar);
   for (int i=0; i<NUM_THREADS; ++i)
      threads[i].join();
}

5. Creation of a thread whose ownership is passed to another object/method.

Void foo()
{
   thread = create_thread(&bar);
   manager.owns(thread);
}

6. Creation of a thread whose ownership is shared between multiple objects.

Void foo()
{
   thread = create_thread(&bar);
   manager1.add(thread);
   manager2.add(thread);
}

Of these usage patterns there's only one that requires reference management (number 6). Hopefully it's fairly obvious that this usage pattern simply won't occur as often as the other usage patterns. So there really isn't a "typical need" for a thread concept, though there is some need.

Since the need isn't typical we must use different criteria for deciding on either a thread_ref or thread design. Possible criteria include ease of use and performance. So let's analyze both of these carefully.

With ease of use we can look at existing experience. The standard C++ objects that represent a system resource, such as std::iostream, are noncopyable, so we know that C++ programmers must at least be experienced with this design. Most C++ developers are also used to smart pointers such as boost::shared_ptr, so we know they can at least adapt to a thread_ref concept with little effort. So existing experience isn't going to lead us to a choice.

The other thing we can look at is how difficult it is to use both types for the six usage patterns above. If we find it overly difficult to use a concept for any of the usage patterns there would be a good argument for choosing the other design. So we'll code all six usage patterns using both designs.

1.

void foo()
{
   thread thrd(&bar);
}

void foo()
{
   thread_ref thrd = create_thread(&bar);
}

2.

void foo()
{
   thread thrd(&bar);
   thrd.join();
}

void foo()
{
   thread_ref thrd =
   create_thread(&bar);thrd->join();
}

3.

void foo()
{
   for (int i=0; i<NUM_THREADS; ++i)
      thread thrd(&bar);
}

void foo()
{
   for (int i=0; i<NUM_THREADS; ++i)
      thread_ref thrd = create_thread(&bar);
}

4.

void foo()
{
   std::auto_ptr<thread> threads[NUM_THREADS];
   for (int i=0; i<NUM_THREADS; ++i)
      threads[i] = std::auto_ptr<thread>(new thread(&bar));
   for (int i= 0; i<NUM_THREADS;
      ++i)threads[i]->join();
}

void foo()
{
   thread_ref threads[NUM_THREADS];
   for (int i=0; i<NUM_THREADS; ++i)
      threads[i] = create_thread(&bar);
   for (int i= 0; i<NUM_THREADS;
      ++i)threads[i]->join();
}

5.

void foo()
{
   thread thrd* = new thread(&bar);
   manager.owns(thread);
}

void foo()
{
   thread_ref thrd = create_thread(&bar);
   manager.owns(thrd);
}

6.

void foo()
{
   boost::shared_ptr<thread> thrd(new thread(&bar));
   manager1.add(thrd);
   manager2.add(thrd);
}

void foo()
{
   thread_ref thrd = create_thread(&bar);
   manager1.add(thrd);
   manager2.add(thrd);
}

This shows the usage patterns being nearly identical in complexity for both designs. The only actual added complexity occurs because of the use of operator new in (4), (5) and (6) and the use of std::auto_ptr and boost::shared_ptr in (4) and (6) respectively. However, that's not really much added complexity, and C++ programmers are used to using these idioms any way. Some may dislike the presence of operator new in user code, but this can be eliminated by proper design of higher level concepts, such as the boost::thread_group class that simplifies example (4) down to:

void foo()
{
   thread_group threads;
   for (int i=0; i<NUM_THREADS; ++i)
      threads.create_thread(&bar);
   threads.join_all();
}

So ease of use is really a wash and not much help in picking a design.

So what about performance? If you look at the above code examples we can analyze the theoretical impact to performance that both designs have. For (1) we can see that platforms that don't have a ref-counted native thread type (POSIX, for instance) will be impacted by a thread_ref design. Even if the native thread type is ref-counted there may be an impact if more state information has to be maintained for concepts foreign to the native API, such as clean up stacks for Win32 implementations. For (2) the performance impact will be identical to (1). The same for (3). For (4) things get a little more interesting and we find that theoretically at least the thread_ref may perform faster since the thread design requires dynamic memory allocation/deallocation. However, in practice there may be dynamic allocation for the thread_ref design as well, it will just be hidden from the user. As long as the implementation has to do dynamic allocations the thread_ref loses again because of the reference management. For (5) we see the same impact as we do for (4). For (6) we still have a possible impact to the thread design because of dynamic allocation but thread_ref no longer suffers because of its reference management, and in fact, theoretically at least, the thread_ref may do a better job of managing the references. All of this indicates that thread wins for (1), (2) and (3), with (4) and (5) the winner depends on the implementation and the platform but the thread design probably has a better chance, and with (6) it will again depend on the implementation and platform but this time we favor thread_ref slightly. Given all of this it's a narrow margin, but the thread design prevails.

Given this analysis, and the fact that noncopyable objects for system resources are the normal designs that C++ programmers are used to dealing with, the Boost.Threads library has gone with a noncopyable design.

Rationale for not providing Event Variables

Event variables are simply far too error-prone. Condition variables are a much safer alternative.

[Note that Graphical User Interface events are a different concept, and are not what is being discussed here.]

Event variables were one of the first synchronization primitives. They are still used today, for example, in the native Windows multithreading API.

Yet both respected computer science researchers and experienced multithreading practitioners believe event variables are so inherently error-prone that they should never be used, and thus should not be part of a multithreading library.

Per Brinch Hansen [Brinch Hansen 73] analyzed event variables in some detail, pointing out [emphasis his] that "event operations force the programmer to be aware of the relative speeds of the sending and receiving processes". His summary:

We must therefore conclude that event variables of the previous type are impractical for system design. The effect of an interaction between two processes must be independent of the speed at which it is carried out.

Experienced programmers using the Windows platform today report that event variables are a continuing source of errors, even after previous bad experiences caused them to be very careful in their use of event variables. Overt problems can be avoided, for example, by teaming the event variable with a mutex, but that may just convert a race condition into another problem, such as excessive resource use. One of the most distressing aspects of the experience reports is the claim that many defects are latent. That is, the programs appear to work correctly, but contain hidden timing dependencies which will cause them to fail when environmental factors or usage patterns change, altering relative thread timings.

The decision to exclude event variables from Boost.Threads has been surprising to some Windows programmers. They have written programs which work using event variables, and wonder what the problem is. It seems similar to the "goto considered harmful" controversy of 30 years ago. It isn't that events, like gotos, can't be made to work, but rather that virtually all programs using alternatives will be easier to write, debug, read, maintain, and be less likely to contain latent defects.

[Rationale provided by Beman Dawes]


Revised 09 January, 2003

© Copyright William E. Kempf 2001-2002. All Rights Reserved.

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