Saturday, August 16, 2008

What is a thread?

Some of my friends are confused by all kinds of concepts around thread, such as system thread, kernel thread, native thread, user-level thread, application thread, java thread, software thread, hardware thread, simultaneous multithread (SMT), hyperthread (HT), helper thread, etc, etc.

So what a thread is?

A thread is nothing but a control flow of execution. It is an concept only valid in control-flow machine, because only then is there control flow. Then what is control flow? Or in other words, how to represent a control flow. In my opinion, only two entities are essential to represent a control flow. They are the program counter and the stack.

Program counter points to next instruction to execute. Stack stores the temporary execution result. To be a meaningful stack, a stack pointer is needed pointing to the next location to store the execution result.

Program counter and the stack can uniquely identify a control flow of an execution. They cannot be shared with other threads (except for some extreme cases). All of other computing resources can be shared between threads, such as heap, code, processor, etc. Hence, program counter and the stack are called the thread context.

That means, if a system provides threading support, it should at least provide a way to distinct one thread context from another, be it in software, hardware or hybrid. If processor hardware provides thread context support, it is hardware thread. Different hardware threads can share same processor pipeline (SMT) or use different pipelines, depending on the design. HT is an implementation of SMT. Any control-flow processor must provide at least one thread context; otherwise, there would be no control flow.

If the processor has only one thread context, threading support still can be provided by software. That is, multiple software threads can multiplex over the same thread context. When a software thread is scheduled to run, its context is loaded from the memory into the hardware thread context. If it is scheduled off the processor, its context is saved into the memory.

Software thread design has an implication. Since the thread context loading/storing (or switch) is conducted by software, it requires the software thread design to guarantee that there are chances to conduct the switch operation (or thread scheduling). An easy way to implement is to leverage hardware interrupt. Once the software receives a hardware interrupt (timer or whatever), it executes the interrupt handler and within the handler, it schedules the threads.

Sometimes, the timer is too long to wait. For example, when a thread is sleeping, before a timer handler is executed, no other thread can be scheduled. This is not desirable. A straightforward solution is, if a thread wants to sleep, it always invokes the scheduler, then the scheduler can switch on another thread.

Now that multiple software threads can share the same hardware context, it is not hard to think that, a software thread context can also be multiplexed by another level of multiple software threads. This is true. So conceptually, software threads can be built with infinite levels, every higher level threads multiplex the contexts of its next level threads.

Also a natural corollary is, a thread is only a thread in your level of discussion. It could contain multiple threads in a higher-level of discussion. Well, although this is true, people do not really build many levels of software threading libraries. Usually there are only two levels, one level shares the hardware context, and the other level shares the software context.

This is reasonable. The most important reason is, all the software threads in one level are treated as a single thread in the next level, so they are scheduled as one thread in the next level. That means, they total only share the time slice of a single thread in the next level. If the next level thread is scheduled off the processor, none of them can be continuing. This is inconvenient.

More inconvenient is, sometimes, only one thread wants to sleep, but all the other threads have to sleep with it together, because they are treated as a single thread in the next level scheduler who sees the sleep operation. This issue can be partially solved with non-blocking sleep. That is, when a thread wants to sleep, it does not really sleep in the sense of the next level scheduler. It only sleeps in the eyes of its level's scheduler. This scheduler will schedule another thread at the same level. From the next-level thread scheduler's point of view, the thread is just continuing without sleep at all. In threading terminology, all the blocking operations (such as sleeping) in one level are implemented as non-blocking in its next level. (Well, this requires the system support for non-blocking operations, such as socket snooping, etc.)

Only operating system kernel really takes control of the execution engine (i.e., the processor or the pipeline). So the time slice concept is only really meaningful to the kernel. That means, only the threading at kernel level can really manipulate all the resources. Higher levels of software threads should always try to leverage the support of kernel threading. This is the fundamental reason why we want at most one additional level of threading above kernel threads.

Kernel threads are exposed to user applications through threading APIs. They are called native threads by the applications, such as NPTL or Linuxthreads in Linux, and WinThreads in Windows. The threading library implemented on top of native threads is called user-level threading. For the inconvenience we discussed above, not so many software today employ user-level threads.

User-level threads have its own advantages in certain scenarios. For example, multiple user threads never run in parallel on multiple processors/cores, because they are actually just single thread from OS' point of view.

Java thread is thread in another dimension. It is actually a language concept. It can implemented in any of threading mechanisms discussed above. Previously before Java, all the threading supports are kind of independent of programming languages. They are just system supports, and any languages can utilize if they want. Java takes a different approach that, it builds threading concept in its language. This is important for program semantic correctness. Hans had a PLDI paper with title "Threads Cannot be Implemented as a Library" [1]. And people are trying to introduce threading as a language construct into more languages.

[1] Hans Boehm, Threads Cannot be Implemented as a Library,