I've been looking through a few notes based on this topic, and although I have an understanding of threads in general, I'm not really to sure about the differences between user-level and kernel-level threads.
I know that processes are basically made up of multiple threads or a single thread, but are these thread of the two prior mentioned types?
From what I understand, kernel-supported threads have access to the kernel for system calls and other uses not available to user-level threads.
So, are user-level threads simply threads created by the programmer when then utilise kernel-supported threads to perform operations that couldn't be normally performed due to its state?
Edit: The question was a little confusing, so I'm answering it two different ways.
OS-level threads vs Green Threads
For clarity, I usually say "OS-level threads" or "native threads" instead of "Kernel-level threads" (which I confused with "kernel threads" in my original answer below.) OS-level threads are created and managed by the OS. Most languages have support for them. (C, recent Java, etc) They are extremely hard to use because you are 100% responsible for preventing problems. In some languages, even the native data structures (such as Hashes or Dictionaries) will break without extra locking code.
The opposite of an OS-thread is a green thread that is managed by your language. These threads are given various names depending on the language (coroutines in C, goroutines in Go, fibers in Ruby, etc). These threads only exist inside your language and not in your OS. Because the language chooses context switches (i.e. at the end of a statement), it prevents TONS of subtle race conditions (such as seeing a partially-copied structure, or needing to lock most data structures). The programmer sees "blocking" calls (i.e. data = file.read() ), but the language translates it into async calls to the OS. The language then allows other green threads to run while waiting for the result.
Green threads are much simpler for the programmer, but their performance varies: If you have a LOT of threads, green threads can be better for both CPU and RAM. On the other hand, most green thread languages can't take advantage of multiple cores. (You can't even buy a single-core computer or phone anymore!). And a bad library can halt the entire language by doing a blocking OS call.
The best of both worlds is to have one OS thread per CPU, and many green threads that are magically moved around onto OS threads. Languages like Go and Erlang can do this.
system calls and other uses not available to user-level threads
This is only half true. Yes, you can easily cause problems if you call the OS yourself (i.e. do something that's blocking.) But the language usually has replacements, so you don't even notice. These replacements do call the kernel, just slightly differently than you think.
Kernel threads vs User Threads
Edit: This is my original answer, but it is about User space threads vs Kernel-only threads, which (in hindsight) probably wasn't the question.
User threads and Kernel threads are exactly the same. (You can see by looking in /proc/ and see that the kernel threads are there too.)
A User thread is one that executes user-space code. But it can call into kernel space at any time. It's still considered a "User" thread, even though it's executing kernel code at elevated security levels.
A Kernel thread is one that only runs kernel code and isn't associated with a user-space process. These are like "UNIX daemons", except they are kernel-only daemons. So you could say that the kernel is a multi-threaded program. For example, there is a kernel thread for swap. This forces all swap issues to get "serialized" into a single stream.
If a user thread needs something, it will call into the kernel, which marks that thread as sleeping. Later, the swap thread finds the data, so it marks the user thread as runnable. Later still, the "user thread" returns from the kernel back to userland as if nothing happened.
In fact, all threads start off in kernel space, because the clone() operation happens in kernel space. (And there's lots of kernel accounting to do before you can 'return' to a new process in user space.)
Before we go into comparison, let us first understand what a thread is. Threads are lightweight processes within the domain of independent processes. They are required because processes are heavy, consume a lot of resources and more importantly,
two separate processes cannot share a memory space.
Let's say you open a text editor. It's an independent process executing in the memory with a separate addressable location. You'll need many resources within this process, such as insert graphics, spell-checks etc. It's not feasible to create separate processes for each of these functionalities and maintain them independently in memory. To avoid this,
multiple threads can be created within a single process, which can
share a common memory space, existing independently within a process.
Now, coming back to your questions, one at a time.
I'm not really to sure about the differences between user-level and kernel-level threads.
Threads are broadly classified as user level threads and kernel level threads based on their domain of execution. There are also cases when one or many user thread maps to one or many kernel threads.
- User Level Threads
User level threads are mostly at the application level where an application creates these threads to sustain its execution in the main memory. Unless required, these thread work in isolation with kernel threads.
These are easier to create since they do not have to refer many registers and context switching is much faster than a kernel level thread.
User level thread, mostly can cause changes at the application level and the kernel level thread continues to execute at its own pace.
- Kernel Level Threads
These threads are mostly independent of the ongoing processes and are executed by the operating system.
These threads are required by the Operating System for tasks like memory management, process management etc.
Since these threads maintain, execute and report the processes required by the operating system; kernel level threads are more expensive to create and manage and context switching of these threads are slow.
Most of the kernel level threads can not be preempted by the user level threads.
MS DOS written for Intel 8088 didn't have dual mode of operation. Thus, a user level process had the ability to corrupt the entire operating system.
- User Level Threads mapped over Kernel Threads
This is perhaps the most interesting part. Many user level threads map over to kernel level thread, which in-turn communicate with the kernel.
Some of the prominent mappings are:
One to One
When one user level thread maps to only one kernel thread.
advantages: each user thread maps to one kernel thread. Even if one of the user thread issues a blocking system call, the other processes remain unaffected.
disadvantages: every user thread requires one kernel thread to interact and kernel threads are expensive to create and manage.
Many to One
When many user threads map to one kernel thread.
advantages: multiple kernel threads are not required since similar user threads can be mapped to one kernel thread.
disadvantage: even if one of the user thread issues a blocking system call, all the other user threads mapped to that kernel thread are blocked.
Also, a good level of concurrency cannot be achieved since the kernel will process only one kernel thread at a time.
Many to Many
When many user threads map to equal or lesser number of kernel threads. The programmer decides how many user threads will map to how many kernel threads. Some of the user threads might map to just one kernel thread.
advantages: a great level of concurrency is achieved. Programmer can decide some potentially dangerous threads which might issue a blocking system call and place them with the one-to-one mapping.
disadvantage: the number of kernel threads, if not decided cautiously can slow down the system.
The other part of your question:
kernel-supported threads have access to the kernel for system calls
and other uses not available to user-level threads.
So, are user-level threads simply threads created by the programmer
when then utilise kernel-supported threads to perform operations that
couldn't be normally performed due to its state?
Partially correct. Almost all the kernel thread have access to system calls and other critical interrupts since kernel threads are responsible for executing the processes of the OS. User thread will not have access to some of these critical features. e.g. a text editor can never shoot a thread which has the ability to change the physical address of the process. But if needed, a user thread can map to kernel thread and issue some of the system calls which it couldn't do as an independent entity. The kernel thread would then map this system call to the kernel and would execute actions, if deemed fit.
Quote from here :
Kernel-Level Threads
To make concurrency cheaper, the execution aspect of process is separated out into threads. As such, the OS now manages threads and processes. All thread operations are implemented in the kernel and the OS schedules all threads in the system. OS managed threads are called kernel-level threads or light weight processes.
NT: Threads
Solaris: Lightweight processes(LWP).
In this method, the kernel knows about and manages the threads. No runtime system is needed in this case. Instead of thread table in each process, the kernel has a thread table that keeps track of all threads in the system. In addition, the kernel also maintains the traditional process table to keep track of processes. Operating Systems kernel provides system call to create and manage threads.
Advantages:
Because kernel has full knowledge of all threads, Scheduler may decide to give more time to a process having large number of threads than process having small number of threads.
Kernel-level threads are especially good for applications that frequently block.
Disadvantages:
The kernel-level threads are slow and inefficient. For instance, threads operations are hundreds of times slower than that of user-level threads.
Since kernel must manage and schedule threads as well as processes. It require a full thread control block (TCB) for each thread to maintain information about threads. As a result there is significant overhead and increased in kernel complexity.
User-Level Threads
Kernel-Level threads make concurrency much cheaper than process because, much less state to allocate and initialize. However, for fine-grained concurrency, kernel-level threads still suffer from too much overhead. Thread operations still require system calls. Ideally, we require thread operations to be as fast as a procedure call. Kernel-Level threads have to be general to support the needs of all programmers, languages, runtimes, etc. For such fine grained concurrency we need still "cheaper" threads.
To make threads cheap and fast, they need to be implemented at user level. User-Level threads are managed entirely by the run-time system (user-level library).The kernel knows nothing about user-level threads and manages them as if they were single-threaded processes.User-Level threads are small and fast, each thread is represented by a PC,register,stack, and small thread control block. Creating a new thread, switiching between threads, and synchronizing threads are done via procedure call. i.e no kernel involvement. User-Level threads are hundred times faster than Kernel-Level threads.
Advantages:
The most obvious advantage of this technique is that a user-level threads package can be implemented on an Operating System that does not support threads.
User-level threads does not require modification to operating systems.
Simple Representation: Each thread is represented simply by a PC, registers, stack and a small control block, all stored in the user process address space.
Simple Management: This simply means that creating a thread, switching between threads and synchronization between threads can all be done without intervention of the kernel.
Fast and Efficient: Thread switching is not much more expensive than a procedure call.
Disadvantages:
User-Level threads are not a perfect solution as with everything else, they are a trade off. Since, User-Level threads are invisible to the OS they are not well integrated with the OS. As a result, Os can make poor decisions like scheduling a process with idle threads, blocking a process whose thread initiated an I/O even though the process has other threads that can run and unscheduling a process with a thread holding a lock. Solving this requires communication between between kernel and user-level thread manager.
There is a lack of coordination between threads and operating system kernel. Therefore, process as whole gets one time slice irrespect of whether process has one thread or 1000 threads within. It is up to each thread to relinquish control to other threads.
User-level threads requires non-blocking systems call i.e., a multithreaded kernel. Otherwise, entire process will blocked in the kernel, even if there are runable threads left in the processes. For example, if one thread causes a page fault, the process blocks.
User Threads
The library provides support for thread creation, scheduling and management with no support from the kernel.
The kernel unaware of user-level threads creation and scheduling are done in user space without kernel intervention.
User-level threads are generally fast to create and manage they have drawbacks however.
If the kernel is single-threaded, then any user-level thread performing a blocking system call will cause the entire process to block, even if other threads are available to run within the application.
User-thread libraries include POSIX Pthreads, Mach C-threads,
and Solaris 2 UI-threads.
Kernel threads
The kernel performs thread creation, scheduling, and management in kernel space.
kernel threads are generally slower to create and manage than are user threads.
the kernel is managing the threads, if a thread performs a blocking system call.
A multiprocessor environment, the kernel can schedule threads on different processors.
5.including Windows NT, Windows 2000, Solaris 2, BeOS, and Tru64 UNIX (formerlyDigital UN1X)-support kernel threads.
Some development environments or languages will add there own threads like feature, that is written to take advantage of some knowledge of the environment, for example a GUI environment could implement some thread functionality which switch between user threads on each event loop.
A game library could have some thread like behaviour for characters. Sometimes the user thread like behaviour can be implemented in a different way, for example I work with cocoa a lot, and it has a timer mechanism which executes your code every x number of seconds, use fraction of a seconds and it like a thread. Ruby has a yield feature which is like cooperative threads. The advantage of user threads is they can switch at more predictable times. With kernel thread every time a thread starts up again, it needs to load any data it was working on, this can take time, with user threads you can switch when you have finished working on some data, so it doesn't need to be reloaded.
I haven't come across user threads that look the same as kernel threads, only thread like mechanisms like the timer, though I have read about them in older text books so I wonder if they were something that was more popular in the past but with the rise of true multithreaded OS's (modern Windows and Mac OS X) and more powerful hardware I wonder if they have gone out of favour.
Related
While going through OPERATING SYSTEM PRINCIPLES, 7TH ED
(By Abraham Silberschatz, Peter Baer Galvin, Greg Gagne), i encountered a
statement in Thread Scheduling Section.It is given as -:
To run on a CPU, user-level threads must ultimately be mapped
to an associated kernel-level thread, although this mapping may
be indirect and may use a lightweight process (LWP).
The first half of the statement i.e
To run on a CPU, user-level threads must ultimately be mapped to an associated kernel-level
is trying to say that When a user level thread is executed ,it will need support from kernel thread like system calls.
But i am completely stuck in other half i.e
although this mapping may
be indirect and may use a lightweight process (LWP)
What does it really mean ???
Please help me out !
You're reading a book that is notoriously crapola. Threads are implemented in two ways.
In the olde days (and still persists on some operating systems) there were just processes. A process consisted of an execution stream and an address space.
When languages that needed thread support (e.g., Ada—"tasks") there was a need to create libraries to implement threads. The libraries used timers to switch among the various threads within the process. This is poor man's threading. The major drawback here is that, even when you have multiple processors, all the threads of a process run on the same processor. The threads are just interleaved execution within a single process that executes on one processors.
These are sometimes called "user level threads." Some books call this the "many-to-one model."
To say
To run on a CPU, user-level threads must ultimately be mapped to an associated kernel-level thread
is highly misleading. There [usually] ARE no kernel threads in this model; just processes. Multiple threads run interleaved in a process. To call this a mapping "to an associated kernel-level thread" is misleading and overly theoretical.
This is mumbo jumbo.
although this mapping may be indirect and may use a lightweight process (LWP)
The next stage in operating system evolution here was for the operating system to support threads directly. Instead of a process being an execution stream + address-space, a process became one-or-more-threads + address-space. Instead of scheduling processes for execution, the OS schedules threads for execution.
Those are kernel threads.
Your book is making the simple complex.
These days the term Light Weight Processes and threads are used interchangeably.
although this mapping may be indirect and may use a lightweight
process (LWP)
I know the above statement is confusing(Notice the 2 mays). I can think only 1 thing which the above statement signifies is that:
Earlier when linux supported only user-level threads, the kernel was unaware of the fact that there are multiple user-level threads, and the way it handled these multiple threads was by associating all of them to a light weight process(which kernel sees as a single scheduling and execution unit) at kernel level.
So associating a kernel-level thread with each user-level thread is kind of direct mapping and associating a single light weight process with each user-level thread is indirect mapping.
I'm learning about threads and processes in an Operating Systems course, and I've come across an apparent contradiction in my textbook (Modern Operating Systems, 4th Ed. by Tanenbaum and Bos). I'm sure there's a something I'm misinterpreting here, it'd be great if someone could clear things up.
On page 106:
Another common thread call is thread_yield, which allows a thread to voluntarily give up the CPU to let another thread run. Such a call is important because there is no clock interrupt to actually enforce multiprogramming as there is with processes
Ok fine - so how I interpret that is that threads will never give up control unless they willingly cede it. Makes sense.
Then on page 116, in an example of threads mishandling shared information:
As an example, consider the errno variable maintained by UNIX. When a process (or a thread) makes a system call that fails, the error code is put into errno. In Fig. 2-19, thread 1 executes the system call access to find out if it has permission to access a certain file. The operating system returns the answer in the global variable errno. After control has returned to thread 1, but before it has a chance to read errno, the scheduler decides that thread 1 has had enough CPU time for the moment and decides to switch to thread 2.
But didn't thread 1 just get pulled from the CPU involuntarily? I thought there was no way to enforce thread switching as there is with process switching?
This makes sense if we're going about process-level threads instead of OS-level threads. The CPU can interrupt a process (regardless of what thread is running), but because the OS is not aware of process-level threads, it cannot interrupt them. If one thread inside the process wants to allow another thread to run, it has to specifically yield to the other thread.
However, most languages these days use OS-level threads, which the OS does know about and can pre-empt.
The confusion is that there are two different ways threads are implemented. In ye olde days there was no thread support at all. The DoD's mandate of the Ada programming language (in which tasks—aka threads—were is an integral part) forced the adoption of threads.
Run time libraries were created (largely to support Ada). That worked within a process. The process maintained a timer that would interrupt a threads and the library would switch among threads much like the operating system switches processes.
Note that this system only allows one thread of a process at a time to execute, even on a multiprocessor system.
Your first example is describing such a library but it is describing a very primitive thread library where thread scheduling is based upon cooperation among the various threads of the process.
Later, operating system started to develop support for threads. Rather than scheduling a process, the operating system schedules threads for execution. A process is then an address space with a collection of threads. Your second example is talking about this kind of thread.
I was looking at the differences between user-level threads and kernel-level threads, which I basically understood.
What's not clear to me is the point of implementing user-level threads at all.
If the kernel is unaware of the existence of multiple threads within a single process, then which benefits could I experience?
I have read a couple of articles that stated user-level implementation of threads is advisable only if such threads do not perform blocking operations (which would cause the entire process to block).
This being said, what's the difference between a sequential execution of all the threads and a "parallel" execution of them, considering they cannot take advantage of multiple processors and independent scheduling?
An answer to a previously asked question (similar to mine) was something like:
No modern operating system actually maps n user-level threads to 1
kernel-level thread.
But for some reason, many people on the Internet state that user-level threads can never take advantage of multiple processors.
Could you help me understand this, please?
I strongly recommend Modern Operating Systems 4th Edition by Andrew S. Tanenbaum (starring in shows such as the debate about Linux; also participating: Linus Torvalds). Costs a whole lot of bucks but it's definitely worth it if you really want to know stuff. For eager students and desperate enthusiasts it's great.
Your questions answered
[...] what's not clear to me is the point of implementing User-level threads
at all.
Read my post. It is comprehensive, I daresay.
If the kernel is unaware of the existence of multiple threads within a
single process, then which benefits could I experience?
Read the section "Disadvantages" below.
I have read a couple of articles that stated that user-level
implementation of threads is advisable only if such threads do not
perform blocking operations (which would cause the entire process to
block).
Read the subsection "No coordination with system calls" in "Disadvantages."
All citations are from the book I recommended in the top of this answer, Chapter 2.2.4, "Implementing Threads in User Space."
Advantages
Enables threads on systems without threads
The first advantage is that user-level threads are a way to work with threads on a system without threads.
The first, and most obvious, advantage is that
a user-level threads package can be implemented on an operating system that does not support threads. All operating systems used to
fall into this category, and even now some still do.
No kernel interaction required
A further benefit is the light overhead when switching threads, as opposed to switching to the kernel mode, doing stuff, switching back, etc. The lighter thread switching is described like this in the book:
When a thread does something that may cause it to become blocked
locally, for example, waiting for another thread in its process to
complete some work, it calls a run-time system procedure. This
procedure checks to see if the thread must be put into blocked state.
If, so it stores the thread’s registers (i.e., its own) [...] and
reloads the machine registers with the new thread’s saved values. As soon as the stack
pointer and program counter have been switched, the new thread comes
to life again automatically. If the machine happens to have an
instruction to store all the registers and another one to load them
all, the entire thread switch can be done in just a handful of in-
structions. Doing thread switching like this is at least an order of
magnitude—maybe more—faster than trapping to the kernel and is a
strong argument in favor of user-level threads packages.
This efficiency is also nice because it spares us from incredibly heavy context switches and all that stuff.
Individually adjusted scheduling algorithms
Also, hence there is no central scheduling algorithm, every process can have its own scheduling algorithm and is way more flexible in its variety of choices. In addition, the "private" scheduling algorithm is way more flexible concerning the information it gets from the threads. The number of information can be adjusted manually and per-process, so it's very finely-grained. This is because, again, there is no central scheduling algorithm needing to fit the needs of every process; it has to be very general and all and must deliver adequate performance in every case. User-level threads allow an extremely specialized scheduling algorithm.
This is only restricted by the disadvantage "No automatic switching to the scheduler."
They [user-level threads] allow each process to have its own
customized scheduling algorithm. For some applications, for example,
those with a garbage-collector thread, not having to worry about a
thread being stopped at an inconvenient moment is a plus. They also
scale better, since kernel threads invariably require some table space
and stack space in the kernel, which can be a problem if there are a
very large number of threads.
Disadvantages
No coordination with system calls
The user-level scheduling algorithm has no idea if some thread has called a blocking read system call. OTOH, a kernel-level scheduling algorithm would've known because it can be notified by the system call; both belong to the kernel code base.
Suppose that a thread reads from the keyboard before any keys have
been hit. Letting the thread actually make the system call is
unacceptable, since this will stop all the threads. One of the main
goals of having threads in the first place was to allow each one to
use blocking calls, but to prevent one blocked thread from affecting
the others. With blocking system calls, it is hard to see how this
goal can be achieved readily.
He goes on that system calls could be made non-blocking but that would be very inconvenient and compatibility to existing OSes would be drastically hurt.
Mr Tanenbaum also says that the library wrappers around the system calls (as found in glibc, for example) could be modified to predict when a system cal blocks using select but he utters that this is inelegant.
Building upon that, he says that threads do block often. Often blocking requires many system calls. And many system calls are bad. And without blocking, threads become less useful:
For applications that are essentially entirely CPU bound and rarely
block, what is the point of having threads at all? No one would
seriously propose computing the first n prime numbers or playing chess
using threads because there is nothing to be gained by doing it that
way.
Page faults block per-process if unaware of threads
The OS has no notion of threads. Therefore, if a page fault occurs, the whole process will be blocked, effectively blocking all user-level threads.
Somewhat analogous to the problem of blocking system calls is the
problem of page faults. [...] If the program calls or jumps to an
instruction that is not in memory, a page fault occurs and the
operating system will go and get the missing instruction (and its
neighbors) from disk. [...] The process is blocked while the necessary
instruction is being located and read in. If a thread causes a page
fault, the kernel, unaware of even the existence of threads, naturally
blocks the entire process until the disk I/O is complete, even though
other threads might be runnable.
I think this can be generalized to all interrupts.
No automatic switching to the scheduler
Since there is no per-process clock interrupt, a thread acquires the CPU forever unless some OS-dependent mechanism (such as a context switch) occurs or it voluntarily releases the CPU.
This prevents usual scheduling algorithms from working, including the Round-Robin algorithm.
[...] if a thread starts running, no other thread in that process
will ever run unless the first thread voluntarily gives up the CPU.
Within a single process, there are no clock interrupts, making it
impossible to schedule processes round-robin fashion (taking turns).
Unless a thread enters the run-time system of its own free will, the scheduler will never get a chance.
He says that a possible solution would be
[...] to have the run-time system request a clock signal (interrupt) once a
second to give it control, but this, too, is crude and messy to
program.
I would even go on further and say that such a "request" would require some system call to happen, whose drawback is already explained in "No coordination with system calls." If no system call then the program would need free access to the timer, which is a security hole and unacceptable in modern OSes.
What's not clear to me is the point of implementing user-level threads at all.
User-level threads largely came into the mainstream due to Ada and its requirement for threads (tasks in Ada terminology). At the time, there were few multiprocessor systems and most multiprocessors were of the master/slave variety. Kernel threads simply did not exist. User threads had to be created to implement languages like Ada.
If the kernel is unaware of the existence of multiple threads within a single process, then which benefits could I experience?
If you have kernel threads, threads multiple threads within a single process can run simultaneously. In user threads, the threads always execute interleaved.
Using threads can simplify some types of programming.
I have read a couple of articles that stated user-level implementation of threads is advisable only if such threads do not perform blocking operations (which would cause the entire process to block).
That is true on Unix and maybe not all unix implementations. User threads on many operating systems function perfectly fine with blocking I/O.
This being said, what's the difference between a sequential execution of all the threads and a "parallel" execution of them, considering they cannot take advantage of multiple processors and independent scheduling?
In user threads. there is never parallel execution. In kernel threads, the can be parallel execution IF there are multiple processors. On a single processor system, there is not much advantage to using kernel threads over single threads (contra: note the blocking I/O issue on Unix and user threads).
But for some reason, many people on the Internet state that user-level threads can never take advantage of multiple processors.
In user threads, the process manages its own "threads" by interleaving execution within itself. The process can only have a thread run in the processor that the process is running in.
If the operating system provides system services to schedule code to run on a different processor, user threads could run on multiple processors.
I conclude by saying that for practicable purposes there are no advantages to user threads over kernel threads. There are those that will assert that there are performance advantages, but for there to be such an advantage it would be system dependent.
Here's what I understand; please correct/add to it:
In pure ULTs, the multithreaded process itself does the thread scheduling. So, the kernel essentially does not notice the difference and considers it a single-thread process. If one thread makes a blocking system call, the entire process is blocked. Even on a multicore processor, only one thread of the process would running at a time, unless the process is blocked. I'm not sure how ULTs are much help though.
In pure KLTs, even if a thread is blocked, the kernel schedules another (ready) thread of the same process. (In case of pure KLTs, I'm assuming the kernel creates all the threads of the process.)
Also, using a combination of ULTs and KLTs, how are ULTs mapped into KLTs?
Your analysis is correct. The OS kernel has no knowledge of user-level threads. From its perspective, a process is an opaque black box that occasionally makes system calls. Consequently, if that program has 100,000 user-level threads but only one kernel thread, then the process can only one run user-level thread at a time because there is only one kernel-level thread associated with it. On the other hand, if a process has multiple kernel-level threads, then it can execute multiple commands in parallel if there is a multicore machine.
A common compromise between these is to have a program request some fixed number of kernel-level threads, then have its own thread scheduler divvy up the user-level threads onto these kernel-level threads as appropriate. That way, multiple ULTs can execute in parallel, and the program can have fine-grained control over how threads execute.
As for how this mapping works - there are a bunch of different schemes. You could imagine that the user program uses any one of multiple different scheduling systems. In fact, if you do this substitution:
Kernel thread <---> Processor core
User thread <---> Kernel thread
Then any scheme the OS could use to map kernel threads onto cores could also be used to map user-level threads onto kernel-level threads.
Hope this helps!
Before anything else, templatetypedef's answer is beautiful; I simply wanted to extend his response a little.
There is one area which I felt the need for expanding a little: combinations of ULT's and KLT's. To understand the importance (what Wikipedia labels hybrid threading), consider the following examples:
Consider a multi-threaded program (multiple KLT's) where there are more KLT's than available logical cores. In order to efficiently use every core, as you mentioned, you want the scheduler to switch out KLT's that are blocking with ones that in a ready state and not blocking. This ensures the core is reducing its amount of idle time. Unfortunately, switching KLT's is expensive for the scheduler and it consumes a relatively large amount of CPU time.
This is one area where hybrid threading can be helpful. Consider a multi-threaded program with multiple KLT's and ULT's. Just as templatetypedef noted, only one ULT can be running at one time for each KLT. If a ULT is blocking, we still want to switch it out for one which is not blocking. Fortunately, ULT's are much more lightweight than KLT's, in the sense that there less resources assigned to a ULT and they require no interaction with the kernel scheduler. Essentially, it is almost always quicker to switch out ULT's than it is to switch out KLT's. As a result, we are able to significantly reduce a cores idle time relative to the first example.
Now, of course, all of this depends on the threading library being used for implementing ULT's. There are two ways (which I can come up with) for "mapping" ULT's to KLT's.
A collection of ULT's for all KLT's
This situation is ideal on a shared memory system. There is essentially a "pool" of ULT's to which each KLT has access. Ideally, the threading library scheduler would assign ULT's to each KLT upon request as opposed to the KLT's accessing the pool individually. The later could cause race conditions or deadlocks if not implemented with locks or something similar.
A collection of ULT's for each KLT (Qthreads)
This situation is ideal on a distributed memory system. Each KLT would have a collection of ULT's to run. The draw back is that the user (or the threading library) would have to divide the ULT's between the KLT's. This could result in load imbalance since it is not guaranteed that all ULT's will have the same amount of work to complete and complete roughly the same amount of time. The solution to this is allowing for ULT migration; that is, migrating ULT's between KLT's.
Is there any comprehensive overview somewhere that discusses all the different types of threads and what their relationship is with the OS and the scheduler? I've heard so much contradicting information about whether you want certain types of threads, or whether thread pooling is a performance gain or a performance hit, or that threads are heavy weight so you should use these other kind of threads that don't map directly to real threads but then how is that different from thread pooling .... I'm paralyzed. How does anyone make sense of it? Assuming the use of a language that actually directly interacts with threads (I'm aware of concurrent languages, implicit parallelism, etc. as an alternative to needing to know this stuff but I'm curious about this at the moment)
Here is my brief summary, please comment and edit at will:
There are no hyperthreads, unless you're talking about Intel's hyperthreading in which case it's just virtual cores.
"Green" usually means "not OS-level" (scheduled/handled by a VM, which may or may not map those unto multiple OS-level threads or processes)
pthreads are an API (Posix Threads)
Kernel threads vs user threads is an implementation level (user threads are implemented in userland, so the kernel is not aware of them and neither is its scheduler), "threads" alone is generally an alias for "kernel threads"
Fibers are system-level coroutines. They're threads, except cooperatively multitasked rather than preemptively.
Well, like with most things, it's common to not just care unless threading is identified as a bottleneck. That is, just use the threading functionality that your platform provides in the usual manner and don't worry about the details, at least in the beginning.
But since you evidently want to know more: Usually, the operating system has a concept of a thread as a unit of execution, which is what the OS scheduler handles. Now, switching between OS-level threads requires a context switch, which can be expensive and can become a performance bottleneck. So instead of mapping programming-language threads directly to OS threads, some threading implementations do everything in user space, so that there is only one OS-level thread that is responsible for all the user-level threads in the application. This is more efficient both performance- and resource-wise, but it has the problem that if you actually have several physical processors, you cannot use more than one of them with user-level threads. So there's one more strategy of allocating threads: have multiple OS-level threads, the number of which relates to the number of physical processors you have, and have each of these be responsible for several user-level threads. These three strategies are often called 1:1 (user threads map 1-to-1 to OS threads), N:1 (all user threads map to 1 OS thread), and M:N (M user threads map to N OS threads).
Thread pooling is a slightly different thing. The idea behind thread pooling is to separate the execution resources from the actual execution, so that you have a number of threads (your resources) available in the thread pool, and when you need some task to be executed, you just pick one thread from the pool and hand the task over to it. So thread pooling is a way to design a multi-threaded application. Another way to design would be to identify the different tasks that will need to be performed (e.g., reading from a network, drawing the UI to the screen), and create a dedicated thread for these tasks. This is mostly orthogonal to whether the threads are user- or OS-level concepts.
Threads are the main building block of a Process in the Windows win32 architecture. You can ignore green threads, fibers, green fibers, pthreads (POSIX). Hyper threads don't exist. It is "hyper threading" which is a CPU architecture thing. You cannot code it. You can ignore it.
This leaves use with threads. Indeed. Only threads. A kernel thread is a thread of the kernel, which lives in the upper 2GB (sometimes upper 1GB) of the virtual memory addess space of a machine. You cannot touch it. So you can ignore it most of the time (unless you are writing kernel mode ring-0 code).
Only user threads are the ones you should be concerned about. They come in two flavors: main thread and auxiliary threads. Each process has at least one main thread, it is created for you when you create a process (CreateProcess API call). Auxiliary threads can do tasks that take long and otherwise interrupt the user experience. In C#/,NET you can use the BackgroundWorker class to easily create and manage threads.
Threads have several properties. This may have lead to all "kinds" of threads. But worker threads are probably the only ones you should be worried about when you start dealing with threads.
I learned a lot reading these slides.
I came across this after looking at Unicorn.