Not understand the concept of LWP in liunx kernel

Question

I study the OS book[operating system concepts] written by Silberschatz recently.

At chapter 5, the book describes the concept of "Contention scope", and mentions that there are two types of contention scope. One is PTHREAD_SCOPE_PROCESS and the other is PTHREAD_SCOPE_SYSTEM.

And for PTHREAD_SCOPE_SYSTEM, the user level thread will bind to one LWP[virtual processor], and this LWP will attaches to one kernel thread.

Actually, I'm not quite understanding the concept of LWP. On the command line terminal I can type the following command:

ps -eLf

And I can see one process[PID] may have several LWP[thread IDs]. By doing some survey, I found that LWP seems simply mean thread or tasks in linux. like the following link:

How Linux handles threads and process scheduling

So I have the following two questions:

Q1 : Is LWP defined as virtual processor in linux? what source code in kernel defines it?

Q2: In the following picture, syslog has the PID [852] and has four threads. And these four LWPs will each bind one kernel thread? Or kernel will just schedule them directly?

The above picture is taken from:

http://www.softprayog.in/tutorials/ps-command-usage-examples-in-linux

Thanks in advance

Answer 1

Linux does not support PTHREAD_SCOPE_PROCESS. See for example http://man7.org/linux/man-pages/man3/pthread_attr_setscope.3.html

POSIX.1 requires that an implementation support at least one of these contention scopes. Linux supports PTHREAD_SCOPE_SYSTEM, but not PTHREAD_SCOPE_PROCESS.

In Linux, a process' multiple threads are like separate processes that happen to share the same address space. Yes, this is a complete oversimplification, they share other resources as well -- notably, signal handlers, file descriptors and various IDs -- and there are other more subtle differences, but they are each full-fledged independently schedulable entities. The terminology used within the kernel is "task". Every process has at least one task. A multithreaded process has more than one.

Q1: LWP is not the same as a virtual processor (whatever you mean by that -- it's not something that's familiar in this context). An LWP corresponds to a task in linux. Although outside kernel development, "process" and "thread" are still the more common terms.

Q2: Each LWP corresponds to a separate task (or user thread), and each is separately schedulable by the kernel. (In linux, the term "kernel thread" has a specific meaning that is quite different: a kernel thread is a thread [really task] which spends its entire life inside the kernel. They're typically used for various internal housekeeping functions.)

For more on the Posix threading model, see this very good explanation: http://www.icir.org/gregor/tools/pthread-scheduling.html that discusses linux and FreeBSD implementations.

And this one, for more general threading concepts: http://timetobleed.com/threading-models-so-many-different-ways-to-get-stuff-done/