threads and LWP in Linux
Is this sentence correct: "All threads in Linux are LWP but not all LWP are threads". Actually, I try to understand thread realisation in Linux. pthread_create call clone syscall, but in man clone, I didn't find any reference to LWP.
So, does Linux have LWP at all?
Answer
From this blog you can find your answer http://www.thegeekstuff.com/2013/11/linux-process-and-threads/
Threads in Linux are nothing but a flow of execution of the process. A process containing multiple execution flows is known as multi-threaded process.
For a non multi-threaded process there is only execution flow that is the main execution flow and hence it is also known as single threaded process. For Linux kernel , there is no concept of thread. Each thread is viewed by kernel as a separate process but these processes are somewhat different from other normal processes. I will explain the difference in following paragraphs.
Threads are often mixed with the term Light Weight Processes or LWPs. The reason dates back to those times when Linux supported threads at user level only. This means that even a multi-threaded application was viewed by kernel as a single process only. This posed big challenges for the library that managed these user level threads because it had to take care of cases that a thread execution did not hinder if any other thread issued a blocking call.
Later on the implementation changed and processes were attached to each thread so that kernel can take care of them. But, as discussed earlier, Linux kernel does not see them as threads, each thread is viewed as a process inside kernel. These processes are known as light weight processes.
The main difference between a light weight process (LWP) and a normal process is that LWPs share the same address space and other resources like open files etc. As some resources are shared so these processes are considered to be light weight as compared to other normal processes and hence the name light weight processes.
So, effectively we can say that threads and light weight processes are the same. It’s just that thread is a term that is used at user level while light weight process is a term used at kernel level.
From implementation point of view, threads are created using functions exposed by POSIX compliant pthread library in Linux. Internally, the clone() function is used to create a normal as well as a light weight process. This means that to create a normal process fork() is used that further calls clone() with appropriate arguments while to create a thread or LWP, a function from pthread library calls clone() with relevant flags. So, the main difference is generated by using different flags that can be passed to clone() function.
Read more about fork() and clone() on their respective man pages.