Running threads in linux

What are Linux Processes, Threads, Light Weight Processes, and Process State

Linux has evolved a lot since its inception. It has become the most widely used operating system when in comes to servers and mission critical work. Though its not easy to understand Linux as a whole but there are aspects which are fundamental to Linux and worth understanding. In this article, we will discuss about Linux processes, threads and light weight processes and understand the difference between them. Towards the end, we will also discuss various states for Linux processes.

Linux Processes

In a very basic form, Linux process can be visualized as running instance of a program. For example, just open a text editor on your Linux box and a text editor process will be born. Here is an example when I opened gedit on my machine :

$ gedit & [1] 5560 $ ps -aef | grep gedit 1000 5560 2684 9 17:34 pts/0 00:00:00 gedit

First command (gedit &) opens gedit window while second ps command (ps -aef | grep gedit) checks if there is an associated process. In the result you can see that there is a process associated with gedit. Processes are fundamental to Linux as each and every work done by the OS is done in terms of and by the processes. Just think of anything and you will see that it is a process. This is because any work that is intended to be done requires system resources ( that are provided by kernel) and it is a process that is viewed by kernel as an entity to which it can provide system resources. Processes have priority based on which kernel context switches them. A process can be pre-empted if a process with higher priority is ready to be executed.

For example, if a process is waiting for a system resource like some text from text file kept on disk then kernel can schedule a higher priority process and get back to the waiting process when data is available. This keeps the ball rolling for an operating system as a whole and gives user a feeling that tasks are being run in parallel. Processes can talk to other processes using Inter process communication methods and can share data using techniques like shared memory. In Linux, fork() is used to create new processes. These new processes are called as child processes and each child process initially shares all the segments like text, stack, heap etc until child tries to make any change to stack or heap. In case of any change, a separate copy of stack and heap segments are prepared for child so that changes remain child specific. The text segment is read-only so both parent and child share the same text segment. C fork function article explains more about fork().

Linux Threads vs Light Weight Processes

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 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 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 alight 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. How to Create Threads in Linux explains more about threads.

Linux Process States

• RUNNING – This state specifies that the process is either in execution or waiting to get executed.
• INTERRUPTIBLE – This state specifies that the process is waiting to get interrupted as it is in sleep mode and waiting for some action to happen that can wake this process up. The action can be a hardware interrupt, signal etc.
• UN-INTERRUPTIBLE – It is just like the INTERRUPTIBLE state, the only difference being that a process in this state cannot be waken up by delivering a signal.
• STOPPED – This state specifies that the process has been stopped. This may happen if a signal like SIGSTOP, SIGTTIN etc is delivered to the process.
• TRACED – This state specifies that the process is being debugged. Whenever the process is stopped by debugger (to help user debug the code) the process enters this state.
• ZOMBIE – This state specifies that the process is terminated but still hanging around in kernel process table because the parent of this process has still not fetched the termination status of this process. Parent uses wait() family of functions to fetch the termination status.
• DEAD – This state specifies that the process is terminated and entry is removed from process table. This state is achieved when the parent successfully fetches the termination status as explained in ZOMBIE state.

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Introduction to Linux Threads – Part I

A thread of execution is often regarded as the smallest unit of processing that a scheduler works on. A process can have multiple threads of execution which are executed asynchronously. This asynchronous execution brings in the capability of each thread handling a particular work or service independently. Hence multiple threads running in a process handle their services which overall constitutes the complete capability of the process.

In this article we will touch base on the fundamentals of threads and build the basic understanding required to learn the practical aspects of Linux threads. Linux Threads Series: part 1 (this article), part 2, part 3.

Why Threads are Required?

Now, one would ask why do we need multiple threads in a process?? Why can’t a process with only one (default) main thread be used in every situation. Well, to answer this lets consider an example : Suppose there is a process, that receiving real time inputs and corresponding to each input it has to produce a certain output. Now, if the process is not multi-threaded ie if the process does not involve multiple threads, then the whole processing in the process becomes synchronous. This means that the process takes an input processes it and produces an output.

The limitation in the above design is that the process cannot accept an input until its done processing the earlier one and in case processing an input takes longer than expected then accepting further inputs goes on hold. To consider the impact of the above limitation, if we map the generic example above with a socket server process that can accept input connection, process them and provide the socket client with output. Now, if in processing any input if the server process takes more than expected time and in the meantime another input (connection request) comes to the socket server then the server process would not be able to accept the new input connection as its already stuck in processing the old input connection. This may lead to a connection time out at the socket client which is not at all desired. This shows that synchronous model of execution cannot be applied everywhere and hence was the requirement of asynchronous model of execution felt which is implemented by using threads.

Difference Between threads and processes

• Processes do not share their address space while threads executing under same process share the address space.
• From the above point its clear that processes execute independent of each other and the synchronization between processes is taken care by kernel only while on the other hand the thread synchronization has to be taken care by the process under which the threads are executing
• Context switching between threads is fast as compared to context switching between processes
• The interaction between two processes is achieved only through the standard inter process communication while threads executing under the same process can communicate easily as they share most of the resources like memory, text segment etc

User threads Vs Kernel Threads

Threads can exist in user space as well as in kernel space.

A user space threads are created, controlled and destroyed using user space thread libraries. These threads are not known to kernel and hence kernel is nowhere involved in their processing. These threads follow co-operative multitasking where-in a thread releases CPU on its own wish ie the scheduler cannot preempt the thread. Th advantages of user space threads is that the switching between two threads does not involve much overhead and is generally very fast while on the negative side since these threads follow co-operative multitasking so if one thread gets block the whole process gets blocked.

A kernel space thread is created, controlled and destroyed by the kernel. For every thread that exists in user space there is a corresponding kernel thread. Since these threads are managed by kernel so they follow preemptive multitasking where-in the scheduler can preempt a thread in execution with a higher priority thread which is ready for execution. The major advantage of kernel threads is that even if one of the thread gets blocked the whole process is not blocked as kernel threads follow preemptive scheduling while on the negative side the context switch is not very fast as compared to user space threads.

If we talk of Linux then kernel threads are optimized to such an extent that they are considered better than user space threads and mostly used in all scenarios except where prime requirement is that of cooperative multitasking.

Problem with Threads

There are some major problems that arise while using threads :

• Many operating system does not implement threads as processes rather they see threads as part of parent process. In this case, what would happen if a thread calls fork() or even worse what if a thread execs a new binary?? These scenarios may have dangerous consequences for example in the later problem the whole parent process could get replaced with the address space of the newly exec’d binary. This is not at all desired. Linux which is POSIX complaint makes sure that calling a fork() duplicates only the thread that has called the fork() function while an exec from any of the thread would stop all the threads in the parent process.
• Another problem that may arise is the concurrency problems. Since threads share all the segments (except the stack segment) and can be preempted at any stage by the scheduler than any global variable or data structure that can be left in inconsistent state by preemption of one thread could cause severe problems when the next high priority thread executes the same function and uses the same variables or data structures.

For the problem 1 mentioned above, all we can say is that its a design issue and design for applications should be done in a way that least problems of this kind arise.

For the problem 2 mentioned above, using locking mechanisms programmer can lock a chunk of code inside a function so that even if a context switch happens (when the function global variable and data structures were in inconsistent state) then also next thread is not able to execute the same code until the locked code block inside the function is unlocked by the previous thread (or the thread that acquired it).

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