Are there threads in linux

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?

1 Answer 1

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.

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4 commands to check thread count per process (threads vs processes) in Linux

Table of Contents

In Linux, some processes are divided into pieces called threads. In one liner, threads are essentially just processes with a shared address space on Linux. In this article we will get some brief overview on threads and processes, also some examples to show threads per process, check thread count per process, check number of threads allowed, count threads and some more related topics.

Threads vs Processes

• A thread is very similar to a process, it has an identifier (TID, or thread ID), and the kernel schedules and runs threads just like processes.
• However, unlike separate processes, which usually do not share system resources such as memory and I/O connections with other processes, all threads inside a single process share their system resources and some memory.
• A process with one thread is single-threaded , and a process with more than one thread is multithreaded .
• All processes start out single-threaded . This starting thread is usually called the main thread. The main thread may then start new threads in order for the process to become multithreaded, similar to the way a process can call fork() to start a new process.
• The primary advantage of a multithreaded process is that when the process has a lot to do, threads can run simultaneously on multiple processors, potentially speeding up computation.
• Although you can also achieve simultaneous computation with multiple processes, threads start faster than processes, and it is often easier and/or more efficient for threads to intercommunicate using their shared memory than it is for processes to communicate over a channel such as a network connection or a pipe.

Show threads per process

1. Using PID task

You can count threads with the list of available sub directories inside /proc/

/task/ . The count of total available sub-directories inside this part is directly proportional to the thread count per process for the provided PID.

For example to check java thread count, I have a Java process for which you can see I have multiple sub-directories so it means this is a multi threaded process. using ls command under this path you can show threads per process for java

But then again I have another process for which as you can see I have single sub-directory hence we know this is a single thread process

2. Using ps command

You can also use » ps » command to show threads per process. With » ps » we can list LWP (Light Weight process) which depicts Thread ID of the respective process and NWLP (Number of Threads).

To show threads per process using ps command you can use below argument

3. Using pstree command

You can also use pstree to show threads per process. Here as you see java thread count and check number of threads for java process

4. Using top command

In top , by default we will not be able to see thread count per process. But when running top , it is possible to change which fields to display and add this column to print thread count per process can be added manually.

• Press f
• This will show a list of fields that top can display. The fields that are displayed in bold are the ones that top will display.
• Use the down arrow to navigate to «nTH» (Number of Threads).
• Press to select «nTH«
• Press ‘s‘ to sort on number of threads.
• Press ‘q‘ to display the data of threads count.

Next you should see a new column at the end of top command with number of thread (nTH) column to show threads per process

Check thread count per process

Next you can use the above explained commands to also check thread count per process by customising them a little bit.

1. Using PID status

To check thread count per process you can use below command. For example here java thread count is 59 threads in my Linux environment

While amsHelper process has single thread

2. Using ps command

We used ps command to show threads per process and count threads, we can also use » ps » command to get LWP and NLWP details, which when combined with » wc » we can count threads per process.

To check thread count per process for a particular PID for example to check java thread count:

Check number of threads allowed in Linux system?

Linux doesn’t have a separate threads per process limit, j ust a limit on the total number of processes on the system . This value controls the maximum number of threads that can be created using fork() . During initialization the kernel sets this value such that even if the maximum number of threads is created

To check number of threads which Linux system can allow

The minimum number of threads that can be written to threads-max is 20 .
The maximum value that can be written to threads-max is given by the constant FUTEX_TID_MASK (0x3fffffff) .
If a value outside of this range is written to threads-max an error EINVAL occurs.

The default value depends on memory size. You can use threads-max to check number of threads allowed in Linux. You can increase thread count per process limit like this:

There is also a limit on the number of processes (an hence threads) that a single user may create, see ulimit for details regarding these limits:

Here, the system is able to create 35,000 threads/processes in total and a single user can create 10000 number of processes.

The logic is very simple here every CPU can execute 1 process at a time, if there are 8 cores that means 8 to 10 processes at a time can be executed easily without any stress but if number of running or runnable threads per CPU increases drastically then there will be performance issue.

What is the maximum processes count allowed in Linux?

Verify the value for kernel.pid_max

Here I can execute 35,000 processes simultaneously in my system that can run in separate memory spaces.

To change the value of kernel.pid_max to 65534:

Lastly I hope the steps from the article to show threads per process, check thread count per process, check number of threads allowed on Linux was helpful. So, let me know your suggestions and feedback using the comment section.

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Are there threads in linux

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How to view threads of a process on Linux

Threads are a popular programming abstraction for parallel execution on modern operating systems. When threads are forked inside a program for multiple flows of execution, these threads share certain resources (e.g., memory address space, open files) among themselves to minimize forking overhead and avoid expensive IPC (inter-process communication) channel. These properties make threads an efficient mechanism for concurrent execution.

In Linux, threads (also called Lightweight Processes (LWP)) created within a program will have the same «thread group ID» as the program’s PID. Each thread will then have its own thread ID (TID). To the Linux kernel’s scheduler, threads are nothing more than standard processes which happen to share certain resources. Classic command-line tools such as ps or top, which display process-level information by default, can be instructed to display thread-level information.

Here are several ways to show threads for a process on Linux:

Using the ps command

The «-T» option for the ps command enables thread views. The following command list all threads created by a process with

The «SID» column represents thread IDs, and «CMD» column shows thread names.

Using the top command

The top command can show a real-time view of individual threads. To enable thread views in the top output, invoke top with «-H» option. This will list all Linux threads. You can also toggle on or off thread view mode while top is running, by pressing ‘H’ key.

Note how in the example above the number of threads on the system is listed.

To restrict the top output to a particular process and check all threads running inside the process:

A more user-friendly way to view threads per process is via htop, an ncurses-based interactive process viewer. This program allows you to monitor individual threads in tree views.

To enable thread views in htop, launch htop, and press F2 to enter htop setup menu. Choose «Display option» under «Setup» column, and toggle on «Tree view» and «Show custom thread names» options. Presss F10 to exit the setup.

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How to Create Threads in Linux (With a C Example Program)

In the part I of the Linux Threads series, we discussed various aspects related to threads in Linux.

In this article we will focus on how a thread is created and identified. We will also present a working C program example that will explain how to do basic threaded programming.

Linux Threads Series: part 1, part 2 (this article), part 3.

Thread Identification

Just as a process is identified through a process ID, a thread is identified by a thread ID. But interestingly, the similarity between the two ends here.

• A process ID is unique across the system where as a thread ID is unique only in context of a single process.
• A process ID is an integer value but the thread ID is not necessarily an integer value. It could well be a structure
• A process ID can be printed very easily while a thread ID is not easy to print.

The above points give an idea about the difference between a process ID and thread ID.

Thread ID is represented by the type ‘pthread_t’. As we already discussed that in most of the cases this type is a structure, so there has to be a function that can compare two thread IDs.

So as you can see that the above function takes two thread IDs and returns nonzero value if both the thread IDs are equal or else it returns zero.

Another case may arise when a thread would want to know its own thread ID. For this case the following function provides the desired service.

So we see that the function ‘pthread_self()’ is used by a thread for printing its own thread ID.

Now, one would ask about the case where the above two function would be required. Suppose there is a case where a link list contains data for different threads. Every node in the list contains a thread ID and the corresponding data. Now whenever a thread tries to fetch its data from linked list, it first gets its own ID by calling ‘pthread_self()’ and then it calls the ‘pthread_equal()’ on every node to see if the node contains data for it or not.

An example of the generic case discussed above would be the one in which a master thread gets the jobs to be processed and then it pushes them into a link list. Now individual worker threads parse the linked list and extract the job assigned to them.

Thread Creation

Normally when a program starts up and becomes a process, it starts with a default thread. So we can say that every process has at least one thread of control. A process can create extra threads using the following function :

The above function requires four arguments, lets first discuss a bit on them :

• The first argument is a pthread_t type address. Once the function is called successfully, the variable whose address is passed as first argument will hold the thread ID of the newly created thread.
• The second argument may contain certain attributes which we want the new thread to contain. It could be priority etc.
• The third argument is a function pointer. This is something to keep in mind that each thread starts with a function and that functions address is passed here as the third argument so that the kernel knows which function to start the thread from.
• As the function (whose address is passed in the third argument above) may accept some arguments also so we can pass these arguments in form of a pointer to a void type. Now, why a void type was chosen? This was because if a function accepts more than one argument then this pointer could be a pointer to a structure that may contain these arguments.

A Practical Thread Example

Following is the example code where we tried to use all the three functions discussed above.

So what this code does is :

• It uses the pthread_create() function to create two threads
• The starting function for both the threads is kept same.
• Inside the function ‘doSomeThing()’, the thread uses pthread_self() and pthread_equal() functions to identify whether the executing thread is the first one or the second one as created.
• Also, Inside the same function ‘doSomeThing()’ a for loop is run so as to simulate some time consuming work.

Now, when the above code is run, following was the output :

As seen in the output, first thread is created and it starts processing, then the second thread is created and then it starts processing. Well one point to be noted here is that the order of execution of threads is not always fixed. It depends on the OS scheduling algorithm.

Note: The whole explanation in this article is done on Posix threads. As can be comprehended from the type, the pthread_t type stands for POSIX threads. If an application wants to test whether POSIX threads are supported or not, then the application can use the macro _POSIX_THREADS for compile time test. To compile a code containing calls to posix APIs, please use the compile option ‘-pthread’.

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