Maximum Number of Threads per Process in Linux

When working with multithreaded applications in Linux, it’s essential to understand the limits on how many threads can be created within a single process. While Linux does not impose a strict “threads per process” limit, several system parameters govern the overall number of threads a process can create. In this post, we’ll explore how Linux handles threading limits, how to check them, and how to modify them if needed.

Understanding Linux Thread Limits

In Linux, threads are implemented as processes that share the same memory space, meaning that threads are essentially lightweight processes. Therefore, Linux doesn’t have a specific limit on the number of threads per process. Instead, the number of threads is governed by general process limits, memory availability, and system configuration.

Key Factors Affecting the Maximum Number of Threads:

1. Total System Processes: The total number of processes (including threads) is controlled by the threads-max kernel parameter.
2. User Process Limits: The maximum number of processes a user can create is governed by ulimit settings.
3. Available Memory and Stack Size: Each thread requires a certain amount of stack memory. The number of threads is limited by the total virtual memory available divided by the per-thread stack size.

How to Check the Maximum Number of Threads

You can check the current maximum number of threads allowed on your system using the following command:

cat /proc/sys/kernel/threads-max

This file represents the system-wide limit on the number of threads that can exist simultaneously.

Example:

$ cat /proc/sys/kernel/threads-max
64456

In this case, the system can have a maximum of 64,456 threads at any given time.

Modifying the Maximum Number of Threads

If you need to increase the number of threads your system can handle, you can modify the threads-max value:

echo 100000 > /proc/sys/kernel/threads-max

This will increase the maximum number of threads to 100,000. However, keep in mind that increasing the number of threads might require you to adjust other system parameters, such as memory limits.

Making the Change Persistent

To make this change persistent across reboots, add the following line to /etc/sysctl.conf:

kernel.threads-max = 100000

Then apply the changes with:

sudo sysctl -p

Stack Size and Its Impact on Thread Limits

Each thread in Linux is allocated a certain amount of stack space, and the default stack size can be viewed and adjusted using the ulimit command. By default, the stack size is often set to 8MB. The number of threads you can create is directly proportional to the total available memory divided by the stack size.

Check the Stack Size

To check the current stack size:

ulimit -s

Example:

$ ulimit -s
8192  # Stack size in kilobytes (8 MB)

Reducing Stack Size

Reducing the stack size per thread can allow more threads to be created, especially in systems with limited memory. For example, to set the stack size to 1MB, use the following command:

ulimit -s 1024

Calculating the Number of Threads

You can estimate the maximum number of threads a process can create by dividing the total virtual memory by the stack size. For example, on a 32-bit system with 3GB of user space and an 8MB stack size:

Number of threads = Total virtual memory / (Stack size)
                 = 3GB / 8MB
                 = 384 threads

On a 64-bit system, the virtual memory space is significantly larger, allowing for far more threads, provided the physical memory and swap space can support them.

User-Level Limits on Processes

In addition to system-wide limits, Linux also imposes per-user limits on the number of processes (and thus threads) that can be created. You can view and modify these limits using the ulimit or getrlimit commands.

View the Current Limit

ulimit -u

This shows the maximum number of processes (including threads) a user can create.

Modify the Limit

You can temporarily increase this limit using:

ulimit -u 100000

To make this change permanent, you can add the following to /etc/security/limits.conf:

* hard nproc 100000
* soft nproc 100000

This increases the maximum number of processes (and thus threads) a user can create.

Example of Increasing Thread Limits

Here’s an example of adjusting system parameters to support a large number of threads (e.g., 100,000 threads):

1. Increase the maximum number of threads:

   echo 120000 > /proc/sys/kernel/threads-max
   

2. Adjust the per-user process limit:

   ulimit -u 120000
   

3. Reduce the default stack size (if needed):

   ulimit -s 1024  # Set stack size to 1MB
   

4. Make changes persistent by modifying /etc/sysctl.conf and /etc/security/limits.conf.

Linux doesn’t impose a hard limit on the number of threads per process. Instead, the limit is determined by several factors, including system-wide process limits, user-level process limits, available memory, and per-thread stack size. By adjusting these parameters, you can increase the number of threads a process can create. However, be mindful of system performance, as creating too many threads can lead to excessive memory consumption and context-switching overhead.