Maximum number of threads per process in Linux?

Linux doesn't have a separate threads per process limit, just a limit on the total number of processes on the system (threads are essentially just processes with a shared address space on Linux) which you can view like this:

cat /proc/sys/kernel/threads-max

The default is the number of memory pages/4. You can increase this like:

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

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

In practical terms, the limit is usually determined by stack space. If each thread gets a 1MB stack (I can't remember if that is the default on Linux), then you a 32-bit system will run out of address space after 3000 threads (assuming that the last gb is reserved to the kernel).

However, you'll most likely experience terrible performance if you use more than a few dozen threads. Sooner or later, you get too much context-switching overhead, too much overhead in the scheduler, and so on. (Creating a large number of threads does little more than eat a lot of memory. But a lot of threads with actual work to do is going to slow you down as they're fighting for the available CPU time)

What are you doing where this limit is even relevant?

@dragosrsupercool

Linux doesn't use the virtual memory to calculate the maximum of thread, but the physical ram installed on the system

 max_threads = totalram_pages / (8 * 8192 / 4096);

http://kavassalis.com/2011/03/linux-and-the-maximum-number-of-processes-threads/

kernel/fork.c

/* The default maximum number of threads is set to a safe
 * value: the thread structures can take up at most half
 * of memory.
 */
max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);

So thread max is different between every system, because the ram installed can be from different sizes, I know Linux doesn't need to increase the virtual memory, because on 32 bit we got 3 GB for user space and 1 GB for the kernel, on 64 bit we got 128 TB of virtual memory, that happen on Solaris, if you want increase the virtual memory you need to add swap space.

To retrieve it:

cat /proc/sys/kernel/threads-max

To set it:

echo 123456789 | sudo tee -a /proc/sys/kernel/threads-max

123456789 = # of threads

It probably shouldn't matter. You are going to get much better performance designing your algorithm to use a fixed number of threads (eg, 4 or 8 if you have 4 or 8 processors). You can do this with work queues, asynchronous IO, or something like libevent.

To set permanently,

vim /etc/sysctl.conf

and add

kernel.threads-max = "value"

I think we missed another restriction which will also block the new thread creation, this is the kernel.pid_max limit.

root@myhost:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.7 LTS
Release:    16.04
Codename:   xenial
root@myhost:~# uname -a
Linux myhost 4.4.0-190-generic #220-Ubuntu SMP Fri Aug 28 23:02:15 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

I find that at least in my system, this threshold kernel.pid_max is 32768. When I launch any simple JVM process, it reports error like below:

java/jstack/jstat ...
#
# There is insufficient memory for the Java Runtime Environment to continue.
# Cannot create GC thread. Out of system resources.
# An error report file with more information is saved as:
# /root/hs_err_pid1390.log

Check the memory, sufficient.

root@lascorehadoop-15a32:~# free -mh
              total        used        free      shared  buff/cache   available
Mem:           125G         11G         41G        1.2G         72G        111G
Swap:            0B          0B          0B

Check the system thread:

~# ps -eLf|wc -l
31506

But I check the system limit by ulimit:

root@myhost:~# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 515471
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 98000
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 515471
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

From the ulimit output, we could see that currently thread number is far less than the maximum user process limit.

In fact, the limit which is reached is the kernel.pid_max

Very easy to check and tuning it: https://www.cyberciti.biz/tips/howto-linux-increase-pid-limits.html

You can see the current value by the following command- cat /proc/sys/kernel/threads-max

You can also set the value like

echo 100500 > /proc/sys/kernel/threads-max

The value you set would be checked against the available RAM pages. If the thread structures occupies more than 1/8th) of the available RAM pages, thread-max would be reduced accordingly.

Yes, to increase the threads number you need to increase the virtual memory or decrease the stack size. In Raspberry Pi I didn’t find a way to increase the virtual memory, if a decrease the stack size from default 8MB to 1MB It is possibly get more than 1000 threads per process but decrease the stack size with the “ulimit -s” command make this for all threads. So, my solution was use “pthread_t” instance “thread class” because the pthread_t let me set the stack size per each thread. Finally, I am available to archive more than 1000 threads per process in Raspberry Pi each one with 1MB of stack.

If you using Suse, you need to use one of these methods

https://www.suse.com/support/kb/doc/?id=000015901

global,

/etc/systemd/system.conf
    DefaultTasksMax=Value

For specific ssh service

/etc/systemd/system/sshd.service.d/override.conf
    TasksMax=Value