Maximum number of threads per process in Linux?

Question

What is the maximum number of threads that can be created by a process under Linux?

How (if possible) can this value be modified?

Answer 1

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.

Answer 2

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.

Answer 3

Depends on your system, just write a sample program [ by creating processes in a loop ] and check using ps axo pid,ppid,rss,vsz,nlwp,cmd. When it can no more create threads check nlwp count [ nlwp is the number threads ] voila you got your fool proof answer instead of going thru books

Answer 4

check the stack size per thread with ulimit, in my case Redhat Linux 2.6:

    ulimit -a
...
    stack size              (kbytes, -s) 10240

Each of your threads will get this amount of memory (10MB) assigned for it's stack. With a 32bit program and a maximum address space of 4GB, that is a maximum of only 4096MB / 10MB = 409 threads !!! Minus program code, minus heap-space will probably lead to an observed max. of 300 threads.

You should be able to raise this by compiling and running on 64bit or setting ulimit -s 8192 or even ulimit -s 4096. But if this is advisable is another discussion...