256 threads maximum?

[Scott Lamb]

Panagiotis Issaris wrote:
> Hi,
> 
> The following simple testprogram tries to create the number of threads 
> given as argv[1].
> When I'm using a 2.6.3 kernel with glibc 2.3.2/NPTL 0.60, the maximum 
> number of threads this application can create is 255. With any number 
> higher it complains about memory allocation failing.

I bet you're using a 32-bit machine with an 8 MiB stack. The top bit is 
reserved for kernel use, IIRC, so a program can only address 2 GiB of 
virtual memory. 2 GiB / 8 MiB = 256. One thread already exists. If you ran a

     $ ulimit -s 4096
     $ ./thr 511

It would succeed, I'd think. And 512 would fail.

I'm not sure why LinuxThreads was able to allocate more threads. Maybe 
it determines the stack size in a different way?

> What am I doing wrong? I know that NPTL can do a lot better then this, 
> since I have been reading reports on NPTL creating thousands of threads 
> ( I think I recall someone stating creating 300000 threads from one 
> process).

You can do a little better by decreasing the stack space per thread, as 
mentioned above. But I think they must have been doing this test on a 
64-bit machine.

Scott