On 06/15/2016 01:43 PM, Peter Zijlstra wrote:

On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote:

quoted

even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should
be more than enough for the foreseeable future.

So what happens if I manage to create 16k+ threads on my 32bit kernel
and get them all to do mmap() or so at the same time.

That doesn't seem too far fetched.

Then again, with double that (the current limit) that doesn't seem
impossible either.

To hit the limit, we need to have all the threads calling down_write() 
at exactly the same instance in time which, I think, is pretty hard to 
do. Also, I don't believe you will ever see a 16k-cpu massive SMP system 
running on 32-bit kernel. I can imagine such a system running on 64-bit 
kernel, but certainly not 32-bit.

Cheers,
Longman