Re: THP backed thread stacks

[Matthew Wilcox]

On Mon, Mar 06, 2023 at 03:57:30PM -0800, Mike Kravetz wrote:
> One of our product teams recently experienced 'memory bloat' in their
> environment.  The application in this environment is the JVM which
> creates hundreds of threads.  Threads are ultimately created via
> pthread_create which also creates the thread stacks.  pthread attributes
> are modified so that stacks are 2MB in size.  It just so happens that
> due to allocation patterns, all their stacks are at 2MB boundaries.  The
> system has THP always set, so a huge page is allocated at the first
> (write) fault when libpthread initializes the stack.

Do you happen to have an strace (or similar) so we can understand what
the application is doing?

My understanding is that for a normal app (like, say, 'cat'), we'll
allow up to an 8MB stack, but we only create a VMA that is 4kB in size
and set the VM_GROWSDOWN flag on it (to allow it to magically grow).
Therefore we won't create a 2MB page because the VMA is too small.

It sounds like the pthread library is maybe creating a 2MB stack as
a 2MB VMA, and that's why we're seeing this behaviour?