Re: THP backed thread stacks

[David Hildenbrand <david@redhat.com>]

On 20.03.23 18:46, William Kucharski wrote:
> 
> 
>> On Mar 20, 2023, at 05:12, David Hildenbrand <david@redhat.com> wrote:
>>
>> On 17.03.23 19:46, Mike Kravetz wrote:
>>> On 03/17/23 17:52, Matthew Wilcox wrote:
>>>> 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?
>>> Yes, pthread stacks create a VMA equal to stack size which is different
>>> than 'main thread' stack.  The 2MB size for pthread stacks created by
>>> JVM is actually them explicitly requesting the size (8MB default).
>>> We have a good understanding of what is happening.  Behavior actually
>>> changed a bit with glibc versions in OL7 vs OL8.  Do note that THP usage
>>> is somewhat out of the control of an application IF they rely on
>>> glibc/pthread to allocate stacks.  Only way for application to make sure
>>> pthread stacks do not use THP would be for them to allocate themselves.
>>> Then, they would need to set up the guard page themselves.  They would
>>> also need to monitor the status of all threads to determine when stacks
>>> could be deleted.  A bunch of extra code that glibc/pthread already does
>>> for free.
>>> Oracle glibc team is also involved, and it 'looks' like they may have
>>> upstream buy in to add a flag to explicitly enable or disable hugepages
>>> on pthread stacks.
>>> It seems like concensus from mm community is that we should not
>>> treat stacks any differently than any other mappings WRT THP.  That is
>>> OK, just wanted to throw it out there.
>>
>> I wonder if this might we one of the cases where we don't want to allocate a THP on first access to fill holes we don't know if they are all going to get used. But we might want to let khugepaged place a THP if all PTEs are already populated. Hm.
>>
>> -- 
>> Thanks,
>>
>> David / dhildenb
> 
> Unless we do decide to start honoring MAP_STACK, we would be setting an interesting precedent here in that stacks would be the only THP allocation that would be denied a large page until it first proved it was actually going to use all the individual PAGESIZE pages comprising one. Should mapping a text page using a THP be likewise deferred until each PAGESIZE page comprising it had been accessed?

IMHO, it's a bit different, because text pages are not anon pages.

I suspect is_stack_mapping() -> VM_STACK -> VM_GROWSUP/VM_GROWSDOWN is 
not always reliable?

-- 
Thanks,

David / dhildenb