Re: THP backed thread stacks

[William Kucharski]

> On Mar 20, 2023, at 05:12, David Hildenbrand <david@xxxxxxxxxx> 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?

Given the main questions of:

1) How to know whether it's a stack allocation

2) How to determine whether the app is consciously trying to allocate the stack via a THP or if it just happened to win the address alignment/size lottery

3) Whether to honor the THP allocation in either case

It seems taking the khugepaged approach would require Yet Another Flag to provide a way for an application that KNOWS a THP-mapped stack would be useful to get it without having to incorporate a loop to touch a byte in every PAGESIZE page in their allocated aligned stack and hope it gets its upgrade.

William Kucharski