Messages in this thread | | | Date | Mon, 20 Mar 2023 18:52:32 +0100 | Subject | Re: THP backed thread stacks | From | David Hildenbrand <> |
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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
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