Messages in this thread | | | Date | Mon, 20 Mar 2023 12:12:21 +0100 | Subject | Re: THP backed thread stacks | From | David Hildenbrand <> |
| |
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
| |