linux-kernel - Re: THP backed thread stacks

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Date:   Mon, 20 Mar 2023 12:12:21 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: THP backed thread stacks

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