From: William Kucharski <william.kucharski@oracle.com>
To: David Hildenbrand <david@redhat.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>,
Matthew Wilcox <willy@infradead.org>,
Linux-MM <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: THP backed thread stacks
Date: Mon, 20 Mar 2023 17:46:22 +0000 [thread overview]
Message-ID: <4A61B05E-62C6-4BC1-9301-CF12FC5A252F@oracle.com> (raw)
In-Reply-To: <655ca9fd-ddc3-0bfa-b442-fc60cda02baa@redhat.com>
> 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?
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
next prev parent reply other threads:[~2023-03-20 17:46 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-06 23:57 THP backed thread stacks Mike Kravetz
2023-03-07 0:15 ` Peter Xu
2023-03-07 0:40 ` Mike Kravetz
2023-03-08 19:02 ` Mike Kravetz
2023-03-09 22:38 ` Zach O'Keefe
2023-03-09 23:33 ` Mike Kravetz
2023-03-10 0:05 ` Zach O'Keefe
2023-03-10 1:40 ` William Kucharski
2023-03-10 11:25 ` David Hildenbrand
2023-03-11 12:24 ` William Kucharski
2023-03-12 0:55 ` Hillf Danton
2023-03-12 4:39 ` William Kucharski
2023-03-10 22:02 ` Yang Shi
2023-03-07 10:10 ` David Hildenbrand
2023-03-07 19:02 ` Mike Kravetz
2023-03-07 13:36 ` Mike Rapoport
2023-03-17 17:52 ` Matthew Wilcox
2023-03-17 18:46 ` Mike Kravetz
2023-03-20 11:12 ` David Hildenbrand
2023-03-20 17:46 ` William Kucharski [this message]
2023-03-20 17:52 ` David Hildenbrand
2023-03-20 18:06 ` Mike Kravetz
2023-03-18 12:58 ` David Laight
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