Re: [PATCH v2] mm: Reduce memory bloat with THP

Kirill A. Shutemov
2018-02-01T10:27:38Z

On Thu, Jan 25, 2018 at 09:13:03PM +0000, Mel Gorman wrote:
> On Thu, Jan 25, 2018 at 11:41:03AM -0800, Nitin Gupta wrote:
> > >> It's not really about memory scarcity but a more efficient use of it.
> > >> Applications may want hugepage benefits without requiring any changes to
> > >> app code which is what THP is supposed to provide, while still avoiding
> > >> memory bloat.
> > >>
> > > I read these links and find that there are mainly two complains:
> > > 1. THP causes latency spikes, because direction compaction slows down THP allocation,
> > > 2. THP bloats memory footprint when jemalloc uses MADV_DONTNEED to return memory ranges smaller than
> > > THP size and fails because of THP.
> > >
> > > The first complain is not related to this patch.
> >
> > I'm trying to address many different THP issues and memory bloat is
> > first among them.
>
> Expecting userspace to get this right is probably going to go sideways.
> It'll be screwed up and be sub-optimal or have odd semantics for existing
> madvise flags. The fact is that an application may not even know if it's
> going to be sparsely using memory in advance if it's a computation load
> modelling from unknown input data.
>
> I suggest you read the old Talluri paper "Superpassing the TLB Performance
> of Superpages with Less Operating System Support" and pay attention to
> Section 4. There it discusses a page reservation scheme whereby on fault
> a naturally aligned set of base pages are reserved and only one correctly
> placed base page is inserted into the faulting address. It was tied into
> a hypothetical piece of hardware that doesn't exist to give best-effort
> support for superpages so it does not directly help you but the initial
> idea is sound. There are holes in the paper from todays perspective but
> it was written in the 90's.
>
> From there, read "Transparent operating system support for superpages"
> by Navarro, particularly chapter 4 paying attention to the parts where
> it talks about opportunism and promotion threshold.
>
> Superficially, it goes like this
>
> 1. On fault, reserve a THP in the allocator and use one base page that
> is correctly-aligned for the faulting addresses. By correctly-aligned,
> I mean that you use base page whose offset would be naturally contiguous
> if it ever was part of a huge page.
> 2. On subsequent faults, attempt to use a base page that is naturally
> aligned to be a THP
> 3. When a "threshold" of base pages are inserted, allocate the remaining
> pages and promote it to a THP
> 4. If there is memory pressure, spill "reserved" pages into the main
> allocation pool and lose the opportunity to promote (which will need
> khugepaged to recover)
>
> By definition, a promotion threshold of 1 would be the existing scheme
> of allocation a THP on the first fault and some users will want that. It
> also should be the default to avoid unexpected overhead. For workloads
> where memory is being sparsely addressed and the increased overhead of
> THP is unwelcome then the threshold should be tuned higher with a maximum
> possible value of HPAGE_PMD_NR.
>
> It's non-trivial to do this because at minimum a page fault has to check
> if there is a potential promotion candidate by checking the PTEs around
> the faulting address searching for a correctly-aligned base page that is
> already inserted. If there is, then check if the correctly aligned base
> page for the current faulting address is free and if so use it. It'll
> also then need to check the remaining PTEs to see if both the promotion
> threshold has been reached and if so, promote it to a THP (or else teach
> khugepaged to do an in-place promotion if possible). In other words,
> implementing the promotion threshold is both hard and it's not free.

"not free" is understatement.

Converting PTE page table to PMD would require down_write(mmap_sem).
Doing it from within page fault path would also mean that we need to drop
down_read(mmap) we hold, re-aquaire it with down_write(), find the vma again
and re-validate that nothing changed in meanwhile...

That's an interesting exercise, but I'm skeptical it would result in anything
practical.

--
Kirill A. Shutemov