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

[Nitin Gupta <nitin.m.gupta@oracle.com>]

On 12/14/17 9:55 PM, Anshuman Khandual wrote:
> On 12/15/2017 06:58 AM, Nitin Gupta wrote:
>> Currently, if the THP enabled policy is "always", or the mode
>> is "madvise" and a region is marked as MADV_HUGEPAGE, a hugepage
>> is allocated on a page fault if the pud or pmd is empty.  This
>> yields the best VA translation performance, but increases memory
>> consumption if some small page ranges within the huge page are
>> never accessed.
> 
> Right, thats as per design.
> 
>>
>> An alternate behavior for such page faults is to install a
>> hugepage only when a region is actually found to be (almost)
>> fully mapped and active.  This is a compromise between
> 
> That is the async method by analyzing page table segment for
> the process by khugepaged and evaluate if a huge page can be
> installed replacing the existing pages.
> 
>> translation performance and memory consumption.  Currently there
>> is no way for an application to choose this compromise for the
>> page fault conditions above.
> 
> Cant we mark the THP enablement mode as "madvise", then switch
> between MADV_HUGEPAGE/MADV_NOHUGEPAGE to implement this ?
> 

Asking applications to issue MADV_HUGEPAGE/NOHUGEPAGE would make THP
much less 'automatic'. With such a scheme applications would have to
track mapping and active status of each hugepage region and manually
issue MADV_HUGEPAGE again to let khugepaged back it with a hugepage.

Compare above with the approach used by this patch: MADV_DONTNEED is
taken as a hint that application still wants transparent hugepages but
wants to be more conservative with memory usage. khugepaged is still
free to collapse pages as it sees fit without explicit
HUGEPAGE/NOHUGEPAGE madvise hints.

>>
>> With this change, when an application issues MADV_DONTNEED on a
>> memory region, the region is marked as "space-efficient". For
> 
> Isn't it that MADV_DONTNEED should be used for a region where
> there are already pages faulted in and page table populated ?
> Are you suggesting that MADV_DONTNEED should be called upon
> a region just after creation to control it's fault behavior ?
> Thats not what MADV_DONTNEED was meant to be.
> 

No, I'm not suggesting MADV_DONTNEED be called on empty region. The
patch just uses these calls, whenever they are made, as a hint to be
more conservative with memory usage for that vma.

>> such regions, a hugepage is not immediately allocated on first
>> write.  Instead, it is left to the khugepaged thread to do
>> delayed hugepage promotion depending on whether the region is
>> actually mapped and active. When application issues
>> MADV_HUGEPAGE, the region is marked again as non-space-efficient
>> wherein hugepage is allocated on first touch
> 
> But MADV_HUGEPAGE/MADV_NOHUGEPAGE combination should do the trick
> as well.
> 

Thanks,
Nitin

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