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

Michal Hocko
2018-01-19T12:50:09Z

On Thu 18-01-18 15:33:16, Nitin Gupta wrote:
> From: Nitin Gupta <nitin.m.gupta@oracle.com>
>
> 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.

Yes, this is true but hardly unexpected for MADV_HUGEPAGE or THP always
users.

> 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
> translation performance and memory consumption. Currently there
> is no way for an application to choose this compromise for the
> page fault conditions above.

Is that really true? We have /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
This is not reflected during the PF of course but you can control the
behavior there as well. Either by the global setting or a per proces
prctl.

> With this change, whenever an application issues MADV_DONTNEED on a
> memory region, the region is marked as "space-efficient". For such
> regions, a hugepage is not immediately allocated on first write.

Kirill didn't like it in the previous version and I do not like this
either. You are adding a very subtle side effect which might completely
unexpected. Consider userspace memory allocator which uses MADV_DONTNEED
to free up unused memory. Now you have put it out of THP usage
basically.

If the memory is used really scarce then we have MADV_NOHUGEPAGE.
--
Michal Hocko
SUSE Labs