On 09.03.21 01:18, Mike Kravetz wrote:
The concurrent use of multiple hugetlb page sizes on a single system
is becoming more common. One of the reasons is better TLB support for
gigantic page sizes on x86 hardware. In addition, hugetlb pages are
being used to back VMs in hosting environments.
When using hugetlb pages to back VMs in such environments, it is
sometimes desirable to preallocate hugetlb pools. This avoids the delay
and uncertainty of allocating hugetlb pages at VM startup. In addition,
preallocating huge pages minimizes the issue of memory fragmentation that
increases the longer the system is up and running.
In such environments, a combination of larger and smaller hugetlb pages
are preallocated in anticipation of backing VMs of various sizes. Over
time, the preallocated pool of smaller hugetlb pages may become
depleted while larger hugetlb pages still remain. In such situations,
it may be desirable to convert larger hugetlb pages to smaller hugetlb
pages.
Converting larger to smaller hugetlb pages can be accomplished today by
first freeing the larger page to the buddy allocator and then allocating
the smaller pages. However, there are two issues with this approach:
1) This process can take quite some time, especially if allocation of
the smaller pages is not immediate and requires migration/compaction.
2) There is no guarantee that the total size of smaller pages allocated
will match the size of the larger page which was freed. This is
because the area freed by the larger page could quickly be
fragmented.
To address these issues, introduce the concept of hugetlb page demotion.
Demotion provides a means of 'in place' splitting a hugetlb page to
pages of a smaller size. For example, on x86 one 1G page can be
demoted to 512 2M pages. Page demotion is controlled via sysfs files.
- demote_size Read only target page size for demotion
- demote Writable number of hugetlb pages to be demoted
Only hugetlb pages which are free at the time of the request can be demoted.
Demotion does not add to the complexity surplus pages. Demotion also honors
reserved huge pages. Therefore, when a value is written to the sysfs demote
file that value is only the maximum number of pages which will be demoted.
It is possible fewer will actually be demoted.
If demote_size is PAGESIZE, demote will simply free pages to the buddy
allocator.
With the vmemmap optimizations you will have to rework the vmemmap
layout. How is that handled? Couldn't it happen that you are half-way
through splitting a PUD into PMDs when you realize that you cannot
allocate vmemmap pages for properly handling the remaining PMDs? What
would happen then?
Or are you planning on making both features mutually exclusive?
Of course, one approach would be first completely restoring the vmemmap
for the whole PUD (allocating more pages than necessary in the end) and
then freeing individual pages again when optimizing the layout per PMD.
--
Thanks,
David / dhildenb
