On Wed 10-03-21 11:46:57, Zi Yan wrote: > On 10 Mar 2021, at 11:23, Michal Hocko wrote: > > > On Mon 08-03-21 16:18:52, Mike Kravetz wrote: > > [...] > >> 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. > > > > I will likely not surprise to show some level of reservation. While your > > concerns about reconfiguration by existing interfaces are quite real is > > this really a problem in practice? How often do you need such a > > reconfiguration? > > > > Is this all really worth the additional code to something as tricky as > > hugetlb code base? > > > >> include/linux/hugetlb.h | 8 ++ > >> mm/hugetlb.c | 199 +++++++++++++++++++++++++++++++++++++++- > >> 2 files changed, 204 insertions(+), 3 deletions(-) > >> > >> -- > >> 2.29.2 > >> > > The high level goal of this patchset seems to enable flexible huge page > allocation from a single pool, when multiple huge page sizes are available > to use. The limitation of existing mechanism is that user has to specify > how many huge pages he/she wants and how many gigantic pages he/she wants > before the actual use. I believe I have understood this part. And I am not questioning that. This seems useful. I am mostly asking whether we need such a flexibility. Mostly because of the additional code and future maintenance complexity which has turned to be a problem for a long time. Each new feature tends to just add on top of the existing complexity. > I just want to throw an idea here, please ignore if it is too crazy. > Could we have a variant buddy allocator for huge page allocations, > which only has available huge page orders in the free list? For example, > if user wants 2MB and 1GB pages, the allocator will only have order-9 and > order-19 pages; when order-9 pages run out, we can split order-19 pages; > if possible, adjacent order-9 pages can be merged back to order-19 pages. I assume you mean to remove those pages from the allocator when they are reserved rather than really used, right? I am not really sure how you want to deal with lower orders consuming/splitting too much from higher orders which then makes those unusable for the use even though they were preallocated for a specific workload. Another worry is that a gap between 2MB and 1GB pages is just too big so a single 2MB request from 1G pool will make the whole 1GB page unusable even when the smaller pool needs few pages. -- Michal Hocko SUSE Labs
