On 10 Mar 2021, at 12:05, Michal Hocko wrote: > 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 totally agree. This patchset looks to me like a partial functional replication of splitting high order free pages to lower order ones in buddy allocator. That is why I had the crazy idea below. > >> 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 No. The allocator maintains all the reserved pages for huge page allocations, replacing existing cma_alloc or alloc_contig_pages. The kernel builds the free list when pages are reserved either during boot time or runtime. > 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. Yeah, the gap between 2MB and 1GB is large. The fragmentation will be a problem. Maybe we do not need it right now, since this patchset does not propose promoting/merging pages. Or we can reuse the existing anti fragmentation mechanisms but with pageblock set to gigantic page size in this pool. I admit my idea is a much intrusive change, but I feel that more functionality replications of core mm are added to hugetlb code, then why not reuse the core mm code. — Best Regards, Yan Zi
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