Re: Compaction & folios

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Wed, Oct 06, 2021 at 06:53:41PM -0400, Kent Overstreet wrote:
> So I have some observations on memory compaction & hugepages.
> 
> Right now, the working assumption in MM is that compaction is hard and
> expensive, and right now it is - because most allocations are order 0, with a
> small subset being hugepage order allocations. This means any time we need a
> hugepage, compaction has to move a bunch of order 0 pages around, and memory
> reclaim is no help here - when we reclaim memory, it's coming back as fragmented
> order 0 pages.
> 
> But what if compaction wasn't such a difficult, expensive operation?
> 
> With folios, and then folios for anonymous pages, we won't see nearly so many
> order 0 allocations anymore - we'll see a spread of allocation sizes based on a
> mixture of application usage patterns - something much closer to a poisson
> distribution, vs. our current very bimodal distribution. And since we won't be
> fragmenting all our allocations up front, memory reclaim will be freeing
> allocations in this same distribution.
> 
> Which means that any time an order n allocation fails, it's likely that we'll
> still have order n-1 pages free - and of those free order n-1 pages, one will
> likely have a buddy that's moveable and hasn't been fragmented - meaning the
> common case is that compaction will have to move _one_ (higher order) page -
> we'll almost never be having to move a bunch of 4k pages.
> 
> Another way of thinking of this is that memory reclaim will be doing most of the
> work that compaction has to do now to allocate a high order page. Compaction
> will go from an expensive, somewhat unreliable operation to one that mostly just
> works - it's going to be _much_ less of a pain point.
> 
> It may turn out that allocating hugepages still doesn't work as reliably as we'd
> like - but folios are still a big help even when we can't allocate a 2MB page,
> because we'll be able to fall back to an order 6 or 7 or 8 allocation, which is
> something we can't do now. And, since multiple CPU vendors now support
> coalescing contiguous PTE entries in the TLB, this will still get us most of the
> performance benefits of using hugepages.

Compaction at the moment built with assumption that compound pages are
PMD-mappable or larger and it doesn't make sense to move them:

		/*
		 * Regardless of being on LRU, compound pages such as THP and
		 * hugetlbfs are not to be compacted unless we are attempting
		 * an allocation much larger than the huge page size (eg CMA).
		 * We can potentially save a lot of iterations if we skip them
		 * at once. The check is racy, but we can consider only valid
		 * values and the only danger is skipping too much.
		 */
		if (PageCompound(page) && !cc->alloc_contig) {
			const unsigned int order = compound_order(page);

			if (likely(order < MAX_ORDER))
				low_pfn += (1UL << order) - 1;
			goto isolate_fail;
		}

It also will apply to folios with direct conversion.

It has to be reworked sooner rather than later if we want to be more
flexible on size of folios or we are risking getting compaction situation
worse.

-- 
 Kirill A. Shutemov