Messages in this thread | | | Date | Wed, 31 Jan 2024 11:43:07 +0100 | Subject | Re: [PATCH v1 0/9] mm/memory: optimize unmap/zap with PTE-mapped THP | From | David Hildenbrand <> |
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On 31.01.24 03:20, Yin Fengwei wrote: > On 1/29/24 22:32, David Hildenbrand wrote: >> This series is based on [1] and must be applied on top of it. >> Similar to what we did with fork(), let's implement PTE batching >> during unmap/zap when processing PTE-mapped THPs. >> >> We collect consecutive PTEs that map consecutive pages of the same large >> folio, making sure that the other PTE bits are compatible, and (a) adjust >> the refcount only once per batch, (b) call rmap handling functions only >> once per batch, (c) perform batch PTE setting/updates and (d) perform TLB >> entry removal once per batch. >> >> Ryan was previously working on this in the context of cont-pte for >> arm64, int latest iteration [2] with a focus on arm6 with cont-pte only. >> This series implements the optimization for all architectures, independent >> of such PTE bits, teaches MMU gather/TLB code to be fully aware of such >> large-folio-pages batches as well, and amkes use of our new rmap batching >> function when removing the rmap. >> >> To achieve that, we have to enlighten MMU gather / page freeing code >> (i.e., everything that consumes encoded_page) to process unmapping >> of consecutive pages that all belong to the same large folio. I'm being >> very careful to not degrade order-0 performance, and it looks like I >> managed to achieve that. > > One possible scenario: > If all the folio is 2M size folio, then one full batch could hold 510M memory. > Is it too much regarding one full batch before just can hold (2M - 4096 * 2) > memory?
Good point, we do have CONFIG_INIT_ON_FREE_DEFAULT_ON. I don't remember if init_on_free or init_on_alloc was used in production systems. In tlb_batch_pages_flush(), there is a cond_resched() to limit the number of entries we process.
So if that is actually problematic, we'd run into a soft-lockup and need another cond_resched() [I have some faint recollection that people are working on removing cond_resched() completely].
One could do some counting in free_pages_and_swap_cache() (where we iterate all entries already) and insert cond_resched+release_pages() for every (e.g., 512) pages.
-- Cheers,
David / dhildenb
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