Messages in this thread | | | Subject | Re: [PATCH 0/7] Free user PTE page table pages | From | Qi Zheng <> | Date | Tue, 20 Jul 2021 12:00:01 +0800 |
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On 7/19/21 7:28 PM, David Hildenbrand wrote: > On 19.07.21 09:34, David Hildenbrand wrote: >> On 18.07.21 06:30, Qi Zheng wrote: >>> Hi, >>> >>> This patch series aims to free user PTE page table pages when all PTE >>> entries >>> are empty. >>> >>> The beginning of this story is that some malloc libraries(e.g. >>> jemalloc or >>> tcmalloc) usually allocate the amount of VAs by mmap() and do not >>> unmap those VAs. >>> They will use madvise(MADV_DONTNEED) to free physical memory if they >>> want. >>> But the page tables do not be freed by madvise(), so it can produce many >>> page tables when the process touches an enormous virtual address space. >> >> ... did you see that I am actually looking into this? >> >> https://lkml.kernel.org/r/bae8b967-c206-819d-774c-f57b94c4b362@redhat.com >> >> and have already spent a significant time on it as part of my research, >> which is *really* unfortunate and makes me quite frustrated at the >> beginning of the week alreadty ... >> >> Ripping out page tables is quite difficult, as we have to stop all page >> table walkers from touching it, including the fast_gup, rmap and page >> faults. This usually involves taking the mmap lock in write. My approach >> does page table reclaim asynchronously from another thread and do not >> rely on reference counts. > > FWIW, I had a quick peek and I like the simplistic approach using > reference counting, although it seems to come with a price. By hooking > using pte_alloc_get_map_lock() instead of pte_alloc_map_lock, we can > handle quite some cases easily. > > There are cases where we might immediately see a reuse after discarding > memory (especially, with virtio-balloon free page reporting), in which > case it's suboptimal to immediately discard instead of waiting a bit if > there is a reuse. However, the performance impact seems to be > comparatively small.
Good point, maybe we can wait a bit in the free_pte_table() in the added optimiztion patch if the frequency of immediate reuse is high.
> > I do wonder if the 1% overhead you're seeing is actually because of > allcoating/freeing or because of the reference count handling on some > hot paths. > > I'm primarily looking into asynchronous reclaim, because it somewhat > makes sense to only reclaim (+ pay a cost) when there is really need to > reclaim memory -- similar to our shrinker infrastructure. >
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