On 28/02/2024 15:12, David Hildenbrand wrote: > On 28.02.24 15:57, Ryan Roberts wrote: >> On 28/02/2024 12:12, David Hildenbrand wrote: >>>>> How relevant is it? Relevant enough that someone decided to put that >>>>> optimization in? I don't know :) >>>> >>>> I'll have one last go at convincing you: Huang Ying (original author) commented >>>> "I believe this should be OK. Better to compare the performance too." at [1]. >>>> That implies to me that perhaps the optimization wasn't in response to a >>>> specific problem after all. Do you have any thoughts, Huang? >>> >>> Might make sense to include that in the patch description! >>> >>>> OK so if we really do need to keep this optimization, here are some ideas: >>>> >>>> Fundamentally, we would like to be able to figure out the size of the swap slot >>>> from the swap entry. Today swap supports 2 sizes; PAGE_SIZE and PMD_SIZE. For >>>> PMD_SIZE, it always uses a full cluster, so can easily add a flag to the >>>> cluster >>>> to mark it as PMD_SIZE. >>>> >>>> Going forwards, we want to support all sizes (power-of-2). Most of the time, a >>>> cluster will contain only one size of THPs, but this is not the case when a THP >>>> in the swapcache gets split or when an order-0 slot gets stolen. We expect >>>> these >>>> cases to be rare. >>>> >>>> 1) Keep the size of the smallest swap entry in the cluster header. Most of the >>>> time it will be the full size of the swap entry, but sometimes it will cover >>>> only a portion. In the latter case you may see a false negative for >>>> swap_page_trans_huge_swapped() meaning we take the slow path, but that is rare. >>>> There is one wrinkle: currently the HUGE flag is cleared in >>>> put_swap_folio(). We >>>> wouldn't want to do the equivalent in the new scheme (i.e. set the whole >>>> cluster >>>> to order-0). I think that is safe, but haven't completely convinced myself yet. >>>> >>>> 2) allocate 4 bits per (small) swap slot to hold the order. This will give >>>> precise information and is conceptually simpler to understand, but will cost >>>> more memory (half as much as the initial swap_map[] again). >>>> >>>> I still prefer to avoid this at all if we can (and would like to hear Huang's >>>> thoughts). But if its a choice between 1 and 2, I prefer 1 - I'll do some >>>> prototyping. >>> >>> Taking a step back: what about we simply batch unmapping of swap entries? >>> >>> That is, if we're unmapping a PTE range, we'll collect swap entries (under PT >>> lock) that reference consecutive swap offsets in the same swap file. >> >> Yes in principle, but there are 4 places where free_swap_and_cache() is called, >> and only 2 of those are really amenable to batching (zap_pte_range() and >> madvise_free_pte_range()). So the other two users will still take the "slow" >> path. Maybe those 2 callsites are the only ones that really matter? I can >> certainly have a stab at this approach. > > We can ignore the s390x one. That s390x code should only apply to KVM guest > memory where ordinary THP are not even supported. (and nobody uses mTHP there yet). > > Long story short: the VM can hint that some memory pages are now unused and the > hypervisor can reclaim them. That's what that callback does (zap guest-provided > guest memory). No need to worry about any batching for now. OK good. > > Then, there is the shmem one in shmem_free_swap(). I really don't know how shmem > handles THP+swapout. > > But looking at shmem_writepage(), we split any large folios before moving them > to the swapcache, so likely we don't care at all, because THP don't apply. Excellent. > >> >>> >>> There, we can then first decrement all the swap counts, and then try minimizing >>> how often we actually have to try reclaiming swap space (lookup folio, see it's >>> a large folio that we cannot reclaim or could reclaim, ...). >>> >>> Might need some fine-tuning in swap code to "advance" to the next entry to try >>> freeing up, but we certainly can do better than what we would do right now. >> >> I'm not sure I've understood this. Isn't advancing just a matter of: >> >> entry = swp_entry(swp_type(entry), swp_offset(entry) + 1); > > I was talking about the advancing swapslot processing after decrementing the > swapcounts. > > Assume you decremented 512 swapcounts and some of them went to 0. AFAIU, you'd > have to start with the first swapslot that has now a swapcount=0 one and try to > reclaim swap. > > Assume you get a small folio, then you'll have to proceed with the next swap > slot and try to reclaim swap. > > Assume you get a large folio, then you can skip more swapslots (depending on > offset into the folio etc). > > If you get what I mean. :) Ahh gottya. I'll have a play.
