Ryan Roberts <ryan.roberts@xxxxxxx> writes: > [...] > >>>> Test 1, sequential swapin/out of 30G zero page on ZRAM: >>>> >>>> Before (us) After (us) >>>> Swapout: 33619409 33886008 >>>> Swapin: 32393771 32465441 (- 0.2%) >>>> Swapout (THP): 7817909 6899938 (+11.8%) >>>> Swapin (THP) : 32452387 33193479 (- 2.2%) >>> >>> If my understanding were correct, we don't have swapin (THP) support, >>> yet. Right? >> >> Yes, this series doesn't change how swapin/swapout works with THP in >> general, but now THP swapout will leave shadows with large order, so >> it needs to be splitted upon swapin, that will slow down later swapin >> by a little bit but I think that's worth it. >> >> If we can do THP swapin in the future, this split on swapin can be >> saved to make the performance even better. > > I'm confused by this (clearly my understanding of how this works is incorrect). > Perhaps you can help me understand: > > When you talk about "shadows" I assume you are referring to the swap cache? It > was my understanding that swapping out a THP would always leave the large folio > in the swap cache, so this is nothing new? > > And on swap-in, if the target page is in the swap cache, even if part of a large > folio, why does it need to be split? I assumed the single page would just be > mapped? (and if all the other pages subsequently fault, then you end up with a > fully mapped large folio back in the process)? > > Perhaps I'm misunderstanding what "shadows" are? Perhaps, shadow is used to support workingset protection/detection on the anonymous LRU list as in the following patchset (merged). https://lore.kernel.org/all/1595490560-15117-5-git-send-email-iamjoonsoo.kim@xxxxxxx/T/#m962395eb5968c74b0c4c8e41d4b0dcdd3f28b2e6 -- Best Regards, Huang, Ying
