Hi Huang, On Fri, Apr 28, 2017 at 08:21:37PM +0800, Huang, Ying wrote: > Minchan Kim <minchan@xxxxxxxxxx> writes: > > > On Thu, Apr 27, 2017 at 03:12:34PM +0800, Huang, Ying wrote: > >> Minchan Kim <minchan@xxxxxxxxxx> writes: > >> > >> > On Tue, Apr 25, 2017 at 08:56:56PM +0800, Huang, Ying wrote: > >> >> From: Huang Ying <ying.huang@xxxxxxxxx> > >> >> > >> >> In this patch, splitting huge page is delayed from almost the first > >> >> step of swapping out to after allocating the swap space for the > >> >> THP (Transparent Huge Page) and adding the THP into the swap cache. > >> >> This will batch the corresponding operation, thus improve THP swap out > >> >> throughput. > >> >> > >> >> This is the first step for the THP swap optimization. The plan is to > >> >> delay splitting the THP step by step and avoid splitting the THP > >> >> finally. > >> >> > >> >> The advantages of the THP swap support include: > >> >> > >> >> - Batch the swap operations for the THP and reduce lock > >> >> acquiring/releasing, including allocating/freeing the swap space, > >> >> adding/deleting to/from the swap cache, and writing/reading the swap > >> >> space, etc. This will help to improve the THP swap performance. > >> >> > >> >> - The THP swap space read/write will be 2M sequential IO. It is > >> >> particularly helpful for the swap read, which usually are 4k random > >> >> IO. This will help to improve the THP swap performance. > >> >> > >> >> - It will help the memory fragmentation, especially when the THP is > >> >> heavily used by the applications. The 2M continuous pages will be > >> >> free up after the THP swapping out. > >> >> > >> >> - It will improve the THP utilization on the system with the swap > >> >> turned on. Because the speed for khugepaged to collapse the normal > >> >> pages into the THP is quite slow. After the THP is split during the > >> >> swapping out, it will take quite long time for the normal pages to > >> >> collapse back into the THP after being swapped in. The high THP > >> >> utilization helps the efficiency of the page based memory management > >> >> too. > >> >> > >> >> There are some concerns regarding THP swap in, mainly because possible > >> >> enlarged read/write IO size (for swap in/out) may put more overhead on > >> >> the storage device. To deal with that, the THP swap in should be > >> >> turned on only when necessary. For example, it can be selected via > >> >> "always/never/madvise" logic, to be turned on globally, turned off > >> >> globally, or turned on only for VMA with MADV_HUGEPAGE, etc. > >> >> > >> >> In this patch, one swap cluster is used to hold the contents of each > >> >> THP swapped out. So, the size of the swap cluster is changed to that > >> >> of the THP (Transparent Huge Page) on x86_64 architecture (512). For > >> >> other architectures which want such THP swap optimization, > >> >> ARCH_USES_THP_SWAP_CLUSTER needs to be selected in the Kconfig file > >> >> for the architecture. In effect, this will enlarge swap cluster size > >> >> by 2 times on x86_64. Which may make it harder to find a free cluster > >> >> when the swap space becomes fragmented. So that, this may reduce the > >> >> continuous swap space allocation and sequential write in theory. The > >> >> performance test in 0day shows no regressions caused by this. > >> > > >> > What about other architecures? > >> > > >> > I mean THP page size on every architectures would be various. > >> > If THP page size is much bigger than 2M, the architecture should > >> > have big swap cluster size for supporting THP swap-out feature. > >> > It means fast empty-swap cluster consumption so that it can suffer > >> > from fragmentation easily which causes THP swap void and swap slot > >> > allocations slow due to not being able to use per-cpu. > >> > > >> > What I suggested was contiguous multiple swap cluster allocations > >> > to meet THP page size. If some of architecure's THP size is 64M > >> > and SWAP_CLUSTER_SIZE is 2M, it should allocate 32 contiguos > >> > swap clusters. For that, swap layer need to manage clusters sort > >> > in order which would be more overhead in CONFIG_THP_SWAP case > >> > but I think it's tradeoff. With that, every architectures can > >> > support THP swap easily without arch-specific something. > >> > >> That may be a good solution for other architectures. But I am afraid I > >> am not the right person to work on that. Because I don't know the > >> requirement of other architectures, and I have no other architectures > >> machines to work on and measure the performance. > > > > IMO, THP swapout is good thing for every architecture so I dobut > > you need to know other architecture's requirement. > > > >> > >> And the swap clusters aren't sorted in order now intentionally to avoid > >> cache line false sharing between the spinlock of struct > >> swap_cluster_info. If we want to sort clusters in order, we need a > >> solution for that. > > > > Does it really matter for this work? IOW, if we couldn't solve it, > > cannot we support THP swapout? I don't think so. That's the least > > of your worries. > > Also, if we have sorted cluster data structure, we need to change > > current single linked list of swap cluster to other one so we would > > need to revisit to see whether it's really problem. > > > >> > >> > If (PAGE_SIZE * 512) swap cluster size were okay for most of > >> > architecture, just increase it. It's orthogonal work regardless of > >> > THP swapout. Then, we don't need to manage swap clusters sort > >> > in order in x86_64 which SWAP_CLUSTER_SIZE is equal to > >> > THP_PAGE_SIZE. It's just a bonus by side-effect. > >> > >> Andrew suggested to make swap cluster size = huge page size (or turn on > >> THP swap optimization) only if we enabled CONFIG_THP_SWAP. So that, THP > >> swap optimization will not be turned on unintentionally. > >> > >> We may adjust default swap cluster size, but I don't think it need to be > >> in this patchset. > > > > That's it. This feature shouldn't be aware of swap cluster size. IOW, > > it would be better to work with every swap cluster size if the align > > between THP and swap cluster size is matched at least. > > Using one swap cluster for each THP is simpler, so why not start from > the simple design? Complex design may be necessary in the future, but > we can work on that at that time. If it were really architecture specific issue, I'm okay with such simple start with major architecture first. However, I don't think it's the case. THP swap: I don't think it's a architecure issue. It's generally good thing once system supports THP. It is also good thing for HDD swap as well as SSD. (In fact, I don't understand why we should have CONFIG_THP_SWAP. I think it should work with CONFIG_TRANSPARENT_HUGEPAGE automatically). Current design is selected for just *simple implementation" and put the heavy drift to make it generalize to others in the future. I don't think it's good thing. But others might have different opinions so I'm not insisting. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
