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| -rw-r--r-- | Documentation/vm/transhuge.txt | 298 |
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diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt new file mode 100644 index 000000000000..0924aaca3302 --- /dev/null +++ b/Documentation/vm/transhuge.txt @@ -0,0 +1,298 @@ += Transparent Hugepage Support = + +== Objective == + +Performance critical computing applications dealing with large memory +working sets are already running on top of libhugetlbfs and in turn +hugetlbfs. Transparent Hugepage Support is an alternative means of +using huge pages for the backing of virtual memory with huge pages +that supports the automatic promotion and demotion of page sizes and +without the shortcomings of hugetlbfs. + +Currently it only works for anonymous memory mappings but in the +future it can expand over the pagecache layer starting with tmpfs. + +The reason applications are running faster is because of two +factors. The first factor is almost completely irrelevant and it's not +of significant interest because it'll also have the downside of +requiring larger clear-page copy-page in page faults which is a +potentially negative effect. The first factor consists in taking a +single page fault for each 2M virtual region touched by userland (so +reducing the enter/exit kernel frequency by a 512 times factor). This +only matters the first time the memory is accessed for the lifetime of +a memory mapping. The second long lasting and much more important +factor will affect all subsequent accesses to the memory for the whole +runtime of the application. The second factor consist of two +components: 1) the TLB miss will run faster (especially with +virtualization using nested pagetables but almost always also on bare +metal without virtualization) and 2) a single TLB entry will be +mapping a much larger amount of virtual memory in turn reducing the +number of TLB misses. With virtualization and nested pagetables the +TLB can be mapped of larger size only if both KVM and the Linux guest +are using hugepages but a significant speedup already happens if only +one of the two is using hugepages just because of the fact the TLB +miss is going to run faster. + +== Design == + +- "graceful fallback": mm components which don't have transparent + hugepage knowledge fall back to breaking a transparent hugepage and + working on the regular pages and their respective regular pmd/pte + mappings + +- if a hugepage allocation fails because of memory fragmentation, + regular pages should be gracefully allocated instead and mixed in + the same vma without any failure or significant delay and without + userland noticing + +- if some task quits and more hugepages become available (either + immediately in the buddy or through the VM), guest physical memory + backed by regular pages should be relocated on hugepages + automatically (with khugepaged) + +- it doesn't require memory reservation and in turn it uses hugepages + whenever possible (the only possible reservation here is kernelcore= + to avoid unmovable pages to fragment all the memory but such a tweak + is not specific to transparent hugepage support and it's a generic + feature that applies to all dynamic high order allocations in the + kernel) + +- this initial support only offers the feature in the anonymous memory + regions but it'd be ideal to move it to tmpfs and the pagecache + later + +Transparent Hugepage Support maximizes the usefulness of free memory +if compared to the reservation approach of hugetlbfs by allowing all +unused memory to be used as cache or other movable (or even unmovable +entities). It doesn't require reservation to prevent hugepage +allocation failures to be noticeable from userland. It allows paging +and all other advanced VM features to be available on the +hugepages. It requires no modifications for applications to take +advantage of it. + +Applications however can be further optimized to take advantage of +this feature, like for example they've been optimized before to avoid +a flood of mmap system calls for every malloc(4k). Optimizing userland +is by far not mandatory and khugepaged already can take care of long +lived page allocations even for hugepage unaware applications that +deals with large amounts of memory. + +In certain cases when hugepages are enabled system wide, application +may end up allocating more memory resources. An application may mmap a +large region but only touch 1 byte of it, in that case a 2M page might +be allocated instead of a 4k page for no good. This is why it's +possible to disable hugepages system-wide and to only have them inside +MADV_HUGEPAGE madvise regions. + +Embedded systems should enable hugepages only inside madvise regions +to eliminate any risk of wasting any precious byte of memory and to +only run faster. + +Applications that gets a lot of benefit from hugepages and that don't +risk to lose memory by using hugepages, should use +madvise(MADV_HUGEPAGE) on their critical mmapped regions. + +== sysfs == + +Transparent Hugepage Support can be entirely disabled (mostly for +debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to +avoid the risk of consuming more memory resources) or enabled system +wide. This can be achieved with one of: + +echo always >/sys/kernel/mm/transparent_hugepage/enabled +echo madvise >/sys/kernel/mm/transparent_hugepage/enabled +echo never >/sys/kernel/mm/transparent_hugepage/enabled + +It's also possible to limit defrag efforts in the VM to generate +hugepages in case they're not immediately free to madvise regions or +to never try to defrag memory and simply fallback to regular pages +unless hugepages are immediately available. Clearly if we spend CPU +time to defrag memory, we would expect to gain even more by the fact +we use hugepages later instead of regular pages. This isn't always +guaranteed, but it may be more likely in case the allocation is for a +MADV_HUGEPAGE region. + +echo always >/sys/kernel/mm/transparent_hugepage/defrag +echo madvise >/sys/kernel/mm/transparent_hugepage/defrag +echo never >/sys/kernel/mm/transparent_hugepage/defrag + +khugepaged will be automatically started when +transparent_hugepage/enabled is set to "always" or "madvise, and it'll +be automatically shutdown if it's set to "never". + +khugepaged runs usually at low frequency so while one may not want to +invoke defrag algorithms synchronously during the page faults, it +should be worth invoking defrag at least in khugepaged. However it's +also possible to disable defrag in khugepaged: + +echo yes >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag +echo no >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag + +You can also control how many pages khugepaged should scan at each +pass: + +/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan + +and how many milliseconds to wait in khugepaged between each pass (you +can set this to 0 to run khugepaged at 100% utilization of one core): + +/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs + +and how many milliseconds to wait in khugepaged if there's an hugepage +allocation failure to throttle the next allocation attempt. + +/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs + +The khugepaged progress can be seen in the number of pages collapsed: + +/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed + +for each pass: + +/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans + +== Boot parameter == + +You can change the sysfs boot time defaults of Transparent Hugepage +Support by passing the parameter "transparent_hugepage=always" or +"transparent_hugepage=madvise" or "transparent_hugepage=never" +(without "") to the kernel command line. + +== Need of application restart == + +The transparent_hugepage/enabled values only affect future +behavior. So to make them effective you need to restart any +application that could have been using hugepages. This also applies to +the regions registered in khugepaged. + +== get_user_pages and follow_page == + +get_user_pages and follow_page if run on a hugepage, will return the +head or tail pages as usual (exactly as they would do on +hugetlbfs). Most gup users will only care about the actual physical +address of the page and its temporary pinning to release after the I/O +is complete, so they won't ever notice the fact the page is huge. But +if any driver is going to mangle over the page structure of the tail +page (like for checking page->mapping or other bits that are relevant +for the head page and not the tail page), it should be updated to jump +to check head page instead (while serializing properly against +split_huge_page() to avoid the head and tail pages to disappear from +under it, see the futex code to see an example of that, hugetlbfs also +needed special handling in futex code for similar reasons). + +NOTE: these aren't new constraints to the GUP API, and they match the +same constrains that applies to hugetlbfs too, so any driver capable +of handling GUP on hugetlbfs will also work fine on transparent +hugepage backed mappings. + +In case you can't handle compound pages if they're returned by +follow_page, the FOLL_SPLIT bit can be specified as parameter to +follow_page, so that it will split the hugepages before returning +them. Migration for example passes FOLL_SPLIT as parameter to +follow_page because it's not hugepage aware and in fact it can't work +at all on hugetlbfs (but it instead works fine on transparent +hugepages thanks to FOLL_SPLIT). migration simply can't deal with +hugepages being returned (as it's not only checking the pfn of the +page and pinning it during the copy but it pretends to migrate the +memory in regular page sizes and with regular pte/pmd mappings). + +== Optimizing the applications == + +To be guaranteed that the kernel will map a 2M page immediately in any +memory region, the mmap region has to be hugepage naturally +aligned. posix_memalign() can provide that guarantee. + +== Hugetlbfs == + +You can use hugetlbfs on a kernel that has transparent hugepage +support enabled just fine as always. No difference can be noted in +hugetlbfs other than there will be less overall fragmentation. All +usual features belonging to hugetlbfs are preserved and +unaffected. libhugetlbfs will also work fine as usual. + +== Graceful fallback == + +Code walking pagetables but unware about huge pmds can simply call +split_huge_page_pmd(mm, pmd) where the pmd is the one returned by +pmd_offset. It's trivial to make the code transparent hugepage aware +by just grepping for "pmd_offset" and adding split_huge_page_pmd where +missing after pmd_offset returns the pmd. Thanks to the graceful +fallback design, with a one liner change, you can avoid to write +hundred if not thousand of lines of complex code to make your code +hugepage aware. + +If you're not walking pagetables but you run into a physical hugepage +but you can't handle it natively in your code, you can split it by +calling split_huge_page(page). This is what the Linux VM does before +it tries to swapout the hugepage for example. + +Example to make mremap.c transparent hugepage aware with a one liner +change: + +diff --git a/mm/mremap.c b/mm/mremap.c +--- a/mm/mremap.c ++++ b/mm/mremap.c +@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru + return NULL; + + pmd = pmd_offset(pud, addr); ++ split_huge_page_pmd(mm, pmd); + if (pmd_none_or_clear_bad(pmd)) + return NULL; + +== Locking in hugepage aware code == + +We want as much code as possible hugepage aware, as calling +split_huge_page() or split_huge_page_pmd() has a cost. + +To make pagetable walks huge pmd aware, all you need to do is to call +pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the +mmap_sem in read (or write) mode to be sure an huge pmd cannot be +created from under you by khugepaged (khugepaged collapse_huge_page +takes the mmap_sem in write mode in addition to the anon_vma lock). If +pmd_trans_huge returns false, you just fallback in the old code +paths. If instead pmd_trans_huge returns true, you have to take the +mm->page_table_lock and re-run pmd_trans_huge. Taking the +page_table_lock will prevent the huge pmd to be converted into a +regular pmd from under you (split_huge_page can run in parallel to the +pagetable walk). If the second pmd_trans_huge returns false, you +should just drop the page_table_lock and fallback to the old code as +before. Otherwise you should run pmd_trans_splitting on the pmd. In +case pmd_trans_splitting returns true, it means split_huge_page is +already in the middle of splitting the page. So if pmd_trans_splitting +returns true it's enough to drop the page_table_lock and call +wait_split_huge_page and then fallback the old code paths. You are +guaranteed by the time wait_split_huge_page returns, the pmd isn't +huge anymore. If pmd_trans_splitting returns false, you can proceed to +process the huge pmd and the hugepage natively. Once finished you can +drop the page_table_lock. + +== compound_lock, get_user_pages and put_page == + +split_huge_page internally has to distribute the refcounts in the head +page to the tail pages before clearing all PG_head/tail bits from the +page structures. It can do that easily for refcounts taken by huge pmd +mappings. But the GUI API as created by hugetlbfs (that returns head +and tail pages if running get_user_pages on an address backed by any +hugepage), requires the refcount to be accounted on the tail pages and +not only in the head pages, if we want to be able to run +split_huge_page while there are gup pins established on any tail +page. Failure to be able to run split_huge_page if there's any gup pin +on any tail page, would mean having to split all hugepages upfront in +get_user_pages which is unacceptable as too many gup users are +performance critical and they must work natively on hugepages like +they work natively on hugetlbfs already (hugetlbfs is simpler because +hugetlbfs pages cannot be splitted so there wouldn't be requirement of +accounting the pins on the tail pages for hugetlbfs). If we wouldn't +account the gup refcounts on the tail pages during gup, we won't know +anymore which tail page is pinned by gup and which is not while we run +split_huge_page. But we still have to add the gup pin to the head page +too, to know when we can free the compound page in case it's never +splitted during its lifetime. That requires changing not just +get_page, but put_page as well so that when put_page runs on a tail +page (and only on a tail page) it will find its respective head page, +and then it will decrease the head page refcount in addition to the +tail page refcount. To obtain a head page reliably and to decrease its +refcount without race conditions, put_page has to serialize against +__split_huge_page_refcount using a special per-page lock called +compound_lock. |