|= 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
|- 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
|- 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
|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
|echo always >/sys/kernel/mm/transparent_hugepage/defrag
|echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
|echo never >/sys/kernel/mm/transparent_hugepage/defrag
|By default kernel tries to use huge zero page on read page fault.
|It's possible to disable huge zero page by writing 0 or enable it
|back by writing 1:
|echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
|echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
|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 by writing 0 or enable
|defrag in khugepaged by writing 1:
|echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
|You can also control how many pages khugepaged should scan at each
|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):
|and how many milliseconds to wait in khugepaged if there's an hugepage
|allocation failure to throttle the next allocation attempt.
|The khugepaged progress can be seen in the number of pages collapsed:
|for each pass:
|max_ptes_none specifies how many extra small pages (that are
|not already mapped) can be allocated when collapsing a group
|of small pages into one large page.
|A higher value leads to use additional memory for programs.
|A lower value leads to gain less thp performance. Value of
|max_ptes_none can waste cpu time very little, you can
|== 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.
|== Monitoring usage ==
|The number of transparent huge pages currently used by the system is
|available by reading the AnonHugePages field in /proc/meminfo. To
|identify what applications are using transparent huge pages, it is
|necessary to read /proc/PID/smaps and count the AnonHugePages fields
|for each mapping. Note that reading the smaps file is expensive and
|reading it frequently will incur overhead.
|There are a number of counters in /proc/vmstat that may be used to
|monitor how successfully the system is providing huge pages for use.
|thp_fault_alloc is incremented every time a huge page is successfully
| allocated to handle a page fault. This applies to both the
| first time a page is faulted and for COW faults.
|thp_collapse_alloc is incremented by khugepaged when it has found
| a range of pages to collapse into one huge page and has
| successfully allocated a new huge page to store the data.
|thp_fault_fallback is incremented if a page fault fails to allocate
| a huge page and instead falls back to using small pages.
|thp_collapse_alloc_failed is incremented if khugepaged found a range
| of pages that should be collapsed into one huge page but failed
| the allocation.
|thp_split is incremented every time a huge page is split into base
| pages. This can happen for a variety of reasons but a common
| reason is that a huge page is old and is being reclaimed.
|thp_zero_page_alloc is incremented every time a huge zero page is
| successfully allocated. It includes allocations which where
| dropped due race with other allocation. Note, it doesn't count
| every map of the huge zero page, only its allocation.
|thp_zero_page_alloc_failed is incremented if kernel fails to allocate
| huge zero page and falls back to using small pages.
|As the system ages, allocating huge pages may be expensive as the
|system uses memory compaction to copy data around memory to free a
|huge page for use. There are some counters in /proc/vmstat to help
|monitor this overhead.
|compact_stall is incremented every time a process stalls to run
| memory compaction so that a huge page is free for use.
|compact_success is incremented if the system compacted memory and
| freed a huge page for use.
|compact_fail is incremented if the system tries to compact memory
| but failed.
|compact_pages_moved is incremented each time a page is moved. If
| this value is increasing rapidly, it implies that the system
| is copying a lot of data to satisfy the huge page allocation.
| It is possible that the cost of copying exceeds any savings
| from reduced TLB misses.
|compact_pagemigrate_failed is incremented when the underlying mechanism
| for moving a page failed.
|compact_blocks_moved is incremented each time memory compaction examines
| a huge page aligned range of pages.
|It is possible to establish how long the stalls were using the function
|tracer to record how long was spent in __alloc_pages_nodemask and
|using the mm_page_alloc tracepoint to identify which allocations were
|for huge pages.
|== 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(vma, addr, 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
|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
|diff --git 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(vma, addr, 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 split 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
|split 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