Documentation / vm / transhuge.txt

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Based on kernel version 3.4. Page generated on 2012-05-21 22:13 EST.

	= 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 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
	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.

• [ vm ]
• 00-INDEX
• active_mm.txt
• balance
• cleancache.txt
• highmem.txt
• hugetlbpage.txt
• hwpoison.txt
• ksm.txt
• locking
• numa
• numa_memory_policy.txt
• overcommit-accounting
• page_migration
• pagemap.txt
• slub.txt
• transhuge.txt
• unevictable-lru.txt
•