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Based on kernel version 4.3. Page generated on 2015-11-02 12:51 EST.

1	= Transparent Hugepage Support =
3	== Objective ==
5	Performance critical computing applications dealing with large memory
6	working sets are already running on top of libhugetlbfs and in turn
7	hugetlbfs. Transparent Hugepage Support is an alternative means of
8	using huge pages for the backing of virtual memory with huge pages
9	that supports the automatic promotion and demotion of page sizes and
10	without the shortcomings of hugetlbfs.
12	Currently it only works for anonymous memory mappings but in the
13	future it can expand over the pagecache layer starting with tmpfs.
15	The reason applications are running faster is because of two
16	factors. The first factor is almost completely irrelevant and it's not
17	of significant interest because it'll also have the downside of
18	requiring larger clear-page copy-page in page faults which is a
19	potentially negative effect. The first factor consists in taking a
20	single page fault for each 2M virtual region touched by userland (so
21	reducing the enter/exit kernel frequency by a 512 times factor). This
22	only matters the first time the memory is accessed for the lifetime of
23	a memory mapping. The second long lasting and much more important
24	factor will affect all subsequent accesses to the memory for the whole
25	runtime of the application. The second factor consist of two
26	components: 1) the TLB miss will run faster (especially with
27	virtualization using nested pagetables but almost always also on bare
28	metal without virtualization) and 2) a single TLB entry will be
29	mapping a much larger amount of virtual memory in turn reducing the
30	number of TLB misses. With virtualization and nested pagetables the
31	TLB can be mapped of larger size only if both KVM and the Linux guest
32	are using hugepages but a significant speedup already happens if only
33	one of the two is using hugepages just because of the fact the TLB
34	miss is going to run faster.
36	== Design ==
38	- "graceful fallback": mm components which don't have transparent
39	  hugepage knowledge fall back to breaking a transparent hugepage and
40	  working on the regular pages and their respective regular pmd/pte
41	  mappings
43	- if a hugepage allocation fails because of memory fragmentation,
44	  regular pages should be gracefully allocated instead and mixed in
45	  the same vma without any failure or significant delay and without
46	  userland noticing
48	- if some task quits and more hugepages become available (either
49	  immediately in the buddy or through the VM), guest physical memory
50	  backed by regular pages should be relocated on hugepages
51	  automatically (with khugepaged)
53	- it doesn't require memory reservation and in turn it uses hugepages
54	  whenever possible (the only possible reservation here is kernelcore=
55	  to avoid unmovable pages to fragment all the memory but such a tweak
56	  is not specific to transparent hugepage support and it's a generic
57	  feature that applies to all dynamic high order allocations in the
58	  kernel)
60	- this initial support only offers the feature in the anonymous memory
61	  regions but it'd be ideal to move it to tmpfs and the pagecache
62	  later
64	Transparent Hugepage Support maximizes the usefulness of free memory
65	if compared to the reservation approach of hugetlbfs by allowing all
66	unused memory to be used as cache or other movable (or even unmovable
67	entities). It doesn't require reservation to prevent hugepage
68	allocation failures to be noticeable from userland. It allows paging
69	and all other advanced VM features to be available on the
70	hugepages. It requires no modifications for applications to take
71	advantage of it.
73	Applications however can be further optimized to take advantage of
74	this feature, like for example they've been optimized before to avoid
75	a flood of mmap system calls for every malloc(4k). Optimizing userland
76	is by far not mandatory and khugepaged already can take care of long
77	lived page allocations even for hugepage unaware applications that
78	deals with large amounts of memory.
80	In certain cases when hugepages are enabled system wide, application
81	may end up allocating more memory resources. An application may mmap a
82	large region but only touch 1 byte of it, in that case a 2M page might
83	be allocated instead of a 4k page for no good. This is why it's
84	possible to disable hugepages system-wide and to only have them inside
85	MADV_HUGEPAGE madvise regions.
87	Embedded systems should enable hugepages only inside madvise regions
88	to eliminate any risk of wasting any precious byte of memory and to
89	only run faster.
91	Applications that gets a lot of benefit from hugepages and that don't
92	risk to lose memory by using hugepages, should use
93	madvise(MADV_HUGEPAGE) on their critical mmapped regions.
95	== sysfs ==
97	Transparent Hugepage Support can be entirely disabled (mostly for
98	debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to
99	avoid the risk of consuming more memory resources) or enabled system
100	wide. This can be achieved with one of:
102	echo always >/sys/kernel/mm/transparent_hugepage/enabled
103	echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
104	echo never >/sys/kernel/mm/transparent_hugepage/enabled
106	It's also possible to limit defrag efforts in the VM to generate
107	hugepages in case they're not immediately free to madvise regions or
108	to never try to defrag memory and simply fallback to regular pages
109	unless hugepages are immediately available. Clearly if we spend CPU
110	time to defrag memory, we would expect to gain even more by the fact
111	we use hugepages later instead of regular pages. This isn't always
112	guaranteed, but it may be more likely in case the allocation is for a
113	MADV_HUGEPAGE region.
115	echo always >/sys/kernel/mm/transparent_hugepage/defrag
116	echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
117	echo never >/sys/kernel/mm/transparent_hugepage/defrag
119	By default kernel tries to use huge zero page on read page fault.
120	It's possible to disable huge zero page by writing 0 or enable it
121	back by writing 1:
123	echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
124	echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
126	khugepaged will be automatically started when
127	transparent_hugepage/enabled is set to "always" or "madvise, and it'll
128	be automatically shutdown if it's set to "never".
130	khugepaged runs usually at low frequency so while one may not want to
131	invoke defrag algorithms synchronously during the page faults, it
132	should be worth invoking defrag at least in khugepaged. However it's
133	also possible to disable defrag in khugepaged by writing 0 or enable
134	defrag in khugepaged by writing 1:
136	echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
137	echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
139	You can also control how many pages khugepaged should scan at each
140	pass:
142	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
144	and how many milliseconds to wait in khugepaged between each pass (you
145	can set this to 0 to run khugepaged at 100% utilization of one core):
147	/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
149	and how many milliseconds to wait in khugepaged if there's an hugepage
150	allocation failure to throttle the next allocation attempt.
152	/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
154	The khugepaged progress can be seen in the number of pages collapsed:
156	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
158	for each pass:
160	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
162	max_ptes_none specifies how many extra small pages (that are
163	not already mapped) can be allocated when collapsing a group
164	of small pages into one large page.
166	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
168	A higher value leads to use additional memory for programs.
169	A lower value leads to gain less thp performance. Value of
170	max_ptes_none can waste cpu time very little, you can
171	ignore it.
173	== Boot parameter ==
175	You can change the sysfs boot time defaults of Transparent Hugepage
176	Support by passing the parameter "transparent_hugepage=always" or
177	"transparent_hugepage=madvise" or "transparent_hugepage=never"
178	(without "") to the kernel command line.
180	== Need of application restart ==
182	The transparent_hugepage/enabled values only affect future
183	behavior. So to make them effective you need to restart any
184	application that could have been using hugepages. This also applies to
185	the regions registered in khugepaged.
187	== Monitoring usage ==
189	The number of transparent huge pages currently used by the system is
190	available by reading the AnonHugePages field in /proc/meminfo. To
191	identify what applications are using transparent huge pages, it is
192	necessary to read /proc/PID/smaps and count the AnonHugePages fields
193	for each mapping. Note that reading the smaps file is expensive and
194	reading it frequently will incur overhead.
196	There are a number of counters in /proc/vmstat that may be used to
197	monitor how successfully the system is providing huge pages for use.
199	thp_fault_alloc is incremented every time a huge page is successfully
200		allocated to handle a page fault. This applies to both the
201		first time a page is faulted and for COW faults.
203	thp_collapse_alloc is incremented by khugepaged when it has found
204		a range of pages to collapse into one huge page and has
205		successfully allocated a new huge page to store the data.
207	thp_fault_fallback is incremented if a page fault fails to allocate
208		a huge page and instead falls back to using small pages.
210	thp_collapse_alloc_failed is incremented if khugepaged found a range
211		of pages that should be collapsed into one huge page but failed
212		the allocation.
214	thp_split is incremented every time a huge page is split into base
215		pages. This can happen for a variety of reasons but a common
216		reason is that a huge page is old and is being reclaimed.
218	thp_zero_page_alloc is incremented every time a huge zero page is
219		successfully allocated. It includes allocations which where
220		dropped due race with other allocation. Note, it doesn't count
221		every map of the huge zero page, only its allocation.
223	thp_zero_page_alloc_failed is incremented if kernel fails to allocate
224		huge zero page and falls back to using small pages.
226	As the system ages, allocating huge pages may be expensive as the
227	system uses memory compaction to copy data around memory to free a
228	huge page for use. There are some counters in /proc/vmstat to help
229	monitor this overhead.
231	compact_stall is incremented every time a process stalls to run
232		memory compaction so that a huge page is free for use.
234	compact_success is incremented if the system compacted memory and
235		freed a huge page for use.
237	compact_fail is incremented if the system tries to compact memory
238		but failed.
240	compact_pages_moved is incremented each time a page is moved. If
241		this value is increasing rapidly, it implies that the system
242		is copying a lot of data to satisfy the huge page allocation.
243		It is possible that the cost of copying exceeds any savings
244		from reduced TLB misses.
246	compact_pagemigrate_failed is incremented when the underlying mechanism
247		for moving a page failed.
249	compact_blocks_moved is incremented each time memory compaction examines
250		a huge page aligned range of pages.
252	It is possible to establish how long the stalls were using the function
253	tracer to record how long was spent in __alloc_pages_nodemask and
254	using the mm_page_alloc tracepoint to identify which allocations were
255	for huge pages.
257	== get_user_pages and follow_page ==
259	get_user_pages and follow_page if run on a hugepage, will return the
260	head or tail pages as usual (exactly as they would do on
261	hugetlbfs). Most gup users will only care about the actual physical
262	address of the page and its temporary pinning to release after the I/O
263	is complete, so they won't ever notice the fact the page is huge. But
264	if any driver is going to mangle over the page structure of the tail
265	page (like for checking page->mapping or other bits that are relevant
266	for the head page and not the tail page), it should be updated to jump
267	to check head page instead (while serializing properly against
268	split_huge_page() to avoid the head and tail pages to disappear from
269	under it, see the futex code to see an example of that, hugetlbfs also
270	needed special handling in futex code for similar reasons).
272	NOTE: these aren't new constraints to the GUP API, and they match the
273	same constrains that applies to hugetlbfs too, so any driver capable
274	of handling GUP on hugetlbfs will also work fine on transparent
275	hugepage backed mappings.
277	In case you can't handle compound pages if they're returned by
278	follow_page, the FOLL_SPLIT bit can be specified as parameter to
279	follow_page, so that it will split the hugepages before returning
280	them. Migration for example passes FOLL_SPLIT as parameter to
281	follow_page because it's not hugepage aware and in fact it can't work
282	at all on hugetlbfs (but it instead works fine on transparent
283	hugepages thanks to FOLL_SPLIT). migration simply can't deal with
284	hugepages being returned (as it's not only checking the pfn of the
285	page and pinning it during the copy but it pretends to migrate the
286	memory in regular page sizes and with regular pte/pmd mappings).
288	== Optimizing the applications ==
290	To be guaranteed that the kernel will map a 2M page immediately in any
291	memory region, the mmap region has to be hugepage naturally
292	aligned. posix_memalign() can provide that guarantee.
294	== Hugetlbfs ==
296	You can use hugetlbfs on a kernel that has transparent hugepage
297	support enabled just fine as always. No difference can be noted in
298	hugetlbfs other than there will be less overall fragmentation. All
299	usual features belonging to hugetlbfs are preserved and
300	unaffected. libhugetlbfs will also work fine as usual.
302	== Graceful fallback ==
304	Code walking pagetables but unware about huge pmds can simply call
305	split_huge_page_pmd(vma, addr, pmd) where the pmd is the one returned by
306	pmd_offset. It's trivial to make the code transparent hugepage aware
307	by just grepping for "pmd_offset" and adding split_huge_page_pmd where
308	missing after pmd_offset returns the pmd. Thanks to the graceful
309	fallback design, with a one liner change, you can avoid to write
310	hundred if not thousand of lines of complex code to make your code
311	hugepage aware.
313	If you're not walking pagetables but you run into a physical hugepage
314	but you can't handle it natively in your code, you can split it by
315	calling split_huge_page(page). This is what the Linux VM does before
316	it tries to swapout the hugepage for example.
318	Example to make mremap.c transparent hugepage aware with a one liner
319	change:
321	diff --git a/mm/mremap.c b/mm/mremap.c
322	--- a/mm/mremap.c
323	+++ b/mm/mremap.c
324	@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
325			return NULL;
327		pmd = pmd_offset(pud, addr);
328	+	split_huge_page_pmd(vma, addr, pmd);
329		if (pmd_none_or_clear_bad(pmd))
330			return NULL;
332	== Locking in hugepage aware code ==
334	We want as much code as possible hugepage aware, as calling
335	split_huge_page() or split_huge_page_pmd() has a cost.
337	To make pagetable walks huge pmd aware, all you need to do is to call
338	pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
339	mmap_sem in read (or write) mode to be sure an huge pmd cannot be
340	created from under you by khugepaged (khugepaged collapse_huge_page
341	takes the mmap_sem in write mode in addition to the anon_vma lock). If
342	pmd_trans_huge returns false, you just fallback in the old code
343	paths. If instead pmd_trans_huge returns true, you have to take the
344	mm->page_table_lock and re-run pmd_trans_huge. Taking the
345	page_table_lock will prevent the huge pmd to be converted into a
346	regular pmd from under you (split_huge_page can run in parallel to the
347	pagetable walk). If the second pmd_trans_huge returns false, you
348	should just drop the page_table_lock and fallback to the old code as
349	before. Otherwise you should run pmd_trans_splitting on the pmd. In
350	case pmd_trans_splitting returns true, it means split_huge_page is
351	already in the middle of splitting the page. So if pmd_trans_splitting
352	returns true it's enough to drop the page_table_lock and call
353	wait_split_huge_page and then fallback the old code paths. You are
354	guaranteed by the time wait_split_huge_page returns, the pmd isn't
355	huge anymore. If pmd_trans_splitting returns false, you can proceed to
356	process the huge pmd and the hugepage natively. Once finished you can
357	drop the page_table_lock.
359	== compound_lock, get_user_pages and put_page ==
361	split_huge_page internally has to distribute the refcounts in the head
362	page to the tail pages before clearing all PG_head/tail bits from the
363	page structures. It can do that easily for refcounts taken by huge pmd
364	mappings. But the GUI API as created by hugetlbfs (that returns head
365	and tail pages if running get_user_pages on an address backed by any
366	hugepage), requires the refcount to be accounted on the tail pages and
367	not only in the head pages, if we want to be able to run
368	split_huge_page while there are gup pins established on any tail
369	page. Failure to be able to run split_huge_page if there's any gup pin
370	on any tail page, would mean having to split all hugepages upfront in
371	get_user_pages which is unacceptable as too many gup users are
372	performance critical and they must work natively on hugepages like
373	they work natively on hugetlbfs already (hugetlbfs is simpler because
374	hugetlbfs pages cannot be split so there wouldn't be requirement of
375	accounting the pins on the tail pages for hugetlbfs). If we wouldn't
376	account the gup refcounts on the tail pages during gup, we won't know
377	anymore which tail page is pinned by gup and which is not while we run
378	split_huge_page. But we still have to add the gup pin to the head page
379	too, to know when we can free the compound page in case it's never
380	split during its lifetime. That requires changing not just
381	get_page, but put_page as well so that when put_page runs on a tail
382	page (and only on a tail page) it will find its respective head page,
383	and then it will decrease the head page refcount in addition to the
384	tail page refcount. To obtain a head page reliably and to decrease its
385	refcount without race conditions, put_page has to serialize against
386	__split_huge_page_refcount using a special per-page lock called
387	compound_lock.
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