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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 EST.

1	= Transparent Hugepage Support =
2	
3	== Objective ==
4	
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.
11	
12	Currently it only works for anonymous memory mappings but in the
13	future it can expand over the pagecache layer starting with tmpfs.
14	
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.
35	
36	== Design ==
37	
38	- "graceful fallback": mm components which don't have transparent hugepage
39	  knowledge fall back to breaking huge pmd mapping into table of ptes and,
40	  if necessary, split a transparent hugepage. Therefore these components
41	  can continue working on the regular pages or regular pte mappings.
42	
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
47	
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)
52	
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)
59	
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
63	
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.
72	
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.
79	
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.
86	
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.
90	
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.
94	
95	== sysfs ==
96	
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:
101	
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
105	
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.
114	
115	echo always >/sys/kernel/mm/transparent_hugepage/defrag
116	echo defer >/sys/kernel/mm/transparent_hugepage/defrag
117	echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
118	echo never >/sys/kernel/mm/transparent_hugepage/defrag
119	
120	"always" means that an application requesting THP will stall on allocation
121	failure and directly reclaim pages and compact memory in an effort to
122	allocate a THP immediately. This may be desirable for virtual machines
123	that benefit heavily from THP use and are willing to delay the VM start
124	to utilise them.
125	
126	"defer" means that an application will wake kswapd in the background
127	to reclaim pages and wake kcompact to compact memory so that THP is
128	available in the near future. It's the responsibility of khugepaged
129	to then install the THP pages later.
130	
131	"madvise" will enter direct reclaim like "always" but only for regions
132	that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
133	
134	"never" should be self-explanatory.
135	
136	By default kernel tries to use huge zero page on read page fault.
137	It's possible to disable huge zero page by writing 0 or enable it
138	back by writing 1:
139	
140	echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
141	echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
142	
143	khugepaged will be automatically started when
144	transparent_hugepage/enabled is set to "always" or "madvise, and it'll
145	be automatically shutdown if it's set to "never".
146	
147	khugepaged runs usually at low frequency so while one may not want to
148	invoke defrag algorithms synchronously during the page faults, it
149	should be worth invoking defrag at least in khugepaged. However it's
150	also possible to disable defrag in khugepaged by writing 0 or enable
151	defrag in khugepaged by writing 1:
152	
153	echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
154	echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
155	
156	You can also control how many pages khugepaged should scan at each
157	pass:
158	
159	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
160	
161	and how many milliseconds to wait in khugepaged between each pass (you
162	can set this to 0 to run khugepaged at 100% utilization of one core):
163	
164	/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
165	
166	and how many milliseconds to wait in khugepaged if there's an hugepage
167	allocation failure to throttle the next allocation attempt.
168	
169	/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
170	
171	The khugepaged progress can be seen in the number of pages collapsed:
172	
173	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
174	
175	for each pass:
176	
177	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
178	
179	max_ptes_none specifies how many extra small pages (that are
180	not already mapped) can be allocated when collapsing a group
181	of small pages into one large page.
182	
183	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
184	
185	A higher value leads to use additional memory for programs.
186	A lower value leads to gain less thp performance. Value of
187	max_ptes_none can waste cpu time very little, you can
188	ignore it.
189	
190	max_ptes_swap specifies how many pages can be brought in from
191	swap when collapsing a group of pages into a transparent huge page.
192	
193	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
194	
195	A higher value can cause excessive swap IO and waste
196	memory. A lower value can prevent THPs from being
197	collapsed, resulting fewer pages being collapsed into
198	THPs, and lower memory access performance.
199	
200	== Boot parameter ==
201	
202	You can change the sysfs boot time defaults of Transparent Hugepage
203	Support by passing the parameter "transparent_hugepage=always" or
204	"transparent_hugepage=madvise" or "transparent_hugepage=never"
205	(without "") to the kernel command line.
206	
207	== Need of application restart ==
208	
209	The transparent_hugepage/enabled values only affect future
210	behavior. So to make them effective you need to restart any
211	application that could have been using hugepages. This also applies to
212	the regions registered in khugepaged.
213	
214	== Monitoring usage ==
215	
216	The number of transparent huge pages currently used by the system is
217	available by reading the AnonHugePages field in /proc/meminfo. To
218	identify what applications are using transparent huge pages, it is
219	necessary to read /proc/PID/smaps and count the AnonHugePages fields
220	for each mapping. Note that reading the smaps file is expensive and
221	reading it frequently will incur overhead.
222	
223	There are a number of counters in /proc/vmstat that may be used to
224	monitor how successfully the system is providing huge pages for use.
225	
226	thp_fault_alloc is incremented every time a huge page is successfully
227		allocated to handle a page fault. This applies to both the
228		first time a page is faulted and for COW faults.
229	
230	thp_collapse_alloc is incremented by khugepaged when it has found
231		a range of pages to collapse into one huge page and has
232		successfully allocated a new huge page to store the data.
233	
234	thp_fault_fallback is incremented if a page fault fails to allocate
235		a huge page and instead falls back to using small pages.
236	
237	thp_collapse_alloc_failed is incremented if khugepaged found a range
238		of pages that should be collapsed into one huge page but failed
239		the allocation.
240	
241	thp_split_page is incremented every time a huge page is split into base
242		pages. This can happen for a variety of reasons but a common
243		reason is that a huge page is old and is being reclaimed.
244		This action implies splitting all PMD the page mapped with.
245	
246	thp_split_page_failed is is incremented if kernel fails to split huge
247		page. This can happen if the page was pinned by somebody.
248	
249	thp_deferred_split_page is incremented when a huge page is put onto split
250		queue. This happens when a huge page is partially unmapped and
251		splitting it would free up some memory. Pages on split queue are
252		going to be split under memory pressure.
253	
254	thp_split_pmd is incremented every time a PMD split into table of PTEs.
255		This can happen, for instance, when application calls mprotect() or
256		munmap() on part of huge page. It doesn't split huge page, only
257		page table entry.
258	
259	thp_zero_page_alloc is incremented every time a huge zero page is
260		successfully allocated. It includes allocations which where
261		dropped due race with other allocation. Note, it doesn't count
262		every map of the huge zero page, only its allocation.
263	
264	thp_zero_page_alloc_failed is incremented if kernel fails to allocate
265		huge zero page and falls back to using small pages.
266	
267	As the system ages, allocating huge pages may be expensive as the
268	system uses memory compaction to copy data around memory to free a
269	huge page for use. There are some counters in /proc/vmstat to help
270	monitor this overhead.
271	
272	compact_stall is incremented every time a process stalls to run
273		memory compaction so that a huge page is free for use.
274	
275	compact_success is incremented if the system compacted memory and
276		freed a huge page for use.
277	
278	compact_fail is incremented if the system tries to compact memory
279		but failed.
280	
281	compact_pages_moved is incremented each time a page is moved. If
282		this value is increasing rapidly, it implies that the system
283		is copying a lot of data to satisfy the huge page allocation.
284		It is possible that the cost of copying exceeds any savings
285		from reduced TLB misses.
286	
287	compact_pagemigrate_failed is incremented when the underlying mechanism
288		for moving a page failed.
289	
290	compact_blocks_moved is incremented each time memory compaction examines
291		a huge page aligned range of pages.
292	
293	It is possible to establish how long the stalls were using the function
294	tracer to record how long was spent in __alloc_pages_nodemask and
295	using the mm_page_alloc tracepoint to identify which allocations were
296	for huge pages.
297	
298	== get_user_pages and follow_page ==
299	
300	get_user_pages and follow_page if run on a hugepage, will return the
301	head or tail pages as usual (exactly as they would do on
302	hugetlbfs). Most gup users will only care about the actual physical
303	address of the page and its temporary pinning to release after the I/O
304	is complete, so they won't ever notice the fact the page is huge. But
305	if any driver is going to mangle over the page structure of the tail
306	page (like for checking page->mapping or other bits that are relevant
307	for the head page and not the tail page), it should be updated to jump
308	to check head page instead. Taking reference on any head/tail page would
309	prevent page from being split by anyone.
310	
311	NOTE: these aren't new constraints to the GUP API, and they match the
312	same constrains that applies to hugetlbfs too, so any driver capable
313	of handling GUP on hugetlbfs will also work fine on transparent
314	hugepage backed mappings.
315	
316	In case you can't handle compound pages if they're returned by
317	follow_page, the FOLL_SPLIT bit can be specified as parameter to
318	follow_page, so that it will split the hugepages before returning
319	them. Migration for example passes FOLL_SPLIT as parameter to
320	follow_page because it's not hugepage aware and in fact it can't work
321	at all on hugetlbfs (but it instead works fine on transparent
322	hugepages thanks to FOLL_SPLIT). migration simply can't deal with
323	hugepages being returned (as it's not only checking the pfn of the
324	page and pinning it during the copy but it pretends to migrate the
325	memory in regular page sizes and with regular pte/pmd mappings).
326	
327	== Optimizing the applications ==
328	
329	To be guaranteed that the kernel will map a 2M page immediately in any
330	memory region, the mmap region has to be hugepage naturally
331	aligned. posix_memalign() can provide that guarantee.
332	
333	== Hugetlbfs ==
334	
335	You can use hugetlbfs on a kernel that has transparent hugepage
336	support enabled just fine as always. No difference can be noted in
337	hugetlbfs other than there will be less overall fragmentation. All
338	usual features belonging to hugetlbfs are preserved and
339	unaffected. libhugetlbfs will also work fine as usual.
340	
341	== Graceful fallback ==
342	
343	Code walking pagetables but unaware about huge pmds can simply call
344	split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
345	pmd_offset. It's trivial to make the code transparent hugepage aware
346	by just grepping for "pmd_offset" and adding split_huge_pmd where
347	missing after pmd_offset returns the pmd. Thanks to the graceful
348	fallback design, with a one liner change, you can avoid to write
349	hundred if not thousand of lines of complex code to make your code
350	hugepage aware.
351	
352	If you're not walking pagetables but you run into a physical hugepage
353	but you can't handle it natively in your code, you can split it by
354	calling split_huge_page(page). This is what the Linux VM does before
355	it tries to swapout the hugepage for example. split_huge_page() can fail
356	if the page is pinned and you must handle this correctly.
357	
358	Example to make mremap.c transparent hugepage aware with a one liner
359	change:
360	
361	diff --git a/mm/mremap.c b/mm/mremap.c
362	--- a/mm/mremap.c
363	+++ b/mm/mremap.c
364	@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
365			return NULL;
366	
367		pmd = pmd_offset(pud, addr);
368	+	split_huge_pmd(vma, pmd, addr);
369		if (pmd_none_or_clear_bad(pmd))
370			return NULL;
371	
372	== Locking in hugepage aware code ==
373	
374	We want as much code as possible hugepage aware, as calling
375	split_huge_page() or split_huge_pmd() has a cost.
376	
377	To make pagetable walks huge pmd aware, all you need to do is to call
378	pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
379	mmap_sem in read (or write) mode to be sure an huge pmd cannot be
380	created from under you by khugepaged (khugepaged collapse_huge_page
381	takes the mmap_sem in write mode in addition to the anon_vma lock). If
382	pmd_trans_huge returns false, you just fallback in the old code
383	paths. If instead pmd_trans_huge returns true, you have to take the
384	page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
385	page table lock will prevent the huge pmd to be converted into a
386	regular pmd from under you (split_huge_pmd can run in parallel to the
387	pagetable walk). If the second pmd_trans_huge returns false, you
388	should just drop the page table lock and fallback to the old code as
389	before. Otherwise you can proceed to process the huge pmd and the
390	hugepage natively. Once finished you can drop the page table lock.
391	
392	== Refcounts and transparent huge pages ==
393	
394	Refcounting on THP is mostly consistent with refcounting on other compound
395	pages:
396	
397	  - get_page()/put_page() and GUP operate in head page's ->_refcount.
398	
399	  - ->_refcount in tail pages is always zero: get_page_unless_zero() never
400	    succeed on tail pages.
401	
402	  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
403	    on relevant sub-page of the compound page.
404	
405	  - map/unmap of the whole compound page accounted in compound_mapcount
406	    (stored in first tail page).
407	
408	PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one.
409	This additional reference is required to get race-free detection of unmap of
410	subpages when we have them mapped with both PMDs and PTEs.
411	
412	This is optimization required to lower overhead of per-subpage mapcount
413	tracking. The alternative is alter ->_mapcount in all subpages on each
414	map/unmap of the whole compound page.
415	
416	We set PG_double_map when a PMD of the page got split for the first time,
417	but still have PMD mapping. The additional references go away with last
418	compound_mapcount.
419	
420	split_huge_page internally has to distribute the refcounts in the head
421	page to the tail pages before clearing all PG_head/tail bits from the page
422	structures. It can be done easily for refcounts taken by page table
423	entries. But we don't have enough information on how to distribute any
424	additional pins (i.e. from get_user_pages). split_huge_page() fails any
425	requests to split pinned huge page: it expects page count to be equal to
426	sum of mapcount of all sub-pages plus one (split_huge_page caller must
427	have reference for head page).
428	
429	split_huge_page uses migration entries to stabilize page->_refcount and
430	page->_mapcount.
431	
432	We safe against physical memory scanners too: the only legitimate way
433	scanner can get reference to a page is get_page_unless_zero().
434	
435	All tail pages have zero ->_refcount until atomic_add(). This prevents the
436	scanner from getting a reference to the tail page up to that point. After the
437	atomic_add() we don't care about the ->_refcount value.  We already known how
438	many references should be uncharged from the head page.
439	
440	For head page get_page_unless_zero() will succeed and we don't mind. It's
441	clear where reference should go after split: it will stay on head page.
442	
443	Note that split_huge_pmd() doesn't have any limitation on refcounting:
444	pmd can be split at any point and never fails.
445	
446	== Partial unmap and deferred_split_huge_page() ==
447	
448	Unmapping part of THP (with munmap() or other way) is not going to free
449	memory immediately. Instead, we detect that a subpage of THP is not in use
450	in page_remove_rmap() and queue the THP for splitting if memory pressure
451	comes. Splitting will free up unused subpages.
452	
453	Splitting the page right away is not an option due to locking context in
454	the place where we can detect partial unmap. It's also might be
455	counterproductive since in many cases partial unmap unmap happens during
456	exit(2) if an THP crosses VMA boundary.
457	
458	Function deferred_split_huge_page() is used to queue page for splitting.
459	The splitting itself will happen when we get memory pressure via shrinker
460	interface.
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