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Based on kernel version 4.8. Page generated on 2016-10-06 23:19 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 and tmpfs/shmem.
13	But in the future it can expand to other filesystems.
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	Transparent Hugepage Support maximizes the usefulness of free memory
61	if compared to the reservation approach of hugetlbfs by allowing all
62	unused memory to be used as cache or other movable (or even unmovable
63	entities). It doesn't require reservation to prevent hugepage
64	allocation failures to be noticeable from userland. It allows paging
65	and all other advanced VM features to be available on the
66	hugepages. It requires no modifications for applications to take
67	advantage of it.
68	
69	Applications however can be further optimized to take advantage of
70	this feature, like for example they've been optimized before to avoid
71	a flood of mmap system calls for every malloc(4k). Optimizing userland
72	is by far not mandatory and khugepaged already can take care of long
73	lived page allocations even for hugepage unaware applications that
74	deals with large amounts of memory.
75	
76	In certain cases when hugepages are enabled system wide, application
77	may end up allocating more memory resources. An application may mmap a
78	large region but only touch 1 byte of it, in that case a 2M page might
79	be allocated instead of a 4k page for no good. This is why it's
80	possible to disable hugepages system-wide and to only have them inside
81	MADV_HUGEPAGE madvise regions.
82	
83	Embedded systems should enable hugepages only inside madvise regions
84	to eliminate any risk of wasting any precious byte of memory and to
85	only run faster.
86	
87	Applications that gets a lot of benefit from hugepages and that don't
88	risk to lose memory by using hugepages, should use
89	madvise(MADV_HUGEPAGE) on their critical mmapped regions.
90	
91	== sysfs ==
92	
93	Transparent Hugepage Support for anonymous memory can be entirely disabled
94	(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
95	regions (to avoid the risk of consuming more memory resources) or enabled
96	system wide. This can be achieved with one of:
97	
98	echo always >/sys/kernel/mm/transparent_hugepage/enabled
99	echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
100	echo never >/sys/kernel/mm/transparent_hugepage/enabled
101	
102	It's also possible to limit defrag efforts in the VM to generate
103	anonymous hugepages in case they're not immediately free to madvise
104	regions or to never try to defrag memory and simply fallback to regular
105	pages unless hugepages are immediately available. Clearly if we spend CPU
106	time to defrag memory, we would expect to gain even more by the fact we
107	use hugepages later instead of regular pages. This isn't always
108	guaranteed, but it may be more likely in case the allocation is for a
109	MADV_HUGEPAGE region.
110	
111	echo always >/sys/kernel/mm/transparent_hugepage/defrag
112	echo defer >/sys/kernel/mm/transparent_hugepage/defrag
113	echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
114	echo never >/sys/kernel/mm/transparent_hugepage/defrag
115	
116	"always" means that an application requesting THP will stall on allocation
117	failure and directly reclaim pages and compact memory in an effort to
118	allocate a THP immediately. This may be desirable for virtual machines
119	that benefit heavily from THP use and are willing to delay the VM start
120	to utilise them.
121	
122	"defer" means that an application will wake kswapd in the background
123	to reclaim pages and wake kcompact to compact memory so that THP is
124	available in the near future. It's the responsibility of khugepaged
125	to then install the THP pages later.
126	
127	"madvise" will enter direct reclaim like "always" but only for regions
128	that are have used madvise(MADV_HUGEPAGE). This is the default behaviour.
129	
130	"never" should be self-explanatory.
131	
132	By default kernel tries to use huge zero page on read page fault to
133	anonymous mapping. It's possible to disable huge zero page by writing 0
134	or enable it back by writing 1:
135	
136	echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
137	echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
138	
139	khugepaged will be automatically started when
140	transparent_hugepage/enabled is set to "always" or "madvise, and it'll
141	be automatically shutdown if it's set to "never".
142	
143	khugepaged runs usually at low frequency so while one may not want to
144	invoke defrag algorithms synchronously during the page faults, it
145	should be worth invoking defrag at least in khugepaged. However it's
146	also possible to disable defrag in khugepaged by writing 0 or enable
147	defrag in khugepaged by writing 1:
148	
149	echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
150	echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
151	
152	You can also control how many pages khugepaged should scan at each
153	pass:
154	
155	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
156	
157	and how many milliseconds to wait in khugepaged between each pass (you
158	can set this to 0 to run khugepaged at 100% utilization of one core):
159	
160	/sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
161	
162	and how many milliseconds to wait in khugepaged if there's an hugepage
163	allocation failure to throttle the next allocation attempt.
164	
165	/sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
166	
167	The khugepaged progress can be seen in the number of pages collapsed:
168	
169	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
170	
171	for each pass:
172	
173	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
174	
175	max_ptes_none specifies how many extra small pages (that are
176	not already mapped) can be allocated when collapsing a group
177	of small pages into one large page.
178	
179	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
180	
181	A higher value leads to use additional memory for programs.
182	A lower value leads to gain less thp performance. Value of
183	max_ptes_none can waste cpu time very little, you can
184	ignore it.
185	
186	max_ptes_swap specifies how many pages can be brought in from
187	swap when collapsing a group of pages into a transparent huge page.
188	
189	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
190	
191	A higher value can cause excessive swap IO and waste
192	memory. A lower value can prevent THPs from being
193	collapsed, resulting fewer pages being collapsed into
194	THPs, and lower memory access performance.
195	
196	== Boot parameter ==
197	
198	You can change the sysfs boot time defaults of Transparent Hugepage
199	Support by passing the parameter "transparent_hugepage=always" or
200	"transparent_hugepage=madvise" or "transparent_hugepage=never"
201	(without "") to the kernel command line.
202	
203	== Hugepages in tmpfs/shmem ==
204	
205	You can control hugepage allocation policy in tmpfs with mount option
206	"huge=". It can have following values:
207	
208	  - "always":
209	    Attempt to allocate huge pages every time we need a new page;
210	
211	  - "never":
212	    Do not allocate huge pages;
213	
214	  - "within_size":
215	    Only allocate huge page if it will be fully within i_size.
216	    Also respect fadvise()/madvise() hints;
217	
218	  - "advise:
219	    Only allocate huge pages if requested with fadvise()/madvise();
220	
221	The default policy is "never".
222	
223	"mount -o remount,huge= /mountpoint" works fine after mount: remounting
224	huge=never will not attempt to break up huge pages at all, just stop more
225	from being allocated.
226	
227	There's also sysfs knob to control hugepage allocation policy for internal
228	shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
229	is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
230	MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
231	
232	In addition to policies listed above, shmem_enabled allows two further
233	values:
234	
235	  - "deny":
236	    For use in emergencies, to force the huge option off from
237	    all mounts;
238	  - "force":
239	    Force the huge option on for all - very useful for testing;
240	
241	== Need of application restart ==
242	
243	The transparent_hugepage/enabled values and tmpfs mount option only affect
244	future behavior. So to make them effective you need to restart any
245	application that could have been using hugepages. This also applies to the
246	regions registered in khugepaged.
247	
248	== Monitoring usage ==
249	
250	The number of anonymous transparent huge pages currently used by the
251	system is available by reading the AnonHugePages field in /proc/meminfo.
252	To identify what applications are using anonymous transparent huge pages,
253	it is necessary to read /proc/PID/smaps and count the AnonHugePages fields
254	for each mapping.
255	
256	The number of file transparent huge pages mapped to userspace is available
257	by reading ShmemPmdMapped and ShmemHugePages fields in /proc/meminfo.
258	To identify what applications are mapping file  transparent huge pages, it
259	is necessary to read /proc/PID/smaps and count the FileHugeMapped fields
260	for each mapping.
261	
262	Note that reading the smaps file is expensive and reading it
263	frequently will incur overhead.
264	
265	There are a number of counters in /proc/vmstat that may be used to
266	monitor how successfully the system is providing huge pages for use.
267	
268	thp_fault_alloc is incremented every time a huge page is successfully
269		allocated to handle a page fault. This applies to both the
270		first time a page is faulted and for COW faults.
271	
272	thp_collapse_alloc is incremented by khugepaged when it has found
273		a range of pages to collapse into one huge page and has
274		successfully allocated a new huge page to store the data.
275	
276	thp_fault_fallback is incremented if a page fault fails to allocate
277		a huge page and instead falls back to using small pages.
278	
279	thp_collapse_alloc_failed is incremented if khugepaged found a range
280		of pages that should be collapsed into one huge page but failed
281		the allocation.
282	
283	thp_file_alloc is incremented every time a file huge page is successfully
284	i	allocated.
285	
286	thp_file_mapped is incremented every time a file huge page is mapped into
287		user address space.
288	
289	thp_split_page is incremented every time a huge page is split into base
290		pages. This can happen for a variety of reasons but a common
291		reason is that a huge page is old and is being reclaimed.
292		This action implies splitting all PMD the page mapped with.
293	
294	thp_split_page_failed is is incremented if kernel fails to split huge
295		page. This can happen if the page was pinned by somebody.
296	
297	thp_deferred_split_page is incremented when a huge page is put onto split
298		queue. This happens when a huge page is partially unmapped and
299		splitting it would free up some memory. Pages on split queue are
300		going to be split under memory pressure.
301	
302	thp_split_pmd is incremented every time a PMD split into table of PTEs.
303		This can happen, for instance, when application calls mprotect() or
304		munmap() on part of huge page. It doesn't split huge page, only
305		page table entry.
306	
307	thp_zero_page_alloc is incremented every time a huge zero page is
308		successfully allocated. It includes allocations which where
309		dropped due race with other allocation. Note, it doesn't count
310		every map of the huge zero page, only its allocation.
311	
312	thp_zero_page_alloc_failed is incremented if kernel fails to allocate
313		huge zero page and falls back to using small pages.
314	
315	As the system ages, allocating huge pages may be expensive as the
316	system uses memory compaction to copy data around memory to free a
317	huge page for use. There are some counters in /proc/vmstat to help
318	monitor this overhead.
319	
320	compact_stall is incremented every time a process stalls to run
321		memory compaction so that a huge page is free for use.
322	
323	compact_success is incremented if the system compacted memory and
324		freed a huge page for use.
325	
326	compact_fail is incremented if the system tries to compact memory
327		but failed.
328	
329	compact_pages_moved is incremented each time a page is moved. If
330		this value is increasing rapidly, it implies that the system
331		is copying a lot of data to satisfy the huge page allocation.
332		It is possible that the cost of copying exceeds any savings
333		from reduced TLB misses.
334	
335	compact_pagemigrate_failed is incremented when the underlying mechanism
336		for moving a page failed.
337	
338	compact_blocks_moved is incremented each time memory compaction examines
339		a huge page aligned range of pages.
340	
341	It is possible to establish how long the stalls were using the function
342	tracer to record how long was spent in __alloc_pages_nodemask and
343	using the mm_page_alloc tracepoint to identify which allocations were
344	for huge pages.
345	
346	== get_user_pages and follow_page ==
347	
348	get_user_pages and follow_page if run on a hugepage, will return the
349	head or tail pages as usual (exactly as they would do on
350	hugetlbfs). Most gup users will only care about the actual physical
351	address of the page and its temporary pinning to release after the I/O
352	is complete, so they won't ever notice the fact the page is huge. But
353	if any driver is going to mangle over the page structure of the tail
354	page (like for checking page->mapping or other bits that are relevant
355	for the head page and not the tail page), it should be updated to jump
356	to check head page instead. Taking reference on any head/tail page would
357	prevent page from being split by anyone.
358	
359	NOTE: these aren't new constraints to the GUP API, and they match the
360	same constrains that applies to hugetlbfs too, so any driver capable
361	of handling GUP on hugetlbfs will also work fine on transparent
362	hugepage backed mappings.
363	
364	In case you can't handle compound pages if they're returned by
365	follow_page, the FOLL_SPLIT bit can be specified as parameter to
366	follow_page, so that it will split the hugepages before returning
367	them. Migration for example passes FOLL_SPLIT as parameter to
368	follow_page because it's not hugepage aware and in fact it can't work
369	at all on hugetlbfs (but it instead works fine on transparent
370	hugepages thanks to FOLL_SPLIT). migration simply can't deal with
371	hugepages being returned (as it's not only checking the pfn of the
372	page and pinning it during the copy but it pretends to migrate the
373	memory in regular page sizes and with regular pte/pmd mappings).
374	
375	== Optimizing the applications ==
376	
377	To be guaranteed that the kernel will map a 2M page immediately in any
378	memory region, the mmap region has to be hugepage naturally
379	aligned. posix_memalign() can provide that guarantee.
380	
381	== Hugetlbfs ==
382	
383	You can use hugetlbfs on a kernel that has transparent hugepage
384	support enabled just fine as always. No difference can be noted in
385	hugetlbfs other than there will be less overall fragmentation. All
386	usual features belonging to hugetlbfs are preserved and
387	unaffected. libhugetlbfs will also work fine as usual.
388	
389	== Graceful fallback ==
390	
391	Code walking pagetables but unaware about huge pmds can simply call
392	split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by
393	pmd_offset. It's trivial to make the code transparent hugepage aware
394	by just grepping for "pmd_offset" and adding split_huge_pmd where
395	missing after pmd_offset returns the pmd. Thanks to the graceful
396	fallback design, with a one liner change, you can avoid to write
397	hundred if not thousand of lines of complex code to make your code
398	hugepage aware.
399	
400	If you're not walking pagetables but you run into a physical hugepage
401	but you can't handle it natively in your code, you can split it by
402	calling split_huge_page(page). This is what the Linux VM does before
403	it tries to swapout the hugepage for example. split_huge_page() can fail
404	if the page is pinned and you must handle this correctly.
405	
406	Example to make mremap.c transparent hugepage aware with a one liner
407	change:
408	
409	diff --git a/mm/mremap.c b/mm/mremap.c
410	--- a/mm/mremap.c
411	+++ b/mm/mremap.c
412	@@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru
413			return NULL;
414	
415		pmd = pmd_offset(pud, addr);
416	+	split_huge_pmd(vma, pmd, addr);
417		if (pmd_none_or_clear_bad(pmd))
418			return NULL;
419	
420	== Locking in hugepage aware code ==
421	
422	We want as much code as possible hugepage aware, as calling
423	split_huge_page() or split_huge_pmd() has a cost.
424	
425	To make pagetable walks huge pmd aware, all you need to do is to call
426	pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the
427	mmap_sem in read (or write) mode to be sure an huge pmd cannot be
428	created from under you by khugepaged (khugepaged collapse_huge_page
429	takes the mmap_sem in write mode in addition to the anon_vma lock). If
430	pmd_trans_huge returns false, you just fallback in the old code
431	paths. If instead pmd_trans_huge returns true, you have to take the
432	page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the
433	page table lock will prevent the huge pmd to be converted into a
434	regular pmd from under you (split_huge_pmd can run in parallel to the
435	pagetable walk). If the second pmd_trans_huge returns false, you
436	should just drop the page table lock and fallback to the old code as
437	before. Otherwise you can proceed to process the huge pmd and the
438	hugepage natively. Once finished you can drop the page table lock.
439	
440	== Refcounts and transparent huge pages ==
441	
442	Refcounting on THP is mostly consistent with refcounting on other compound
443	pages:
444	
445	  - get_page()/put_page() and GUP operate in head page's ->_refcount.
446	
447	  - ->_refcount in tail pages is always zero: get_page_unless_zero() never
448	    succeed on tail pages.
449	
450	  - map/unmap of the pages with PTE entry increment/decrement ->_mapcount
451	    on relevant sub-page of the compound page.
452	
453	  - map/unmap of the whole compound page accounted in compound_mapcount
454	    (stored in first tail page). For file huge pages, we also increment
455	    ->_mapcount of all sub-pages in order to have race-free detection of
456	    last unmap of subpages.
457	
458	PageDoubleMap() indicates that the page is *possibly* mapped with PTEs.
459	
460	For anonymous pages PageDoubleMap() also indicates ->_mapcount in all
461	subpages is offset up by one. This additional reference is required to
462	get race-free detection of unmap of subpages when we have them mapped with
463	both PMDs and PTEs.
464	
465	This is optimization required to lower overhead of per-subpage mapcount
466	tracking. The alternative is alter ->_mapcount in all subpages on each
467	map/unmap of the whole compound page.
468	
469	For anonymous pages, we set PG_double_map when a PMD of the page got split
470	for the first time, but still have PMD mapping. The additional references
471	go away with last compound_mapcount.
472	
473	File pages get PG_double_map set on first map of the page with PTE and
474	goes away when the page gets evicted from page cache.
475	
476	split_huge_page internally has to distribute the refcounts in the head
477	page to the tail pages before clearing all PG_head/tail bits from the page
478	structures. It can be done easily for refcounts taken by page table
479	entries. But we don't have enough information on how to distribute any
480	additional pins (i.e. from get_user_pages). split_huge_page() fails any
481	requests to split pinned huge page: it expects page count to be equal to
482	sum of mapcount of all sub-pages plus one (split_huge_page caller must
483	have reference for head page).
484	
485	split_huge_page uses migration entries to stabilize page->_refcount and
486	page->_mapcount of anonymous pages. File pages just got unmapped.
487	
488	We safe against physical memory scanners too: the only legitimate way
489	scanner can get reference to a page is get_page_unless_zero().
490	
491	All tail pages have zero ->_refcount until atomic_add(). This prevents the
492	scanner from getting a reference to the tail page up to that point. After the
493	atomic_add() we don't care about the ->_refcount value.  We already known how
494	many references should be uncharged from the head page.
495	
496	For head page get_page_unless_zero() will succeed and we don't mind. It's
497	clear where reference should go after split: it will stay on head page.
498	
499	Note that split_huge_pmd() doesn't have any limitation on refcounting:
500	pmd can be split at any point and never fails.
501	
502	== Partial unmap and deferred_split_huge_page() ==
503	
504	Unmapping part of THP (with munmap() or other way) is not going to free
505	memory immediately. Instead, we detect that a subpage of THP is not in use
506	in page_remove_rmap() and queue the THP for splitting if memory pressure
507	comes. Splitting will free up unused subpages.
508	
509	Splitting the page right away is not an option due to locking context in
510	the place where we can detect partial unmap. It's also might be
511	counterproductive since in many cases partial unmap unmap happens during
512	exit(2) if an THP crosses VMA boundary.
513	
514	Function deferred_split_huge_page() is used to queue page for splitting.
515	The splitting itself will happen when we get memory pressure via shrinker
516	interface.
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