1 ============================ 1 ============================
2 Transparent Hugepage Support 2 Transparent Hugepage Support
3 ============================ 3 ============================
4 4
5 Objective 5 Objective
6 ========= 6 =========
7 7
8 Performance critical computing applications de 8 Performance critical computing applications dealing with large memory
9 working sets are already running on top of lib 9 working sets are already running on top of libhugetlbfs and in turn
10 hugetlbfs. Transparent HugePage Support (THP) 10 hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of
11 using huge pages for the backing of virtual me 11 using huge pages for the backing of virtual memory with huge pages
12 that supports the automatic promotion and demo 12 that supports the automatic promotion and demotion of page sizes and
13 without the shortcomings of hugetlbfs. 13 without the shortcomings of hugetlbfs.
14 14
15 Currently THP only works for anonymous memory 15 Currently THP only works for anonymous memory mappings and tmpfs/shmem.
16 But in the future it can expand to other files 16 But in the future it can expand to other filesystems.
17 17
18 .. note:: 18 .. note::
19 in the examples below we presume that the b 19 in the examples below we presume that the basic page size is 4K and
20 the huge page size is 2M, although the actu 20 the huge page size is 2M, although the actual numbers may vary
21 depending on the CPU architecture. 21 depending on the CPU architecture.
22 22
23 The reason applications are running faster is 23 The reason applications are running faster is because of two
24 factors. The first factor is almost completely 24 factors. The first factor is almost completely irrelevant and it's not
25 of significant interest because it'll also hav 25 of significant interest because it'll also have the downside of
26 requiring larger clear-page copy-page in page 26 requiring larger clear-page copy-page in page faults which is a
27 potentially negative effect. The first factor 27 potentially negative effect. The first factor consists in taking a
28 single page fault for each 2M virtual region t 28 single page fault for each 2M virtual region touched by userland (so
29 reducing the enter/exit kernel frequency by a 29 reducing the enter/exit kernel frequency by a 512 times factor). This
30 only matters the first time the memory is acce 30 only matters the first time the memory is accessed for the lifetime of
31 a memory mapping. The second long lasting and 31 a memory mapping. The second long lasting and much more important
32 factor will affect all subsequent accesses to 32 factor will affect all subsequent accesses to the memory for the whole
33 runtime of the application. The second factor 33 runtime of the application. The second factor consist of two
34 components: 34 components:
35 35
36 1) the TLB miss will run faster (especially wi 36 1) the TLB miss will run faster (especially with virtualization using
37 nested pagetables but almost always also on 37 nested pagetables but almost always also on bare metal without
38 virtualization) 38 virtualization)
39 39
40 2) a single TLB entry will be mapping a much l 40 2) a single TLB entry will be mapping a much larger amount of virtual
41 memory in turn reducing the number of TLB m 41 memory in turn reducing the number of TLB misses. With
42 virtualization and nested pagetables the TL 42 virtualization and nested pagetables the TLB can be mapped of
43 larger size only if both KVM and the Linux 43 larger size only if both KVM and the Linux guest are using
44 hugepages but a significant speedup already 44 hugepages but a significant speedup already happens if only one of
45 the two is using hugepages just because of 45 the two is using hugepages just because of the fact the TLB miss is
46 going to run faster. 46 going to run faster.
47 47
48 Modern kernels support "multi-size THP" (mTHP) 48 Modern kernels support "multi-size THP" (mTHP), which introduces the
49 ability to allocate memory in blocks that are 49 ability to allocate memory in blocks that are bigger than a base page
50 but smaller than traditional PMD-size (as desc 50 but smaller than traditional PMD-size (as described above), in
51 increments of a power-of-2 number of pages. mT 51 increments of a power-of-2 number of pages. mTHP can back anonymous
52 memory (for example 16K, 32K, 64K, etc). These 52 memory (for example 16K, 32K, 64K, etc). These THPs continue to be
53 PTE-mapped, but in many cases can still provid 53 PTE-mapped, but in many cases can still provide similar benefits to
54 those outlined above: Page faults are signific 54 those outlined above: Page faults are significantly reduced (by a
55 factor of e.g. 4, 8, 16, etc), but latency spi 55 factor of e.g. 4, 8, 16, etc), but latency spikes are much less
56 prominent because the size of each page isn't 56 prominent because the size of each page isn't as huge as the PMD-sized
57 variant and there is less memory to clear in e 57 variant and there is less memory to clear in each page fault. Some
58 architectures also employ TLB compression mech 58 architectures also employ TLB compression mechanisms to squeeze more
59 entries in when a set of PTEs are virtually an 59 entries in when a set of PTEs are virtually and physically contiguous
60 and approporiately aligned. In this case, TLB 60 and approporiately aligned. In this case, TLB misses will occur less
61 often. 61 often.
62 62
63 THP can be enabled system wide or restricted t 63 THP can be enabled system wide or restricted to certain tasks or even
64 memory ranges inside task's address space. Unl 64 memory ranges inside task's address space. Unless THP is completely
65 disabled, there is ``khugepaged`` daemon that 65 disabled, there is ``khugepaged`` daemon that scans memory and
66 collapses sequences of basic pages into PMD-si 66 collapses sequences of basic pages into PMD-sized huge pages.
67 67
68 The THP behaviour is controlled via :ref:`sysf 68 The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
69 interface and using madvise(2) and prctl(2) sy 69 interface and using madvise(2) and prctl(2) system calls.
70 70
71 Transparent Hugepage Support maximizes the use 71 Transparent Hugepage Support maximizes the usefulness of free memory
72 if compared to the reservation approach of hug 72 if compared to the reservation approach of hugetlbfs by allowing all
73 unused memory to be used as cache or other mov 73 unused memory to be used as cache or other movable (or even unmovable
74 entities). It doesn't require reservation to p 74 entities). It doesn't require reservation to prevent hugepage
75 allocation failures to be noticeable from user 75 allocation failures to be noticeable from userland. It allows paging
76 and all other advanced VM features to be avail 76 and all other advanced VM features to be available on the
77 hugepages. It requires no modifications for ap 77 hugepages. It requires no modifications for applications to take
78 advantage of it. 78 advantage of it.
79 79
80 Applications however can be further optimized 80 Applications however can be further optimized to take advantage of
81 this feature, like for example they've been op 81 this feature, like for example they've been optimized before to avoid
82 a flood of mmap system calls for every malloc( 82 a flood of mmap system calls for every malloc(4k). Optimizing userland
83 is by far not mandatory and khugepaged already 83 is by far not mandatory and khugepaged already can take care of long
84 lived page allocations even for hugepage unawa 84 lived page allocations even for hugepage unaware applications that
85 deals with large amounts of memory. 85 deals with large amounts of memory.
86 86
87 In certain cases when hugepages are enabled sy 87 In certain cases when hugepages are enabled system wide, application
88 may end up allocating more memory resources. A 88 may end up allocating more memory resources. An application may mmap a
89 large region but only touch 1 byte of it, in t 89 large region but only touch 1 byte of it, in that case a 2M page might
90 be allocated instead of a 4k page for no good. 90 be allocated instead of a 4k page for no good. This is why it's
91 possible to disable hugepages system-wide and 91 possible to disable hugepages system-wide and to only have them inside
92 MADV_HUGEPAGE madvise regions. 92 MADV_HUGEPAGE madvise regions.
93 93
94 Embedded systems should enable hugepages only 94 Embedded systems should enable hugepages only inside madvise regions
95 to eliminate any risk of wasting any precious 95 to eliminate any risk of wasting any precious byte of memory and to
96 only run faster. 96 only run faster.
97 97
98 Applications that gets a lot of benefit from h 98 Applications that gets a lot of benefit from hugepages and that don't
99 risk to lose memory by using hugepages, should 99 risk to lose memory by using hugepages, should use
100 madvise(MADV_HUGEPAGE) on their critical mmapp 100 madvise(MADV_HUGEPAGE) on their critical mmapped regions.
101 101
102 .. _thp_sysfs: 102 .. _thp_sysfs:
103 103
104 sysfs 104 sysfs
105 ===== 105 =====
106 106
107 Global THP controls 107 Global THP controls
108 ------------------- 108 -------------------
109 109
110 Transparent Hugepage Support for anonymous mem 110 Transparent Hugepage Support for anonymous memory can be entirely disabled
111 (mostly for debugging purposes) or only enable 111 (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
112 regions (to avoid the risk of consuming more m 112 regions (to avoid the risk of consuming more memory resources) or enabled
113 system wide. This can be achieved per-supporte 113 system wide. This can be achieved per-supported-THP-size with one of::
114 114
115 echo always >/sys/kernel/mm/transparen 115 echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
116 echo madvise >/sys/kernel/mm/transpare 116 echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
117 echo never >/sys/kernel/mm/transparent 117 echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
118 118
119 where <size> is the hugepage size being addres 119 where <size> is the hugepage size being addressed, the available sizes
120 for which vary by system. 120 for which vary by system.
121 121
122 For example:: 122 For example::
123 123
124 echo always >/sys/kernel/mm/transparen 124 echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
125 125
126 Alternatively it is possible to specify that a 126 Alternatively it is possible to specify that a given hugepage size
127 will inherit the top-level "enabled" value:: 127 will inherit the top-level "enabled" value::
128 128
129 echo inherit >/sys/kernel/mm/transpare 129 echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled
130 130
131 For example:: 131 For example::
132 132
133 echo inherit >/sys/kernel/mm/transpare 133 echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
134 134
135 The top-level setting (for use with "inherit") 135 The top-level setting (for use with "inherit") can be set by issuing
136 one of the following commands:: 136 one of the following commands::
137 137
138 echo always >/sys/kernel/mm/transparen 138 echo always >/sys/kernel/mm/transparent_hugepage/enabled
139 echo madvise >/sys/kernel/mm/transpare 139 echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
140 echo never >/sys/kernel/mm/transparent 140 echo never >/sys/kernel/mm/transparent_hugepage/enabled
141 141
142 By default, PMD-sized hugepages have enabled=" 142 By default, PMD-sized hugepages have enabled="inherit" and all other
143 hugepage sizes have enabled="never". If enabli 143 hugepage sizes have enabled="never". If enabling multiple hugepage
144 sizes, the kernel will select the most appropr 144 sizes, the kernel will select the most appropriate enabled size for a
145 given allocation. 145 given allocation.
146 146
147 It's also possible to limit defrag efforts in 147 It's also possible to limit defrag efforts in the VM to generate
148 anonymous hugepages in case they're not immedi 148 anonymous hugepages in case they're not immediately free to madvise
149 regions or to never try to defrag memory and s 149 regions or to never try to defrag memory and simply fallback to regular
150 pages unless hugepages are immediately availab 150 pages unless hugepages are immediately available. Clearly if we spend CPU
151 time to defrag memory, we would expect to gain 151 time to defrag memory, we would expect to gain even more by the fact we
152 use hugepages later instead of regular pages. 152 use hugepages later instead of regular pages. This isn't always
153 guaranteed, but it may be more likely in case 153 guaranteed, but it may be more likely in case the allocation is for a
154 MADV_HUGEPAGE region. 154 MADV_HUGEPAGE region.
155 155
156 :: 156 ::
157 157
158 echo always >/sys/kernel/mm/transparen 158 echo always >/sys/kernel/mm/transparent_hugepage/defrag
159 echo defer >/sys/kernel/mm/transparent 159 echo defer >/sys/kernel/mm/transparent_hugepage/defrag
160 echo defer+madvise >/sys/kernel/mm/tra 160 echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag
161 echo madvise >/sys/kernel/mm/transpare 161 echo madvise >/sys/kernel/mm/transparent_hugepage/defrag
162 echo never >/sys/kernel/mm/transparent 162 echo never >/sys/kernel/mm/transparent_hugepage/defrag
163 163
164 always 164 always
165 means that an application requesting T 165 means that an application requesting THP will stall on
166 allocation failure and directly reclai 166 allocation failure and directly reclaim pages and compact
167 memory in an effort to allocate a THP 167 memory in an effort to allocate a THP immediately. This may be
168 desirable for virtual machines that be 168 desirable for virtual machines that benefit heavily from THP
169 use and are willing to delay the VM st 169 use and are willing to delay the VM start to utilise them.
170 170
171 defer 171 defer
172 means that an application will wake ks 172 means that an application will wake kswapd in the background
173 to reclaim pages and wake kcompactd to 173 to reclaim pages and wake kcompactd to compact memory so that
174 THP is available in the near future. I 174 THP is available in the near future. It's the responsibility
175 of khugepaged to then install the THP 175 of khugepaged to then install the THP pages later.
176 176
177 defer+madvise 177 defer+madvise
178 will enter direct reclaim and compacti 178 will enter direct reclaim and compaction like ``always``, but
179 only for regions that have used madvis 179 only for regions that have used madvise(MADV_HUGEPAGE); all
180 other regions will wake kswapd in the 180 other regions will wake kswapd in the background to reclaim
181 pages and wake kcompactd to compact me 181 pages and wake kcompactd to compact memory so that THP is
182 available in the near future. 182 available in the near future.
183 183
184 madvise 184 madvise
185 will enter direct reclaim like ``alway 185 will enter direct reclaim like ``always`` but only for regions
186 that are have used madvise(MADV_HUGEPA 186 that are have used madvise(MADV_HUGEPAGE). This is the default
187 behaviour. 187 behaviour.
188 188
189 never 189 never
190 should be self-explanatory. 190 should be self-explanatory.
191 191
192 By default kernel tries to use huge, PMD-mappa 192 By default kernel tries to use huge, PMD-mappable zero page on read
193 page fault to anonymous mapping. It's possible 193 page fault to anonymous mapping. It's possible to disable huge zero
194 page by writing 0 or enable it back by writing 194 page by writing 0 or enable it back by writing 1::
195 195
196 echo 0 >/sys/kernel/mm/transparent_hug 196 echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page
197 echo 1 >/sys/kernel/mm/transparent_hug 197 echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page
198 198
199 Some userspace (such as a test program, or an 199 Some userspace (such as a test program, or an optimized memory
200 allocation library) may want to know the size 200 allocation library) may want to know the size (in bytes) of a
201 PMD-mappable transparent hugepage:: 201 PMD-mappable transparent hugepage::
202 202
203 cat /sys/kernel/mm/transparent_hugepag 203 cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size
204 204
205 All THPs at fault and collapse time will be ad !! 205 khugepaged will be automatically started when one or more hugepage
206 and will therefore be split under memory presu !! 206 sizes are enabled (either by directly setting "always" or "madvise",
207 "underused". A THP is underused if the number !! 207 or by setting "inherit" while the top-level enabled is set to "always"
208 the THP is above max_ptes_none (see below). It !! 208 or "madvise"), and it'll be automatically shutdown when the last
209 this behaviour by writing 0 to shrink_underuse !! 209 hugepage size is disabled (either by directly setting "never", or by
210 1 to it:: !! 210 setting "inherit" while the top-level enabled is set to "never").
211 <<
212 echo 0 > /sys/kernel/mm/transparent_hu <<
213 echo 1 > /sys/kernel/mm/transparent_hu <<
214 <<
215 khugepaged will be automatically started when <<
216 (either of the per-size anon control or the to <<
217 to "always" or "madvise"), and it'll be automa <<
218 PMD-sized THP is disabled (when both the per-s <<
219 top-level control are "never") <<
220 211
221 Khugepaged controls 212 Khugepaged controls
222 ------------------- 213 -------------------
223 214
224 .. note:: 215 .. note::
225 khugepaged currently only searches for oppo 216 khugepaged currently only searches for opportunities to collapse to
226 PMD-sized THP and no attempt is made to col 217 PMD-sized THP and no attempt is made to collapse to other THP
227 sizes. 218 sizes.
228 219
229 khugepaged runs usually at low frequency so wh 220 khugepaged runs usually at low frequency so while one may not want to
230 invoke defrag algorithms synchronously during 221 invoke defrag algorithms synchronously during the page faults, it
231 should be worth invoking defrag at least in kh 222 should be worth invoking defrag at least in khugepaged. However it's
232 also possible to disable defrag in khugepaged 223 also possible to disable defrag in khugepaged by writing 0 or enable
233 defrag in khugepaged by writing 1:: 224 defrag in khugepaged by writing 1::
234 225
235 echo 0 >/sys/kernel/mm/transparent_hug 226 echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
236 echo 1 >/sys/kernel/mm/transparent_hug 227 echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag
237 228
238 You can also control how many pages khugepaged 229 You can also control how many pages khugepaged should scan at each
239 pass:: 230 pass::
240 231
241 /sys/kernel/mm/transparent_hugepage/kh 232 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan
242 233
243 and how many milliseconds to wait in khugepage 234 and how many milliseconds to wait in khugepaged between each pass (you
244 can set this to 0 to run khugepaged at 100% ut 235 can set this to 0 to run khugepaged at 100% utilization of one core)::
245 236
246 /sys/kernel/mm/transparent_hugepage/kh 237 /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs
247 238
248 and how many milliseconds to wait in khugepage 239 and how many milliseconds to wait in khugepaged if there's an hugepage
249 allocation failure to throttle the next alloca 240 allocation failure to throttle the next allocation attempt::
250 241
251 /sys/kernel/mm/transparent_hugepage/kh 242 /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs
252 243
253 The khugepaged progress can be seen in the num 244 The khugepaged progress can be seen in the number of pages collapsed (note
254 that this counter may not be an exact count of 245 that this counter may not be an exact count of the number of pages
255 collapsed, since "collapsed" could mean multip 246 collapsed, since "collapsed" could mean multiple things: (1) A PTE mapping
256 being replaced by a PMD mapping, or (2) All 4K 247 being replaced by a PMD mapping, or (2) All 4K physical pages replaced by
257 one 2M hugepage. Each may happen independently 248 one 2M hugepage. Each may happen independently, or together, depending on
258 the type of memory and the failures that occur 249 the type of memory and the failures that occur. As such, this value should
259 be interpreted roughly as a sign of progress, 250 be interpreted roughly as a sign of progress, and counters in /proc/vmstat
260 consulted for more accurate accounting):: 251 consulted for more accurate accounting)::
261 252
262 /sys/kernel/mm/transparent_hugepage/kh 253 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed
263 254
264 for each pass:: 255 for each pass::
265 256
266 /sys/kernel/mm/transparent_hugepage/kh 257 /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans
267 258
268 ``max_ptes_none`` specifies how many extra sma 259 ``max_ptes_none`` specifies how many extra small pages (that are
269 not already mapped) can be allocated when coll 260 not already mapped) can be allocated when collapsing a group
270 of small pages into one large page:: 261 of small pages into one large page::
271 262
272 /sys/kernel/mm/transparent_hugepage/kh 263 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
273 264
274 A higher value leads to use additional memory 265 A higher value leads to use additional memory for programs.
275 A lower value leads to gain less thp performan 266 A lower value leads to gain less thp performance. Value of
276 max_ptes_none can waste cpu time very little, 267 max_ptes_none can waste cpu time very little, you can
277 ignore it. 268 ignore it.
278 269
279 ``max_ptes_swap`` specifies how many pages can 270 ``max_ptes_swap`` specifies how many pages can be brought in from
280 swap when collapsing a group of pages into a t 271 swap when collapsing a group of pages into a transparent huge page::
281 272
282 /sys/kernel/mm/transparent_hugepage/kh 273 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap
283 274
284 A higher value can cause excessive swap IO and 275 A higher value can cause excessive swap IO and waste
285 memory. A lower value can prevent THPs from be 276 memory. A lower value can prevent THPs from being
286 collapsed, resulting fewer pages being collaps 277 collapsed, resulting fewer pages being collapsed into
287 THPs, and lower memory access performance. 278 THPs, and lower memory access performance.
288 279
289 ``max_ptes_shared`` specifies how many pages c 280 ``max_ptes_shared`` specifies how many pages can be shared across multiple
290 processes. khugepaged might treat pages of THP !! 281 processes. Exceeding the number would block the collapse::
291 that THP is shared. Exceeding the number would <<
292 282
293 /sys/kernel/mm/transparent_hugepage/kh 283 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared
294 284
295 A higher value may increase memory footprint f 285 A higher value may increase memory footprint for some workloads.
296 286
297 Boot parameters !! 287 Boot parameter
298 =============== !! 288 ==============
299 289
300 You can change the sysfs boot time default for !! 290 You can change the sysfs boot time defaults of Transparent Hugepage
301 control by passing the parameter ``transparent !! 291 Support by passing the parameter ``transparent_hugepage=always`` or
302 ``transparent_hugepage=madvise`` or ``transpar !! 292 ``transparent_hugepage=madvise`` or ``transparent_hugepage=never``
303 kernel command line. !! 293 to the kernel command line.
304 <<
305 Alternatively, each supported anonymous THP si <<
306 passing ``thp_anon=<size>[KMG],<size>[KMG]:<st <<
307 where ``<size>`` is the THP size (must be a po <<
308 supported anonymous THP) and ``<state>`` is o <<
309 ``never`` or ``inherit``. <<
310 <<
311 For example, the following will set 16K, 32K, <<
312 set 128K, 512K to ``inherit``, set 256K to ``m <<
313 to ``never``:: <<
314 <<
315 thp_anon=16K-64K:always;128K,512K:inhe <<
316 <<
317 ``thp_anon=`` may be specified multiple times <<
318 required. If ``thp_anon=`` is specified at lea <<
319 not explicitly configured on the command line <<
320 ``never``. <<
321 <<
322 ``transparent_hugepage`` setting only affects <<
323 ``thp_anon`` is not specified, PMD_ORDER THP w <<
324 However, if a valid ``thp_anon`` setting is pr <<
325 PMD_ORDER THP policy will be overridden. If th <<
326 is not defined within a valid ``thp_anon``, it <<
327 ``never``. <<
328 294
329 Hugepages in tmpfs/shmem 295 Hugepages in tmpfs/shmem
330 ======================== 296 ========================
331 297
332 You can control hugepage allocation policy in 298 You can control hugepage allocation policy in tmpfs with mount option
333 ``huge=``. It can have following values: 299 ``huge=``. It can have following values:
334 300
335 always 301 always
336 Attempt to allocate huge pages every time 302 Attempt to allocate huge pages every time we need a new page;
337 303
338 never 304 never
339 Do not allocate huge pages; 305 Do not allocate huge pages;
340 306
341 within_size 307 within_size
342 Only allocate huge page if it will be full 308 Only allocate huge page if it will be fully within i_size.
343 Also respect fadvise()/madvise() hints; 309 Also respect fadvise()/madvise() hints;
344 310
345 advise 311 advise
346 Only allocate huge pages if requested with 312 Only allocate huge pages if requested with fadvise()/madvise();
347 313
348 The default policy is ``never``. 314 The default policy is ``never``.
349 315
350 ``mount -o remount,huge= /mountpoint`` works f 316 ``mount -o remount,huge= /mountpoint`` works fine after mount: remounting
351 ``huge=never`` will not attempt to break up hu 317 ``huge=never`` will not attempt to break up huge pages at all, just stop more
352 from being allocated. 318 from being allocated.
353 319
354 There's also sysfs knob to control hugepage al 320 There's also sysfs knob to control hugepage allocation policy for internal
355 shmem mount: /sys/kernel/mm/transparent_hugepa 321 shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount
356 is used for SysV SHM, memfds, shared anonymous 322 is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or
357 MAP_ANONYMOUS), GPU drivers' DRM objects, Ashm 323 MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem.
358 324
359 In addition to policies listed above, shmem_en 325 In addition to policies listed above, shmem_enabled allows two further
360 values: 326 values:
361 327
362 deny 328 deny
363 For use in emergencies, to force the huge 329 For use in emergencies, to force the huge option off from
364 all mounts; 330 all mounts;
365 force 331 force
366 Force the huge option on for all - very us 332 Force the huge option on for all - very useful for testing;
367 333
368 Shmem can also use "multi-size THP" (mTHP) by <<
369 control mTHP allocation: <<
370 '/sys/kernel/mm/transparent_hugepage/hugepages <<
371 and its value for each mTHP is essentially con <<
372 setting. An 'inherit' option is added to ensu <<
373 global settings. Conversely, the options 'for <<
374 which are rather testing artifacts from the ol <<
375 <<
376 always <<
377 Attempt to allocate <size> huge pages ever <<
378 <<
379 inherit <<
380 Inherit the top-level "shmem_enabled" valu <<
381 have enabled="inherit" and all other hugep <<
382 <<
383 never <<
384 Do not allocate <size> huge pages; <<
385 <<
386 within_size <<
387 Only allocate <size> huge page if it will <<
388 Also respect fadvise()/madvise() hints; <<
389 <<
390 advise <<
391 Only allocate <size> huge pages if request <<
392 <<
393 Need of application restart 334 Need of application restart
394 =========================== 335 ===========================
395 336
396 The transparent_hugepage/enabled and 337 The transparent_hugepage/enabled and
397 transparent_hugepage/hugepages-<size>kB/enable 338 transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount
398 option only affect future behavior. So to make 339 option only affect future behavior. So to make them effective you need
399 to restart any application that could have bee 340 to restart any application that could have been using hugepages. This
400 also applies to the regions registered in khug 341 also applies to the regions registered in khugepaged.
401 342
402 Monitoring usage 343 Monitoring usage
403 ================ 344 ================
404 345
>> 346 .. note::
>> 347 Currently the below counters only record events relating to
>> 348 PMD-sized THP. Events relating to other THP sizes are not included.
>> 349
405 The number of PMD-sized anonymous transparent 350 The number of PMD-sized anonymous transparent huge pages currently used by the
406 system is available by reading the AnonHugePag 351 system is available by reading the AnonHugePages field in ``/proc/meminfo``.
407 To identify what applications are using PMD-si 352 To identify what applications are using PMD-sized anonymous transparent huge
408 pages, it is necessary to read ``/proc/PID/sma 353 pages, it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages
409 fields for each mapping. (Note that AnonHugePa 354 fields for each mapping. (Note that AnonHugePages only applies to traditional
410 PMD-sized THP for historical reasons and shoul 355 PMD-sized THP for historical reasons and should have been called
411 AnonHugePmdMapped). 356 AnonHugePmdMapped).
412 357
413 The number of file transparent huge pages mapp 358 The number of file transparent huge pages mapped to userspace is available
414 by reading ShmemPmdMapped and ShmemHugePages f 359 by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``.
415 To identify what applications are mapping file 360 To identify what applications are mapping file transparent huge pages, it
416 is necessary to read ``/proc/PID/smaps`` and c 361 is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields
417 for each mapping. 362 for each mapping.
418 363
419 Note that reading the smaps file is expensive 364 Note that reading the smaps file is expensive and reading it
420 frequently will incur overhead. 365 frequently will incur overhead.
421 366
422 There are a number of counters in ``/proc/vmst 367 There are a number of counters in ``/proc/vmstat`` that may be used to
423 monitor how successfully the system is providi 368 monitor how successfully the system is providing huge pages for use.
424 369
425 thp_fault_alloc 370 thp_fault_alloc
426 is incremented every time a huge page 371 is incremented every time a huge page is successfully
427 allocated and charged to handle a page !! 372 allocated to handle a page fault.
428 373
429 thp_collapse_alloc 374 thp_collapse_alloc
430 is incremented by khugepaged when it h 375 is incremented by khugepaged when it has found
431 a range of pages to collapse into one 376 a range of pages to collapse into one huge page and has
432 successfully allocated a new huge page 377 successfully allocated a new huge page to store the data.
433 378
434 thp_fault_fallback 379 thp_fault_fallback
435 is incremented if a page fault fails t !! 380 is incremented if a page fault fails to allocate
436 a huge page and instead falls back to 381 a huge page and instead falls back to using small pages.
437 382
438 thp_fault_fallback_charge 383 thp_fault_fallback_charge
439 is incremented if a page fault fails t 384 is incremented if a page fault fails to charge a huge page and
440 instead falls back to using small page 385 instead falls back to using small pages even though the
441 allocation was successful. 386 allocation was successful.
442 387
443 thp_collapse_alloc_failed 388 thp_collapse_alloc_failed
444 is incremented if khugepaged found a r 389 is incremented if khugepaged found a range
445 of pages that should be collapsed into 390 of pages that should be collapsed into one huge page but failed
446 the allocation. 391 the allocation.
447 392
448 thp_file_alloc 393 thp_file_alloc
449 is incremented every time a shmem huge !! 394 is incremented every time a file huge page is successfully
450 allocated (Note that despite being nam !! 395 allocated.
451 measures only shmem). <<
452 396
453 thp_file_fallback 397 thp_file_fallback
454 is incremented if a shmem huge page is !! 398 is incremented if a file huge page is attempted to be allocated
455 but fails and instead falls back to us !! 399 but fails and instead falls back to using small pages.
456 despite being named after "file", the <<
457 400
458 thp_file_fallback_charge 401 thp_file_fallback_charge
459 is incremented if a shmem huge page ca !! 402 is incremented if a file huge page cannot be charged and instead
460 falls back to using small pages even t 403 falls back to using small pages even though the allocation was
461 successful. (Note that despite being n !! 404 successful.
462 counter measures only shmem). <<
463 405
464 thp_file_mapped 406 thp_file_mapped
465 is incremented every time a file or sh !! 407 is incremented every time a file huge page is mapped into
466 user address space. 408 user address space.
467 409
468 thp_split_page 410 thp_split_page
469 is incremented every time a huge page 411 is incremented every time a huge page is split into base
470 pages. This can happen for a variety o 412 pages. This can happen for a variety of reasons but a common
471 reason is that a huge page is old and 413 reason is that a huge page is old and is being reclaimed.
472 This action implies splitting all PMD 414 This action implies splitting all PMD the page mapped with.
473 415
474 thp_split_page_failed 416 thp_split_page_failed
475 is incremented if kernel fails to spli 417 is incremented if kernel fails to split huge
476 page. This can happen if the page was 418 page. This can happen if the page was pinned by somebody.
477 419
478 thp_deferred_split_page 420 thp_deferred_split_page
479 is incremented when a huge page is put 421 is incremented when a huge page is put onto split
480 queue. This happens when a huge page i 422 queue. This happens when a huge page is partially unmapped and
481 splitting it would free up some memory 423 splitting it would free up some memory. Pages on split queue are
482 going to be split under memory pressur 424 going to be split under memory pressure.
483 425
484 thp_underused_split_page <<
485 is incremented when a huge page on the <<
486 because it was underused. A THP is und <<
487 zero pages in the THP is above a certa <<
488 (/sys/kernel/mm/transparent_hugepage/k <<
489 <<
490 thp_split_pmd 426 thp_split_pmd
491 is incremented every time a PMD split 427 is incremented every time a PMD split into table of PTEs.
492 This can happen, for instance, when ap 428 This can happen, for instance, when application calls mprotect() or
493 munmap() on part of huge page. It does 429 munmap() on part of huge page. It doesn't split huge page, only
494 page table entry. 430 page table entry.
495 431
496 thp_zero_page_alloc 432 thp_zero_page_alloc
497 is incremented every time a huge zero 433 is incremented every time a huge zero page used for thp is
498 successfully allocated. Note, it doesn 434 successfully allocated. Note, it doesn't count every map of
499 the huge zero page, only its allocatio 435 the huge zero page, only its allocation.
500 436
501 thp_zero_page_alloc_failed 437 thp_zero_page_alloc_failed
502 is incremented if kernel fails to allo 438 is incremented if kernel fails to allocate
503 huge zero page and falls back to using 439 huge zero page and falls back to using small pages.
504 440
505 thp_swpout 441 thp_swpout
506 is incremented every time a huge page 442 is incremented every time a huge page is swapout in one
507 piece without splitting. 443 piece without splitting.
508 444
509 thp_swpout_fallback 445 thp_swpout_fallback
510 is incremented if a huge page has to b 446 is incremented if a huge page has to be split before swapout.
511 Usually because failed to allocate som 447 Usually because failed to allocate some continuous swap space
512 for the huge page. 448 for the huge page.
513 <<
514 In /sys/kernel/mm/transparent_hugepage/hugepag <<
515 also individual counters for each huge page si <<
516 monitor the system's effectiveness in providin <<
517 counter has its own corresponding file. <<
518 <<
519 anon_fault_alloc <<
520 is incremented every time a huge page <<
521 allocated and charged to handle a page <<
522 <<
523 anon_fault_fallback <<
524 is incremented if a page fault fails t <<
525 a huge page and instead falls back to <<
526 lower orders or small pages. <<
527 <<
528 anon_fault_fallback_charge <<
529 is incremented if a page fault fails t <<
530 instead falls back to using huge pages <<
531 small pages even though the allocation <<
532 <<
533 swpout <<
534 is incremented every time a huge page <<
535 piece without splitting. <<
536 <<
537 swpout_fallback <<
538 is incremented if a huge page has to b <<
539 Usually because failed to allocate som <<
540 for the huge page. <<
541 <<
542 shmem_alloc <<
543 is incremented every time a shmem huge <<
544 allocated. <<
545 <<
546 shmem_fallback <<
547 is incremented if a shmem huge page is <<
548 but fails and instead falls back to us <<
549 <<
550 shmem_fallback_charge <<
551 is incremented if a shmem huge page ca <<
552 falls back to using small pages even t <<
553 successful. <<
554 <<
555 split <<
556 is incremented every time a huge page <<
557 smaller orders. This can happen for a <<
558 common reason is that a huge page is o <<
559 <<
560 split_failed <<
561 is incremented if kernel fails to spli <<
562 page. This can happen if the page was <<
563 <<
564 split_deferred <<
565 is incremented when a huge page is put <<
566 This happens when a huge page is parti <<
567 it would free up some memory. Pages on <<
568 be split under memory pressure, if spl <<
569 <<
570 nr_anon <<
571 the number of anonymous THP we have in <<
572 might be currently entirely mapped or h <<
573 subpages. <<
574 <<
575 nr_anon_partially_mapped <<
576 the number of anonymous THP which are l <<
577 wasting memory, and have been queued fo <<
578 Note that in corner some cases (e.g., f <<
579 an anonymous THP as "partially mapped" <<
580 is not actually partially mapped anymor <<
581 449
582 As the system ages, allocating huge pages may 450 As the system ages, allocating huge pages may be expensive as the
583 system uses memory compaction to copy data aro 451 system uses memory compaction to copy data around memory to free a
584 huge page for use. There are some counters in 452 huge page for use. There are some counters in ``/proc/vmstat`` to help
585 monitor this overhead. 453 monitor this overhead.
586 454
587 compact_stall 455 compact_stall
588 is incremented every time a process st 456 is incremented every time a process stalls to run
589 memory compaction so that a huge page 457 memory compaction so that a huge page is free for use.
590 458
591 compact_success 459 compact_success
592 is incremented if the system compacted 460 is incremented if the system compacted memory and
593 freed a huge page for use. 461 freed a huge page for use.
594 462
595 compact_fail 463 compact_fail
596 is incremented if the system tries to 464 is incremented if the system tries to compact memory
597 but failed. 465 but failed.
598 466
599 It is possible to establish how long the stall 467 It is possible to establish how long the stalls were using the function
600 tracer to record how long was spent in __alloc 468 tracer to record how long was spent in __alloc_pages() and
601 using the mm_page_alloc tracepoint to identify 469 using the mm_page_alloc tracepoint to identify which allocations were
602 for huge pages. 470 for huge pages.
603 471
604 Optimizing the applications 472 Optimizing the applications
605 =========================== 473 ===========================
606 474
607 To be guaranteed that the kernel will map a TH 475 To be guaranteed that the kernel will map a THP immediately in any
608 memory region, the mmap region has to be hugep 476 memory region, the mmap region has to be hugepage naturally
609 aligned. posix_memalign() can provide that gua 477 aligned. posix_memalign() can provide that guarantee.
610 478
611 Hugetlbfs 479 Hugetlbfs
612 ========= 480 =========
613 481
614 You can use hugetlbfs on a kernel that has tra 482 You can use hugetlbfs on a kernel that has transparent hugepage
615 support enabled just fine as always. No differ 483 support enabled just fine as always. No difference can be noted in
616 hugetlbfs other than there will be less overal 484 hugetlbfs other than there will be less overall fragmentation. All
617 usual features belonging to hugetlbfs are pres 485 usual features belonging to hugetlbfs are preserved and
618 unaffected. libhugetlbfs will also work fine a 486 unaffected. libhugetlbfs will also work fine as usual.
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