1============================ 2Transparent Hugepage Support 3============================ 4 5Objective 6========= 7 8Performance critical computing applications dealing with large memory 9working sets are already running on top of libhugetlbfs and in turn 10hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of 11using huge pages for the backing of virtual memory with huge pages 12that supports the automatic promotion and demotion of page sizes and 13without the shortcomings of hugetlbfs. 14 15Currently THP only works for anonymous memory mappings and tmpfs/shmem. 16But in the future it can expand to other filesystems. 17 18.. note:: 19 in the examples below we presume that the basic page size is 4K and 20 the huge page size is 2M, although the actual numbers may vary 21 depending on the CPU architecture. 22 23The reason applications are running faster is because of two 24factors. The first factor is almost completely irrelevant and it's not 25of significant interest because it'll also have the downside of 26requiring larger clear-page copy-page in page faults which is a 27potentially negative effect. The first factor consists in taking a 28single page fault for each 2M virtual region touched by userland (so 29reducing the enter/exit kernel frequency by a 512 times factor). This 30only matters the first time the memory is accessed for the lifetime of 31a memory mapping. The second long lasting and much more important 32factor will affect all subsequent accesses to the memory for the whole 33runtime of the application. The second factor consist of two 34components: 35 361) the TLB miss will run faster (especially with virtualization using 37 nested pagetables but almost always also on bare metal without 38 virtualization) 39 402) a single TLB entry will be mapping a much larger amount of virtual 41 memory in turn reducing the number of TLB misses. With 42 virtualization and nested pagetables the TLB can be mapped of 43 larger size only if both KVM and the Linux guest are using 44 hugepages but a significant speedup already happens if only one of 45 the two is using hugepages just because of the fact the TLB miss is 46 going to run faster. 47 48Modern kernels support "multi-size THP" (mTHP), which introduces the 49ability to allocate memory in blocks that are bigger than a base page 50but smaller than traditional PMD-size (as described above), in 51increments of a power-of-2 number of pages. mTHP can back anonymous 52memory (for example 16K, 32K, 64K, etc). These THPs continue to be 53PTE-mapped, but in many cases can still provide similar benefits to 54those outlined above: Page faults are significantly reduced (by a 55factor of e.g. 4, 8, 16, etc), but latency spikes are much less 56prominent because the size of each page isn't as huge as the PMD-sized 57variant and there is less memory to clear in each page fault. Some 58architectures also employ TLB compression mechanisms to squeeze more 59entries in when a set of PTEs are virtually and physically contiguous 60and approporiately aligned. In this case, TLB misses will occur less 61often. 62 63THP can be enabled system wide or restricted to certain tasks or even 64memory ranges inside task's address space. Unless THP is completely 65disabled, there is ``khugepaged`` daemon that scans memory and 66collapses sequences of basic pages into PMD-sized huge pages. 67 68The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` 69interface and using madvise(2) and prctl(2) system calls. 70 71Transparent Hugepage Support maximizes the usefulness of free memory 72if compared to the reservation approach of hugetlbfs by allowing all 73unused memory to be used as cache or other movable (or even unmovable 74entities). It doesn't require reservation to prevent hugepage 75allocation failures to be noticeable from userland. It allows paging 76and all other advanced VM features to be available on the 77hugepages. It requires no modifications for applications to take 78advantage of it. 79 80Applications however can be further optimized to take advantage of 81this feature, like for example they've been optimized before to avoid 82a flood of mmap system calls for every malloc(4k). Optimizing userland 83is by far not mandatory and khugepaged already can take care of long 84lived page allocations even for hugepage unaware applications that 85deals with large amounts of memory. 86 87In certain cases when hugepages are enabled system wide, application 88may end up allocating more memory resources. An application may mmap a 89large region but only touch 1 byte of it, in that case a 2M page might 90be allocated instead of a 4k page for no good. This is why it's 91possible to disable hugepages system-wide and to only have them inside 92MADV_HUGEPAGE madvise regions. 93 94Embedded systems should enable hugepages only inside madvise regions 95to eliminate any risk of wasting any precious byte of memory and to 96only run faster. 97 98Applications that gets a lot of benefit from hugepages and that don't 99risk to lose memory by using hugepages, should use 100madvise(MADV_HUGEPAGE) on their critical mmapped regions. 101 102.. _thp_sysfs: 103 104sysfs 105===== 106 107Global THP controls 108------------------- 109 110Transparent Hugepage Support for anonymous memory can be disabled 111(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE 112regions (to avoid the risk of consuming more memory resources) or enabled 113system wide. This can be achieved per-supported-THP-size with one of:: 114 115 echo always >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled 116 echo madvise >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled 117 echo never >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled 118 119where <size> is the hugepage size being addressed, the available sizes 120for which vary by system. 121 122.. note:: Setting "never" in all sysfs THP controls does **not** disable 123 Transparent Huge Pages globally. This is because ``madvise(..., 124 MADV_COLLAPSE)`` ignores these settings and collapses ranges to 125 PMD-sized huge pages unconditionally. 126 127For example:: 128 129 echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled 130 131Alternatively it is possible to specify that a given hugepage size 132will inherit the top-level "enabled" value:: 133 134 echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/enabled 135 136For example:: 137 138 echo inherit >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled 139 140The top-level setting (for use with "inherit") can be set by issuing 141one of the following commands:: 142 143 echo always >/sys/kernel/mm/transparent_hugepage/enabled 144 echo madvise >/sys/kernel/mm/transparent_hugepage/enabled 145 echo never >/sys/kernel/mm/transparent_hugepage/enabled 146 147By default, PMD-sized hugepages have enabled="inherit" and all other 148hugepage sizes have enabled="never". If enabling multiple hugepage 149sizes, the kernel will select the most appropriate enabled size for a 150given allocation. 151 152It's also possible to limit defrag efforts in the VM to generate 153anonymous hugepages in case they're not immediately free to madvise 154regions or to never try to defrag memory and simply fallback to regular 155pages unless hugepages are immediately available. Clearly if we spend CPU 156time to defrag memory, we would expect to gain even more by the fact we 157use hugepages later instead of regular pages. This isn't always 158guaranteed, but it may be more likely in case the allocation is for a 159MADV_HUGEPAGE region. 160 161:: 162 163 echo always >/sys/kernel/mm/transparent_hugepage/defrag 164 echo defer >/sys/kernel/mm/transparent_hugepage/defrag 165 echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag 166 echo madvise >/sys/kernel/mm/transparent_hugepage/defrag 167 echo never >/sys/kernel/mm/transparent_hugepage/defrag 168 169always 170 means that an application requesting THP will stall on 171 allocation failure and directly reclaim pages and compact 172 memory in an effort to allocate a THP immediately. This may be 173 desirable for virtual machines that benefit heavily from THP 174 use and are willing to delay the VM start to utilise them. 175 176defer 177 means that an application will wake kswapd in the background 178 to reclaim pages and wake kcompactd to compact memory so that 179 THP is available in the near future. It's the responsibility 180 of khugepaged to then install the THP pages later. 181 182defer+madvise 183 will enter direct reclaim and compaction like ``always``, but 184 only for regions that have used madvise(MADV_HUGEPAGE); all 185 other regions will wake kswapd in the background to reclaim 186 pages and wake kcompactd to compact memory so that THP is 187 available in the near future. 188 189madvise 190 will enter direct reclaim like ``always`` but only for regions 191 that are have used madvise(MADV_HUGEPAGE). This is the default 192 behaviour. 193 194never 195 should be self-explanatory. Note that ``madvise(..., 196 MADV_COLLAPSE)`` can still cause transparent huge pages to be 197 obtained even if this mode is specified everywhere. 198 199By default kernel tries to use huge, PMD-mappable zero page on read 200page fault to anonymous mapping. It's possible to disable huge zero 201page by writing 0 or enable it back by writing 1:: 202 203 echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page 204 echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page 205 206Some userspace (such as a test program, or an optimized memory 207allocation library) may want to know the size (in bytes) of a 208PMD-mappable transparent hugepage:: 209 210 cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size 211 212All THPs at fault and collapse time will be added to _deferred_list, 213and will therefore be split under memory presure if they are considered 214"underused". A THP is underused if the number of zero-filled pages in 215the THP is above max_ptes_none (see below). It is possible to disable 216this behaviour by writing 0 to shrink_underused, and enable it by writing 2171 to it:: 218 219 echo 0 > /sys/kernel/mm/transparent_hugepage/shrink_underused 220 echo 1 > /sys/kernel/mm/transparent_hugepage/shrink_underused 221 222khugepaged will be automatically started when PMD-sized THP is enabled 223(either of the per-size anon control or the top-level control are set 224to "always" or "madvise"), and it'll be automatically shutdown when 225PMD-sized THP is disabled (when both the per-size anon control and the 226top-level control are "never") 227 228process THP controls 229-------------------- 230 231A process can control its own THP behaviour using the ``PR_SET_THP_DISABLE`` 232and ``PR_GET_THP_DISABLE`` pair of prctl(2) calls. The THP behaviour set using 233``PR_SET_THP_DISABLE`` is inherited across fork(2) and execve(2). These calls 234support the following arguments:: 235 236 prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0): 237 This will disable THPs completely for the process, irrespective 238 of global THP controls or madvise(..., MADV_COLLAPSE) being used. 239 240 prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0): 241 This will disable THPs for the process except when the usage of THPs is 242 advised. Consequently, THPs will only be used when: 243 - Global THP controls are set to "always" or "madvise" and 244 madvise(..., MADV_HUGEPAGE) or madvise(..., MADV_COLLAPSE) is used. 245 - Global THP controls are set to "never" and madvise(..., MADV_COLLAPSE) 246 is used. This is the same behavior as if THPs would not be disabled on 247 a process level. 248 Note that MADV_COLLAPSE is currently always rejected if 249 madvise(..., MADV_NOHUGEPAGE) is set on an area. 250 251 prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0): 252 This will re-enable THPs for the process, as if they were never disabled. 253 Whether THPs will actually be used depends on global THP controls and 254 madvise() calls. 255 256 prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0): 257 This returns a value whose bits indicate how THP-disable is configured: 258 Bits 259 1 0 Value Description 260 |0|0| 0 No THP-disable behaviour specified. 261 |0|1| 1 THP is entirely disabled for this process. 262 |1|1| 3 THP-except-advised mode is set for this process. 263 264Khugepaged controls 265------------------- 266 267.. note:: 268 khugepaged currently only searches for opportunities to collapse to 269 PMD-sized THP and no attempt is made to collapse to other THP 270 sizes. 271 272khugepaged runs usually at low frequency so while one may not want to 273invoke defrag algorithms synchronously during the page faults, it 274should be worth invoking defrag at least in khugepaged. However it's 275also possible to disable defrag in khugepaged by writing 0 or enable 276defrag in khugepaged by writing 1:: 277 278 echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag 279 echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag 280 281You can also control how many pages khugepaged should scan at each 282pass:: 283 284 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan 285 286and how many milliseconds to wait in khugepaged between each pass (you 287can set this to 0 to run khugepaged at 100% utilization of one core):: 288 289 /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs 290 291and how many milliseconds to wait in khugepaged if there's an hugepage 292allocation failure to throttle the next allocation attempt:: 293 294 /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs 295 296The khugepaged progress can be seen in the number of pages collapsed (note 297that this counter may not be an exact count of the number of pages 298collapsed, since "collapsed" could mean multiple things: (1) A PTE mapping 299being replaced by a PMD mapping, or (2) All 4K physical pages replaced by 300one 2M hugepage. Each may happen independently, or together, depending on 301the type of memory and the failures that occur. As such, this value should 302be interpreted roughly as a sign of progress, and counters in /proc/vmstat 303consulted for more accurate accounting):: 304 305 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed 306 307for each pass:: 308 309 /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans 310 311``max_ptes_none`` specifies how many extra small pages (that are 312not already mapped) can be allocated when collapsing a group 313of small pages into one large page:: 314 315 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none 316 317A higher value leads to use additional memory for programs. 318A lower value leads to gain less thp performance. Value of 319max_ptes_none can waste cpu time very little, you can 320ignore it. 321 322``max_ptes_swap`` specifies how many pages can be brought in from 323swap when collapsing a group of pages into a transparent huge page:: 324 325 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap 326 327A higher value can cause excessive swap IO and waste 328memory. A lower value can prevent THPs from being 329collapsed, resulting fewer pages being collapsed into 330THPs, and lower memory access performance. 331 332``max_ptes_shared`` specifies how many pages can be shared across multiple 333processes. khugepaged might treat pages of THPs as shared if any page of 334that THP is shared. Exceeding the number would block the collapse:: 335 336 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_shared 337 338A higher value may increase memory footprint for some workloads. 339 340Boot parameters 341=============== 342 343You can change the sysfs boot time default for the top-level "enabled" 344control by passing the parameter ``transparent_hugepage=always`` or 345``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the 346kernel command line. 347 348Alternatively, each supported anonymous THP size can be controlled by 349passing ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``, 350where ``<size>`` is the THP size (must be a power of 2 of PAGE_SIZE and 351supported anonymous THP) and ``<state>`` is one of ``always``, ``madvise``, 352``never`` or ``inherit``. 353 354For example, the following will set 16K, 32K, 64K THP to ``always``, 355set 128K, 512K to ``inherit``, set 256K to ``madvise`` and 1M, 2M 356to ``never``:: 357 358 thp_anon=16K-64K:always;128K,512K:inherit;256K:madvise;1M-2M:never 359 360``thp_anon=`` may be specified multiple times to configure all THP sizes as 361required. If ``thp_anon=`` is specified at least once, any anon THP sizes 362not explicitly configured on the command line are implicitly set to 363``never``. 364 365``transparent_hugepage`` setting only affects the global toggle. If 366``thp_anon`` is not specified, PMD_ORDER THP will default to ``inherit``. 367However, if a valid ``thp_anon`` setting is provided by the user, the 368PMD_ORDER THP policy will be overridden. If the policy for PMD_ORDER 369is not defined within a valid ``thp_anon``, its policy will default to 370``never``. 371 372Similarly to ``transparent_hugepage``, you can control the hugepage 373allocation policy for the internal shmem mount by using the kernel parameter 374``transparent_hugepage_shmem=<policy>``, where ``<policy>`` is one of the 375seven valid policies for shmem (``always``, ``within_size``, ``advise``, 376``never``, ``deny``, and ``force``). 377 378Similarly to ``transparent_hugepage_shmem``, you can control the default 379hugepage allocation policy for the tmpfs mount by using the kernel parameter 380``transparent_hugepage_tmpfs=<policy>``, where ``<policy>`` is one of the 381four valid policies for tmpfs (``always``, ``within_size``, ``advise``, 382``never``). The tmpfs mount default policy is ``never``. 383 384In the same manner as ``thp_anon`` controls each supported anonymous THP 385size, ``thp_shmem`` controls each supported shmem THP size. ``thp_shmem`` 386has the same format as ``thp_anon``, but also supports the policy 387``within_size``. 388 389``thp_shmem=`` may be specified multiple times to configure all THP sizes 390as required. If ``thp_shmem=`` is specified at least once, any shmem THP 391sizes not explicitly configured on the command line are implicitly set to 392``never``. 393 394``transparent_hugepage_shmem`` setting only affects the global toggle. If 395``thp_shmem`` is not specified, PMD_ORDER hugepage will default to 396``inherit``. However, if a valid ``thp_shmem`` setting is provided by the 397user, the PMD_ORDER hugepage policy will be overridden. If the policy for 398PMD_ORDER is not defined within a valid ``thp_shmem``, its policy will 399default to ``never``. 400 401Hugepages in tmpfs/shmem 402======================== 403 404Traditionally, tmpfs only supported a single huge page size ("PMD"). Today, 405it also supports smaller sizes just like anonymous memory, often referred 406to as "multi-size THP" (mTHP). Huge pages of any size are commonly 407represented in the kernel as "large folios". 408 409While there is fine control over the huge page sizes to use for the internal 410shmem mount (see below), ordinary tmpfs mounts will make use of all available 411huge page sizes without any control over the exact sizes, behaving more like 412other file systems. 413 414tmpfs mounts 415------------ 416 417The THP allocation policy for tmpfs mounts can be adjusted using the mount 418option: ``huge=``. It can have following values: 419 420always 421 Attempt to allocate huge pages every time we need a new page; 422 Always try PMD-sized huge pages first, and fall back to smaller-sized 423 huge pages if the PMD-sized huge page allocation fails; 424 425never 426 Do not allocate huge pages. Note that ``madvise(..., MADV_COLLAPSE)`` 427 can still cause transparent huge pages to be obtained even if this mode 428 is specified everywhere; 429 430within_size 431 Only allocate huge page if it will be fully within i_size; 432 Always try PMD-sized huge pages first, and fall back to smaller-sized 433 huge pages if the PMD-sized huge page allocation fails; 434 Also respect madvise() hints; 435 436advise 437 Only allocate huge pages if requested with madvise(); 438 439Remember, that the kernel may use huge pages of all available sizes, and 440that no fine control as for the internal tmpfs mount is available. 441 442The default policy in the past was ``never``, but it can now be adjusted 443using the kernel parameter ``transparent_hugepage_tmpfs=<policy>``. 444 445``mount -o remount,huge= /mountpoint`` works fine after mount: remounting 446``huge=never`` will not attempt to break up huge pages at all, just stop more 447from being allocated. 448 449In addition to policies listed above, the sysfs knob 450/sys/kernel/mm/transparent_hugepage/shmem_enabled will affect the 451allocation policy of tmpfs mounts, when set to the following values: 452 453deny 454 For use in emergencies, to force the huge option off from 455 all mounts; 456force 457 Force the huge option on for all - very useful for testing; 458 459shmem / internal tmpfs 460---------------------- 461The mount internal tmpfs mount is used for SysV SHM, memfds, shared anonymous 462mmaps (of /dev/zero or MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem. 463 464To control the THP allocation policy for this internal tmpfs mount, the 465sysfs knob /sys/kernel/mm/transparent_hugepage/shmem_enabled and the knobs 466per THP size in 467'/sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/shmem_enabled' 468can be used. 469 470The global knob has the same semantics as the ``huge=`` mount options 471for tmpfs mounts, except that the different huge page sizes can be controlled 472individually, and will only use the setting of the global knob when the 473per-size knob is set to 'inherit'. 474 475The options 'force' and 'deny' are dropped for the individual sizes, which 476are rather testing artifacts from the old ages. 477 478always 479 Attempt to allocate <size> huge pages every time we need a new page; 480 481inherit 482 Inherit the top-level "shmem_enabled" value. By default, PMD-sized hugepages 483 have enabled="inherit" and all other hugepage sizes have enabled="never"; 484 485never 486 Do not allocate <size> huge pages. Note that ``madvise(..., 487 MADV_COLLAPSE)`` can still cause transparent huge pages to be obtained 488 even if this mode is specified everywhere; 489 490within_size 491 Only allocate <size> huge page if it will be fully within i_size. 492 Also respect madvise() hints; 493 494advise 495 Only allocate <size> huge pages if requested with madvise(); 496 497Need of application restart 498=========================== 499 500The transparent_hugepage/enabled and 501transparent_hugepage/hugepages-<size>kB/enabled values and tmpfs mount 502option only affect future behavior. So to make them effective you need 503to restart any application that could have been using hugepages. This 504also applies to the regions registered in khugepaged. 505 506Monitoring usage 507================ 508 509The number of PMD-sized anonymous transparent huge pages currently used by the 510system is available by reading the AnonHugePages field in ``/proc/meminfo``. 511To identify what applications are using PMD-sized anonymous transparent huge 512pages, it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages 513fields for each mapping. (Note that AnonHugePages only applies to traditional 514PMD-sized THP for historical reasons and should have been called 515AnonHugePmdMapped). 516 517The number of file transparent huge pages mapped to userspace is available 518by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``. 519To identify what applications are mapping file transparent huge pages, it 520is necessary to read ``/proc/PID/smaps`` and count the FilePmdMapped fields 521for each mapping. 522 523Note that reading the smaps file is expensive and reading it 524frequently will incur overhead. 525 526There are a number of counters in ``/proc/vmstat`` that may be used to 527monitor how successfully the system is providing huge pages for use. 528 529thp_fault_alloc 530 is incremented every time a huge page is successfully 531 allocated and charged to handle a page fault. 532 533thp_collapse_alloc 534 is incremented by khugepaged when it has found 535 a range of pages to collapse into one huge page and has 536 successfully allocated a new huge page to store the data. 537 538thp_fault_fallback 539 is incremented if a page fault fails to allocate or charge 540 a huge page and instead falls back to using small pages. 541 542thp_fault_fallback_charge 543 is incremented if a page fault fails to charge a huge page and 544 instead falls back to using small pages even though the 545 allocation was successful. 546 547thp_collapse_alloc_failed 548 is incremented if khugepaged found a range 549 of pages that should be collapsed into one huge page but failed 550 the allocation. 551 552thp_file_alloc 553 is incremented every time a shmem huge page is successfully 554 allocated (Note that despite being named after "file", the counter 555 measures only shmem). 556 557thp_file_fallback 558 is incremented if a shmem huge page is attempted to be allocated 559 but fails and instead falls back to using small pages. (Note that 560 despite being named after "file", the counter measures only shmem). 561 562thp_file_fallback_charge 563 is incremented if a shmem huge page cannot be charged and instead 564 falls back to using small pages even though the allocation was 565 successful. (Note that despite being named after "file", the 566 counter measures only shmem). 567 568thp_file_mapped 569 is incremented every time a file or shmem huge page is mapped into 570 user address space. 571 572thp_split_page 573 is incremented every time a huge page is split into base 574 pages. This can happen for a variety of reasons but a common 575 reason is that a huge page is old and is being reclaimed. 576 This action implies splitting all PMD the page mapped with. 577 578thp_split_page_failed 579 is incremented if kernel fails to split huge 580 page. This can happen if the page was pinned by somebody. 581 582thp_deferred_split_page 583 is incremented when a huge page is put onto split 584 queue. This happens when a huge page is partially unmapped and 585 splitting it would free up some memory. Pages on split queue are 586 going to be split under memory pressure. 587 588thp_underused_split_page 589 is incremented when a huge page on the split queue was split 590 because it was underused. A THP is underused if the number of 591 zero pages in the THP is above a certain threshold 592 (/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none). 593 594thp_split_pmd 595 is incremented every time a PMD split into table of PTEs. 596 This can happen, for instance, when application calls mprotect() or 597 munmap() on part of huge page. It doesn't split huge page, only 598 page table entry. 599 600thp_zero_page_alloc 601 is incremented every time a huge zero page used for thp is 602 successfully allocated. Note, it doesn't count every map of 603 the huge zero page, only its allocation. 604 605thp_zero_page_alloc_failed 606 is incremented if kernel fails to allocate 607 huge zero page and falls back to using small pages. 608 609thp_swpout 610 is incremented every time a huge page is swapout in one 611 piece without splitting. 612 613thp_swpout_fallback 614 is incremented if a huge page has to be split before swapout. 615 Usually because failed to allocate some continuous swap space 616 for the huge page. 617 618In /sys/kernel/mm/transparent_hugepage/hugepages-<size>kB/stats, There are 619also individual counters for each huge page size, which can be utilized to 620monitor the system's effectiveness in providing huge pages for usage. Each 621counter has its own corresponding file. 622 623anon_fault_alloc 624 is incremented every time a huge page is successfully 625 allocated and charged to handle a page fault. 626 627anon_fault_fallback 628 is incremented if a page fault fails to allocate or charge 629 a huge page and instead falls back to using huge pages with 630 lower orders or small pages. 631 632anon_fault_fallback_charge 633 is incremented if a page fault fails to charge a huge page and 634 instead falls back to using huge pages with lower orders or 635 small pages even though the allocation was successful. 636 637zswpout 638 is incremented every time a huge page is swapped out to zswap in one 639 piece without splitting. 640 641swpin 642 is incremented every time a huge page is swapped in from a non-zswap 643 swap device in one piece. 644 645swpin_fallback 646 is incremented if swapin fails to allocate or charge a huge page 647 and instead falls back to using huge pages with lower orders or 648 small pages. 649 650swpin_fallback_charge 651 is incremented if swapin fails to charge a huge page and instead 652 falls back to using huge pages with lower orders or small pages 653 even though the allocation was successful. 654 655swpout 656 is incremented every time a huge page is swapped out to a non-zswap 657 swap device in one piece without splitting. 658 659swpout_fallback 660 is incremented if a huge page has to be split before swapout. 661 Usually because failed to allocate some continuous swap space 662 for the huge page. 663 664shmem_alloc 665 is incremented every time a shmem huge page is successfully 666 allocated. 667 668shmem_fallback 669 is incremented if a shmem huge page is attempted to be allocated 670 but fails and instead falls back to using small pages. 671 672shmem_fallback_charge 673 is incremented if a shmem huge page cannot be charged and instead 674 falls back to using small pages even though the allocation was 675 successful. 676 677split 678 is incremented every time a huge page is successfully split into 679 smaller orders. This can happen for a variety of reasons but a 680 common reason is that a huge page is old and is being reclaimed. 681 682split_failed 683 is incremented if kernel fails to split huge 684 page. This can happen if the page was pinned by somebody. 685 686split_deferred 687 is incremented when a huge page is put onto split queue. 688 This happens when a huge page is partially unmapped and splitting 689 it would free up some memory. Pages on split queue are going to 690 be split under memory pressure, if splitting is possible. 691 692nr_anon 693 the number of anonymous THP we have in the whole system. These THPs 694 might be currently entirely mapped or have partially unmapped/unused 695 subpages. 696 697nr_anon_partially_mapped 698 the number of anonymous THP which are likely partially mapped, possibly 699 wasting memory, and have been queued for deferred memory reclamation. 700 Note that in corner some cases (e.g., failed migration), we might detect 701 an anonymous THP as "partially mapped" and count it here, even though it 702 is not actually partially mapped anymore. 703 704As the system ages, allocating huge pages may be expensive as the 705system uses memory compaction to copy data around memory to free a 706huge page for use. There are some counters in ``/proc/vmstat`` to help 707monitor this overhead. 708 709compact_stall 710 is incremented every time a process stalls to run 711 memory compaction so that a huge page is free for use. 712 713compact_success 714 is incremented if the system compacted memory and 715 freed a huge page for use. 716 717compact_fail 718 is incremented if the system tries to compact memory 719 but failed. 720 721It is possible to establish how long the stalls were using the function 722tracer to record how long was spent in __alloc_pages() and 723using the mm_page_alloc tracepoint to identify which allocations were 724for huge pages. 725 726Optimizing the applications 727=========================== 728 729To be guaranteed that the kernel will map a THP immediately in any 730memory region, the mmap region has to be hugepage naturally 731aligned. posix_memalign() can provide that guarantee. 732 733Hugetlbfs 734========= 735 736You can use hugetlbfs on a kernel that has transparent hugepage 737support enabled just fine as always. No difference can be noted in 738hugetlbfs other than there will be less overall fragmentation. All 739usual features belonging to hugetlbfs are preserved and 740unaffected. libhugetlbfs will also work fine as usual. 741