Based on kernel version 4.7.2. Page generated on 2016-08-22 22:48 EST.
1 = Transparent Hugepage Support = 2 3 == Objective == 4 5 Performance critical computing applications dealing with large memory 6 working sets are already running on top of libhugetlbfs and in turn 7 hugetlbfs. Transparent Hugepage Support is an alternative means of 8 using huge pages for the backing of virtual memory with huge pages 9 that supports the automatic promotion and demotion of page sizes and 10 without the shortcomings of hugetlbfs. 11 12 Currently it only works for anonymous memory mappings but in the 13 future it can expand over the pagecache layer starting with tmpfs. 14 15 The reason applications are running faster is because of two 16 factors. The first factor is almost completely irrelevant and it's not 17 of significant interest because it'll also have the downside of 18 requiring larger clear-page copy-page in page faults which is a 19 potentially negative effect. The first factor consists in taking a 20 single page fault for each 2M virtual region touched by userland (so 21 reducing the enter/exit kernel frequency by a 512 times factor). This 22 only matters the first time the memory is accessed for the lifetime of 23 a memory mapping. The second long lasting and much more important 24 factor will affect all subsequent accesses to the memory for the whole 25 runtime of the application. The second factor consist of two 26 components: 1) the TLB miss will run faster (especially with 27 virtualization using nested pagetables but almost always also on bare 28 metal without virtualization) and 2) a single TLB entry will be 29 mapping a much larger amount of virtual memory in turn reducing the 30 number of TLB misses. With virtualization and nested pagetables the 31 TLB can be mapped of larger size only if both KVM and the Linux guest 32 are using hugepages but a significant speedup already happens if only 33 one of the two is using hugepages just because of the fact the TLB 34 miss is going to run faster. 35 36 == Design == 37 38 - "graceful fallback": mm components which don't have transparent hugepage 39 knowledge fall back to breaking huge pmd mapping into table of ptes and, 40 if necessary, split a transparent hugepage. Therefore these components 41 can continue working on the regular pages or regular pte mappings. 42 43 - if a hugepage allocation fails because of memory fragmentation, 44 regular pages should be gracefully allocated instead and mixed in 45 the same vma without any failure or significant delay and without 46 userland noticing 47 48 - if some task quits and more hugepages become available (either 49 immediately in the buddy or through the VM), guest physical memory 50 backed by regular pages should be relocated on hugepages 51 automatically (with khugepaged) 52 53 - it doesn't require memory reservation and in turn it uses hugepages 54 whenever possible (the only possible reservation here is kernelcore= 55 to avoid unmovable pages to fragment all the memory but such a tweak 56 is not specific to transparent hugepage support and it's a generic 57 feature that applies to all dynamic high order allocations in the 58 kernel) 59 60 - this initial support only offers the feature in the anonymous memory 61 regions but it'd be ideal to move it to tmpfs and the pagecache 62 later 63 64 Transparent Hugepage Support maximizes the usefulness of free memory 65 if compared to the reservation approach of hugetlbfs by allowing all 66 unused memory to be used as cache or other movable (or even unmovable 67 entities). It doesn't require reservation to prevent hugepage 68 allocation failures to be noticeable from userland. It allows paging 69 and all other advanced VM features to be available on the 70 hugepages. It requires no modifications for applications to take 71 advantage of it. 72 73 Applications however can be further optimized to take advantage of 74 this feature, like for example they've been optimized before to avoid 75 a flood of mmap system calls for every malloc(4k). Optimizing userland 76 is by far not mandatory and khugepaged already can take care of long 77 lived page allocations even for hugepage unaware applications that 78 deals with large amounts of memory. 79 80 In certain cases when hugepages are enabled system wide, application 81 may end up allocating more memory resources. An application may mmap a 82 large region but only touch 1 byte of it, in that case a 2M page might 83 be allocated instead of a 4k page for no good. This is why it's 84 possible to disable hugepages system-wide and to only have them inside 85 MADV_HUGEPAGE madvise regions. 86 87 Embedded systems should enable hugepages only inside madvise regions 88 to eliminate any risk of wasting any precious byte of memory and to 89 only run faster. 90 91 Applications that gets a lot of benefit from hugepages and that don't 92 risk to lose memory by using hugepages, should use 93 madvise(MADV_HUGEPAGE) on their critical mmapped regions. 94 95 == sysfs == 96 97 Transparent Hugepage Support can be entirely disabled (mostly for 98 debugging purposes) or only enabled inside MADV_HUGEPAGE regions (to 99 avoid the risk of consuming more memory resources) or enabled system 100 wide. This can be achieved with one of: 101 102 echo always >/sys/kernel/mm/transparent_hugepage/enabled 103 echo madvise >/sys/kernel/mm/transparent_hugepage/enabled 104 echo never >/sys/kernel/mm/transparent_hugepage/enabled 105 106 It's also possible to limit defrag efforts in the VM to generate 107 hugepages in case they're not immediately free to madvise regions or 108 to never try to defrag memory and simply fallback to regular pages 109 unless hugepages are immediately available. Clearly if we spend CPU 110 time to defrag memory, we would expect to gain even more by the fact 111 we use hugepages later instead of regular pages. This isn't always 112 guaranteed, but it may be more likely in case the allocation is for a 113 MADV_HUGEPAGE region. 114 115 echo always >/sys/kernel/mm/transparent_hugepage/defrag 116 echo defer >/sys/kernel/mm/transparent_hugepage/defrag 117 echo madvise >/sys/kernel/mm/transparent_hugepage/defrag 118 echo never >/sys/kernel/mm/transparent_hugepage/defrag 119 120 "always" means that an application requesting THP will stall on allocation 121 failure and directly reclaim pages and compact memory in an effort to 122 allocate a THP immediately. This may be desirable for virtual machines 123 that benefit heavily from THP use and are willing to delay the VM start 124 to utilise them. 125 126 "defer" means that an application will wake kswapd in the background 127 to reclaim pages and wake kcompact to compact memory so that THP is 128 available in the near future. It's the responsibility of khugepaged 129 to then install the THP pages later. 130 131 "madvise" will enter direct reclaim like "always" but only for regions 132 that are have used madvise(MADV_HUGEPAGE). This is the default behaviour. 133 134 "never" should be self-explanatory. 135 136 By default kernel tries to use huge zero page on read page fault. 137 It's possible to disable huge zero page by writing 0 or enable it 138 back by writing 1: 139 140 echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page 141 echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page 142 143 khugepaged will be automatically started when 144 transparent_hugepage/enabled is set to "always" or "madvise, and it'll 145 be automatically shutdown if it's set to "never". 146 147 khugepaged runs usually at low frequency so while one may not want to 148 invoke defrag algorithms synchronously during the page faults, it 149 should be worth invoking defrag at least in khugepaged. However it's 150 also possible to disable defrag in khugepaged by writing 0 or enable 151 defrag in khugepaged by writing 1: 152 153 echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag 154 echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag 155 156 You can also control how many pages khugepaged should scan at each 157 pass: 158 159 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan 160 161 and how many milliseconds to wait in khugepaged between each pass (you 162 can set this to 0 to run khugepaged at 100% utilization of one core): 163 164 /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs 165 166 and how many milliseconds to wait in khugepaged if there's an hugepage 167 allocation failure to throttle the next allocation attempt. 168 169 /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs 170 171 The khugepaged progress can be seen in the number of pages collapsed: 172 173 /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed 174 175 for each pass: 176 177 /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans 178 179 max_ptes_none specifies how many extra small pages (that are 180 not already mapped) can be allocated when collapsing a group 181 of small pages into one large page. 182 183 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none 184 185 A higher value leads to use additional memory for programs. 186 A lower value leads to gain less thp performance. Value of 187 max_ptes_none can waste cpu time very little, you can 188 ignore it. 189 190 max_ptes_swap specifies how many pages can be brought in from 191 swap when collapsing a group of pages into a transparent huge page. 192 193 /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap 194 195 A higher value can cause excessive swap IO and waste 196 memory. A lower value can prevent THPs from being 197 collapsed, resulting fewer pages being collapsed into 198 THPs, and lower memory access performance. 199 200 == Boot parameter == 201 202 You can change the sysfs boot time defaults of Transparent Hugepage 203 Support by passing the parameter "transparent_hugepage=always" or 204 "transparent_hugepage=madvise" or "transparent_hugepage=never" 205 (without "") to the kernel command line. 206 207 == Need of application restart == 208 209 The transparent_hugepage/enabled values only affect future 210 behavior. So to make them effective you need to restart any 211 application that could have been using hugepages. This also applies to 212 the regions registered in khugepaged. 213 214 == Monitoring usage == 215 216 The number of transparent huge pages currently used by the system is 217 available by reading the AnonHugePages field in /proc/meminfo. To 218 identify what applications are using transparent huge pages, it is 219 necessary to read /proc/PID/smaps and count the AnonHugePages fields 220 for each mapping. Note that reading the smaps file is expensive and 221 reading it frequently will incur overhead. 222 223 There are a number of counters in /proc/vmstat that may be used to 224 monitor how successfully the system is providing huge pages for use. 225 226 thp_fault_alloc is incremented every time a huge page is successfully 227 allocated to handle a page fault. This applies to both the 228 first time a page is faulted and for COW faults. 229 230 thp_collapse_alloc is incremented by khugepaged when it has found 231 a range of pages to collapse into one huge page and has 232 successfully allocated a new huge page to store the data. 233 234 thp_fault_fallback is incremented if a page fault fails to allocate 235 a huge page and instead falls back to using small pages. 236 237 thp_collapse_alloc_failed is incremented if khugepaged found a range 238 of pages that should be collapsed into one huge page but failed 239 the allocation. 240 241 thp_split_page is incremented every time a huge page is split into base 242 pages. This can happen for a variety of reasons but a common 243 reason is that a huge page is old and is being reclaimed. 244 This action implies splitting all PMD the page mapped with. 245 246 thp_split_page_failed is is incremented if kernel fails to split huge 247 page. This can happen if the page was pinned by somebody. 248 249 thp_deferred_split_page is incremented when a huge page is put onto split 250 queue. This happens when a huge page is partially unmapped and 251 splitting it would free up some memory. Pages on split queue are 252 going to be split under memory pressure. 253 254 thp_split_pmd is incremented every time a PMD split into table of PTEs. 255 This can happen, for instance, when application calls mprotect() or 256 munmap() on part of huge page. It doesn't split huge page, only 257 page table entry. 258 259 thp_zero_page_alloc is incremented every time a huge zero page is 260 successfully allocated. It includes allocations which where 261 dropped due race with other allocation. Note, it doesn't count 262 every map of the huge zero page, only its allocation. 263 264 thp_zero_page_alloc_failed is incremented if kernel fails to allocate 265 huge zero page and falls back to using small pages. 266 267 As the system ages, allocating huge pages may be expensive as the 268 system uses memory compaction to copy data around memory to free a 269 huge page for use. There are some counters in /proc/vmstat to help 270 monitor this overhead. 271 272 compact_stall is incremented every time a process stalls to run 273 memory compaction so that a huge page is free for use. 274 275 compact_success is incremented if the system compacted memory and 276 freed a huge page for use. 277 278 compact_fail is incremented if the system tries to compact memory 279 but failed. 280 281 compact_pages_moved is incremented each time a page is moved. If 282 this value is increasing rapidly, it implies that the system 283 is copying a lot of data to satisfy the huge page allocation. 284 It is possible that the cost of copying exceeds any savings 285 from reduced TLB misses. 286 287 compact_pagemigrate_failed is incremented when the underlying mechanism 288 for moving a page failed. 289 290 compact_blocks_moved is incremented each time memory compaction examines 291 a huge page aligned range of pages. 292 293 It is possible to establish how long the stalls were using the function 294 tracer to record how long was spent in __alloc_pages_nodemask and 295 using the mm_page_alloc tracepoint to identify which allocations were 296 for huge pages. 297 298 == get_user_pages and follow_page == 299 300 get_user_pages and follow_page if run on a hugepage, will return the 301 head or tail pages as usual (exactly as they would do on 302 hugetlbfs). Most gup users will only care about the actual physical 303 address of the page and its temporary pinning to release after the I/O 304 is complete, so they won't ever notice the fact the page is huge. But 305 if any driver is going to mangle over the page structure of the tail 306 page (like for checking page->mapping or other bits that are relevant 307 for the head page and not the tail page), it should be updated to jump 308 to check head page instead. Taking reference on any head/tail page would 309 prevent page from being split by anyone. 310 311 NOTE: these aren't new constraints to the GUP API, and they match the 312 same constrains that applies to hugetlbfs too, so any driver capable 313 of handling GUP on hugetlbfs will also work fine on transparent 314 hugepage backed mappings. 315 316 In case you can't handle compound pages if they're returned by 317 follow_page, the FOLL_SPLIT bit can be specified as parameter to 318 follow_page, so that it will split the hugepages before returning 319 them. Migration for example passes FOLL_SPLIT as parameter to 320 follow_page because it's not hugepage aware and in fact it can't work 321 at all on hugetlbfs (but it instead works fine on transparent 322 hugepages thanks to FOLL_SPLIT). migration simply can't deal with 323 hugepages being returned (as it's not only checking the pfn of the 324 page and pinning it during the copy but it pretends to migrate the 325 memory in regular page sizes and with regular pte/pmd mappings). 326 327 == Optimizing the applications == 328 329 To be guaranteed that the kernel will map a 2M page immediately in any 330 memory region, the mmap region has to be hugepage naturally 331 aligned. posix_memalign() can provide that guarantee. 332 333 == Hugetlbfs == 334 335 You can use hugetlbfs on a kernel that has transparent hugepage 336 support enabled just fine as always. No difference can be noted in 337 hugetlbfs other than there will be less overall fragmentation. All 338 usual features belonging to hugetlbfs are preserved and 339 unaffected. libhugetlbfs will also work fine as usual. 340 341 == Graceful fallback == 342 343 Code walking pagetables but unaware about huge pmds can simply call 344 split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by 345 pmd_offset. It's trivial to make the code transparent hugepage aware 346 by just grepping for "pmd_offset" and adding split_huge_pmd where 347 missing after pmd_offset returns the pmd. Thanks to the graceful 348 fallback design, with a one liner change, you can avoid to write 349 hundred if not thousand of lines of complex code to make your code 350 hugepage aware. 351 352 If you're not walking pagetables but you run into a physical hugepage 353 but you can't handle it natively in your code, you can split it by 354 calling split_huge_page(page). This is what the Linux VM does before 355 it tries to swapout the hugepage for example. split_huge_page() can fail 356 if the page is pinned and you must handle this correctly. 357 358 Example to make mremap.c transparent hugepage aware with a one liner 359 change: 360 361 diff --git a/mm/mremap.c b/mm/mremap.c 362 --- a/mm/mremap.c 363 +++ b/mm/mremap.c 364 @@ -41,6 +41,7 @@ static pmd_t *get_old_pmd(struct mm_stru 365 return NULL; 366 367 pmd = pmd_offset(pud, addr); 368 + split_huge_pmd(vma, pmd, addr); 369 if (pmd_none_or_clear_bad(pmd)) 370 return NULL; 371 372 == Locking in hugepage aware code == 373 374 We want as much code as possible hugepage aware, as calling 375 split_huge_page() or split_huge_pmd() has a cost. 376 377 To make pagetable walks huge pmd aware, all you need to do is to call 378 pmd_trans_huge() on the pmd returned by pmd_offset. You must hold the 379 mmap_sem in read (or write) mode to be sure an huge pmd cannot be 380 created from under you by khugepaged (khugepaged collapse_huge_page 381 takes the mmap_sem in write mode in addition to the anon_vma lock). If 382 pmd_trans_huge returns false, you just fallback in the old code 383 paths. If instead pmd_trans_huge returns true, you have to take the 384 page table lock (pmd_lock()) and re-run pmd_trans_huge. Taking the 385 page table lock will prevent the huge pmd to be converted into a 386 regular pmd from under you (split_huge_pmd can run in parallel to the 387 pagetable walk). If the second pmd_trans_huge returns false, you 388 should just drop the page table lock and fallback to the old code as 389 before. Otherwise you can proceed to process the huge pmd and the 390 hugepage natively. Once finished you can drop the page table lock. 391 392 == Refcounts and transparent huge pages == 393 394 Refcounting on THP is mostly consistent with refcounting on other compound 395 pages: 396 397 - get_page()/put_page() and GUP operate in head page's ->_refcount. 398 399 - ->_refcount in tail pages is always zero: get_page_unless_zero() never 400 succeed on tail pages. 401 402 - map/unmap of the pages with PTE entry increment/decrement ->_mapcount 403 on relevant sub-page of the compound page. 404 405 - map/unmap of the whole compound page accounted in compound_mapcount 406 (stored in first tail page). 407 408 PageDoubleMap() indicates that ->_mapcount in all subpages is offset up by one. 409 This additional reference is required to get race-free detection of unmap of 410 subpages when we have them mapped with both PMDs and PTEs. 411 412 This is optimization required to lower overhead of per-subpage mapcount 413 tracking. The alternative is alter ->_mapcount in all subpages on each 414 map/unmap of the whole compound page. 415 416 We set PG_double_map when a PMD of the page got split for the first time, 417 but still have PMD mapping. The additional references go away with last 418 compound_mapcount. 419 420 split_huge_page internally has to distribute the refcounts in the head 421 page to the tail pages before clearing all PG_head/tail bits from the page 422 structures. It can be done easily for refcounts taken by page table 423 entries. But we don't have enough information on how to distribute any 424 additional pins (i.e. from get_user_pages). split_huge_page() fails any 425 requests to split pinned huge page: it expects page count to be equal to 426 sum of mapcount of all sub-pages plus one (split_huge_page caller must 427 have reference for head page). 428 429 split_huge_page uses migration entries to stabilize page->_refcount and 430 page->_mapcount. 431 432 We safe against physical memory scanners too: the only legitimate way 433 scanner can get reference to a page is get_page_unless_zero(). 434 435 All tail pages have zero ->_refcount until atomic_add(). This prevents the 436 scanner from getting a reference to the tail page up to that point. After the 437 atomic_add() we don't care about the ->_refcount value. We already known how 438 many references should be uncharged from the head page. 439 440 For head page get_page_unless_zero() will succeed and we don't mind. It's 441 clear where reference should go after split: it will stay on head page. 442 443 Note that split_huge_pmd() doesn't have any limitation on refcounting: 444 pmd can be split at any point and never fails. 445 446 == Partial unmap and deferred_split_huge_page() == 447 448 Unmapping part of THP (with munmap() or other way) is not going to free 449 memory immediately. Instead, we detect that a subpage of THP is not in use 450 in page_remove_rmap() and queue the THP for splitting if memory pressure 451 comes. Splitting will free up unused subpages. 452 453 Splitting the page right away is not an option due to locking context in 454 the place where we can detect partial unmap. It's also might be 455 counterproductive since in many cases partial unmap unmap happens during 456 exit(2) if an THP crosses VMA boundary. 457 458 Function deferred_split_huge_page() is used to queue page for splitting. 459 The splitting itself will happen when we get memory pressure via shrinker 460 interface.