diff options
| -rw-r--r-- | Documentation/admin-guide/kernel-parameters.txt | 3 | ||||
| -rw-r--r-- | Documentation/admin-guide/mm/index.rst | 1 | ||||
| -rw-r--r-- | Documentation/admin-guide/mm/transhuge.rst | 418 | ||||
| -rw-r--r-- | Documentation/vm/transhuge.rst | 414 |
4 files changed, 423 insertions, 413 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 42f3e2884e7c..8d24270644a1 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4313,7 +4313,8 @@ Format: [always|madvise|never] Can be used to control the default behavior of the system with respect to transparent hugepages. - See Documentation/vm/transhuge.rst for more details. + See Documentation/admin-guide/mm/transhuge.rst + for more details. tsc= Disable clocksource stability checks for TSC. Format: <string> diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst index a69aa69af255..8454be638108 100644 --- a/Documentation/admin-guide/mm/index.rst +++ b/Documentation/admin-guide/mm/index.rst @@ -27,4 +27,5 @@ the Linux memory management. numa_memory_policy pagemap soft-dirty + transhuge userfaultfd diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst new file mode 100644 index 000000000000..7ab93a8404b9 --- /dev/null +++ b/Documentation/admin-guide/mm/transhuge.rst @@ -0,0 +1,418 @@ +.. _admin_guide_transhuge: + +============================ +Transparent Hugepage Support +============================ + +Objective +========= + +Performance critical computing applications dealing with large memory +working sets are already running on top of libhugetlbfs and in turn +hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of +using huge pages for the backing of virtual memory with huge pages +that supports the automatic promotion and demotion of page sizes and +without the shortcomings of hugetlbfs. + +Currently THP only works for anonymous memory mappings and tmpfs/shmem. +But in the future it can expand to other filesystems. + +.. note:: + in the examples below we presume that the basic page size is 4K and + the huge page size is 2M, although the actual numbers may vary + depending on the CPU architecture. + +The reason applications are running faster is because of two +factors. The first factor is almost completely irrelevant and it's not +of significant interest because it'll also have the downside of +requiring larger clear-page copy-page in page faults which is a +potentially negative effect. The first factor consists in taking a +single page fault for each 2M virtual region touched by userland (so +reducing the enter/exit kernel frequency by a 512 times factor). This +only matters the first time the memory is accessed for the lifetime of +a memory mapping. The second long lasting and much more important +factor will affect all subsequent accesses to the memory for the whole +runtime of the application. The second factor consist of two +components: + +1) the TLB miss will run faster (especially with virtualization using + nested pagetables but almost always also on bare metal without + virtualization) + +2) a single TLB entry will be mapping a much larger amount of virtual + memory in turn reducing the number of TLB misses. With + virtualization and nested pagetables the TLB can be mapped of + larger size only if both KVM and the Linux guest are using + hugepages but a significant speedup already happens if only one of + the two is using hugepages just because of the fact the TLB miss is + going to run faster. + +THP can be enabled system wide or restricted to certain tasks or even +memory ranges inside task's address space. Unless THP is completely +disabled, there is ``khugepaged`` daemon that scans memory and +collapses sequences of basic pages into huge pages. + +The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` +interface and using madivse(2) and prctl(2) system calls. + +Transparent Hugepage Support maximizes the usefulness of free memory +if compared to the reservation approach of hugetlbfs by allowing all +unused memory to be used as cache or other movable (or even unmovable +entities). It doesn't require reservation to prevent hugepage +allocation failures to be noticeable from userland. It allows paging +and all other advanced VM features to be available on the +hugepages. It requires no modifications for applications to take +advantage of it. + +Applications however can be further optimized to take advantage of +this feature, like for example they've been optimized before to avoid +a flood of mmap system calls for every malloc(4k). Optimizing userland +is by far not mandatory and khugepaged already can take care of long +lived page allocations even for hugepage unaware applications that +deals with large amounts of memory. + +In certain cases when hugepages are enabled system wide, application +may end up allocating more memory resources. An application may mmap a +large region but only touch 1 byte of it, in that case a 2M page might +be allocated instead of a 4k page for no good. This is why it's +possible to disable hugepages system-wide and to only have them inside +MADV_HUGEPAGE madvise regions. + +Embedded systems should enable hugepages only inside madvise regions +to eliminate any risk of wasting any precious byte of memory and to +only run faster. + +Applications that gets a lot of benefit from hugepages and that don't +risk to lose memory by using hugepages, should use +madvise(MADV_HUGEPAGE) on their critical mmapped regions. + +.. _thp_sysfs: + +sysfs +===== + +Global THP controls +------------------- + +Transparent Hugepage Support for anonymous memory can be entirely disabled +(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE +regions (to avoid the risk of consuming more memory resources) or enabled +system wide. This can be achieved with one of:: + + echo always >/sys/kernel/mm/transparent_hugepage/enabled + echo madvise >/sys/kernel/mm/transparent_hugepage/enabled + echo never >/sys/kernel/mm/transparent_hugepage/enabled + +It's also possible to limit defrag efforts in the VM to generate +anonymous hugepages in case they're not immediately free to madvise +regions or to never try to defrag memory and simply fallback to regular +pages unless hugepages are immediately available. Clearly if we spend CPU +time to defrag memory, we would expect to gain even more by the fact we +use hugepages later instead of regular pages. This isn't always +guaranteed, but it may be more likely in case the allocation is for a +MADV_HUGEPAGE region. + +:: + + echo always >/sys/kernel/mm/transparent_hugepage/defrag + echo defer >/sys/kernel/mm/transparent_hugepage/defrag + echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag + echo madvise >/sys/kernel/mm/transparent_hugepage/defrag + echo never >/sys/kernel/mm/transparent_hugepage/defrag + +always + means that an application requesting THP will stall on + allocation failure and directly reclaim pages and compact + memory in an effort to allocate a THP immediately. This may be + desirable for virtual machines that benefit heavily from THP + use and are willing to delay the VM start to utilise them. + +defer + means that an application will wake kswapd in the background + to reclaim pages and wake kcompactd to compact memory so that + THP is available in the near future. It's the responsibility + of khugepaged to then install the THP pages later. + +defer+madvise + will enter direct reclaim and compaction like ``always``, but + only for regions that have used madvise(MADV_HUGEPAGE); all + other regions will wake kswapd in the background to reclaim + pages and wake kcompactd to compact memory so that THP is + available in the near future. + +madvise + will enter direct reclaim like ``always`` but only for regions + that are have used madvise(MADV_HUGEPAGE). This is the default + behaviour. + +never + should be self-explanatory. + +By default kernel tries to use huge zero page on read page fault to +anonymous mapping. It's possible to disable huge zero page by writing 0 +or enable it back by writing 1:: + + echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page + echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page + +Some userspace (such as a test program, or an optimized memory allocation +library) may want to know the size (in bytes) of a transparent hugepage:: + + cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size + +khugepaged will be automatically started when +transparent_hugepage/enabled is set to "always" or "madvise, and it'll +be automatically shutdown if it's set to "never". + +Khugepaged controls +------------------- + +khugepaged runs usually at low frequency so while one may not want to +invoke defrag algorithms synchronously during the page faults, it +should be worth invoking defrag at least in khugepaged. However it's +also possible to disable defrag in khugepaged by writing 0 or enable +defrag in khugepaged by writing 1:: + + echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag + echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag + +You can also control how many pages khugepaged should scan at each +pass:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan + +and how many milliseconds to wait in khugepaged between each pass (you +can set this to 0 to run khugepaged at 100% utilization of one core):: + + /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs + +and how many milliseconds to wait in khugepaged if there's an hugepage +allocation failure to throttle the next allocation attempt:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs + +The khugepaged progress can be seen in the number of pages collapsed:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed + +for each pass:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans + +``max_ptes_none`` specifies how many extra small pages (that are +not already mapped) can be allocated when collapsing a group +of small pages into one large page:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none + +A higher value leads to use additional memory for programs. +A lower value leads to gain less thp performance. Value of +max_ptes_none can waste cpu time very little, you can +ignore it. + +``max_ptes_swap`` specifies how many pages can be brought in from +swap when collapsing a group of pages into a transparent huge page:: + + /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap + +A higher value can cause excessive swap IO and waste +memory. A lower value can prevent THPs from being +collapsed, resulting fewer pages being collapsed into +THPs, and lower memory access performance. + +Boot parameter +============== + +You can change the sysfs boot time defaults of Transparent Hugepage +Support by passing the parameter ``transparent_hugepage=always`` or +``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` +to the kernel command line. + +Hugepages in tmpfs/shmem +======================== + +You can control hugepage allocation policy in tmpfs with mount option +``huge=``. It can have following values: + +always + Attempt to allocate huge pages every time we need a new page; + +never + Do not allocate huge pages; + +within_size + Only allocate huge page if it will be fully within i_size. + Also respect fadvise()/madvise() hints; + +advise + Only allocate huge pages if requested with fadvise()/madvise(); + +The default policy is ``never``. + +``mount -o remount,huge= /mountpoint`` works fine after mount: remounting +``huge=never`` will not attempt to break up huge pages at all, just stop more +from being allocated. + +There's also sysfs knob to control hugepage allocation policy for internal +shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount +is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or +MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem. + +In addition to policies listed above, shmem_enabled allows two further +values: + +deny + For use in emergencies, to force the huge option off from + all mounts; +force + Force the huge option on for all - very useful for testing; + +Need of application restart +=========================== + +The transparent_hugepage/enabled values and tmpfs mount option only affect +future behavior. So to make them effective you need to restart any +application that could have been using hugepages. This also applies to the +regions registered in khugepaged. + +Monitoring usage +================ + +The number of anonymous transparent huge pages currently used by the +system is available by reading the AnonHugePages field in ``/proc/meminfo``. +To identify what applications are using anonymous transparent huge pages, +it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages fields +for each mapping. + +The number of file transparent huge pages mapped to userspace is available +by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``. +To identify what applications are mapping file transparent huge pages, it +is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields +for each mapping. + +Note that reading the smaps file is expensive and reading it +frequently will incur overhead. + +There are a number of counters in ``/proc/vmstat`` that may be used to +monitor how successfully the system is providing huge pages for use. + +thp_fault_alloc + is incremented every time a huge page is successfully + allocated to handle a page fault. This applies to both the + first time a page is faulted and for COW faults. + +thp_collapse_alloc + is incremented by khugepaged when it has found + a range of pages to collapse into one huge page and has + successfully allocated a new huge page to store the data. + +thp_fault_fallback + is incremented if a page fault fails to allocate + a huge page and instead falls back to using small pages. + +thp_collapse_alloc_failed + is incremented if khugepaged found a range + of pages that should be collapsed into one huge page but failed + the allocation. + +thp_file_alloc + is incremented every time a file huge page is successfully + allocated. + +thp_file_mapped + is incremented every time a file huge page is mapped into + user address space. + +thp_split_page + is incremented every time a huge page is split into base + pages. This can happen for a variety of reasons but a common + reason is that a huge page is old and is being reclaimed. + This action implies splitting all PMD the page mapped with. + +thp_split_page_failed + is incremented if kernel fails to split huge + page. This can happen if the page was pinned by somebody. + +thp_deferred_split_page + is incremented when a huge page is put onto split + queue. This happens when a huge page is partially unmapped and + splitting it would free up some memory. Pages on split queue are + going to be split under memory pressure. + +thp_split_pmd + is incremented every time a PMD split into table of PTEs. + This can happen, for instance, when application calls mprotect() or + munmap() on part of huge page. It doesn't split huge page, only + page table entry. + +thp_zero_page_alloc + is incremented every time a huge zero page is + successfully allocated. It includes allocations which where + dropped due race with other allocation. Note, it doesn't count + every map of the huge zero page, only its allocation. + +thp_zero_page_alloc_failed + is incremented if kernel fails to allocate + huge zero page and falls back to using small pages. + +thp_swpout + is incremented every time a huge page is swapout in one + piece without splitting. + +thp_swpout_fallback + is incremented if a huge page has to be split before swapout. + Usually because failed to allocate some continuous swap space + for the huge page. + +As the system ages, allocating huge pages may be expensive as the +system uses memory compaction to copy data around memory to free a +huge page for use. There are some counters in ``/proc/vmstat`` to help +monitor this overhead. + +compact_stall + is incremented every time a process stalls to run + memory compaction so that a huge page is free for use. + +compact_success + is incremented if the system compacted memory and + freed a huge page for use. + +compact_fail + is incremented if the system tries to compact memory + but failed. + +compact_pages_moved + is incremented each time a page is moved. If + this value is increasing rapidly, it implies that the system + is copying a lot of data to satisfy the huge page allocation. + It is possible that the cost of copying exceeds any savings + from reduced TLB misses. + +compact_pagemigrate_failed + is incremented when the underlying mechanism + for moving a page failed. + +compact_blocks_moved + is incremented each time memory compaction examines + a huge page aligned range of pages. + +It is possible to establish how long the stalls were using the function +tracer to record how long was spent in __alloc_pages_nodemask and +using the mm_page_alloc tracepoint to identify which allocations were +for huge pages. + +Optimizing the applications +=========================== + +To be guaranteed that the kernel will map a 2M page immediately in any +memory region, the mmap region has to be hugepage naturally +aligned. posix_memalign() can provide that guarantee. + +Hugetlbfs +========= + +You can use hugetlbfs on a kernel that has transparent hugepage +support enabled just fine as always. No difference can be noted in +hugetlbfs other than there will be less overall fragmentation. All +usual features belonging to hugetlbfs are preserved and +unaffected. libhugetlbfs will also work fine as usual. diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst index 47c7e4742bc2..a8cf6809e36e 100644 --- a/Documentation/vm/transhuge.rst +++ b/Documentation/vm/transhuge.rst @@ -4,418 +4,8 @@ Transparent Hugepage Support ============================ -Objective -========= - -Performance critical computing applications dealing with large memory -working sets are already running on top of libhugetlbfs and in turn -hugetlbfs. Transparent HugePage Support (THP) is an alternative mean of -using huge pages for the backing of virtual memory with huge pages -that supports the automatic promotion and demotion of page sizes and -without the shortcomings of hugetlbfs. - -Currently THP only works for anonymous memory mappings and tmpfs/shmem. -But in the future it can expand to other filesystems. - -.. note:: - in the examples below we presume that the basic page size is 4K and - the huge page size is 2M, although the actual numbers may vary - depending on the CPU architecture. - -The reason applications are running faster is because of two -factors. The first factor is almost completely irrelevant and it's not -of significant interest because it'll also have the downside of -requiring larger clear-page copy-page in page faults which is a -potentially negative effect. The first factor consists in taking a -single page fault for each 2M virtual region touched by userland (so -reducing the enter/exit kernel frequency by a 512 times factor). This -only matters the first time the memory is accessed for the lifetime of -a memory mapping. The second long lasting and much more important -factor will affect all subsequent accesses to the memory for the whole -runtime of the application. The second factor consist of two -components: - -1) the TLB miss will run faster (especially with virtualization using - nested pagetables but almost always also on bare metal without - virtualization) - -2) a single TLB entry will be mapping a much larger amount of virtual - memory in turn reducing the number of TLB misses. With - virtualization and nested pagetables the TLB can be mapped of - larger size only if both KVM and the Linux guest are using - hugepages but a significant speedup already happens if only one of - the two is using hugepages just because of the fact the TLB miss is - going to run faster. - -THP can be enabled system wide or restricted to certain tasks or even -memory ranges inside task's address space. Unless THP is completely -disabled, there is ``khugepaged`` daemon that scans memory and -collapses sequences of basic pages into huge pages. - -The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` -interface and using madivse(2) and prctl(2) system calls. - -Transparent Hugepage Support maximizes the usefulness of free memory -if compared to the reservation approach of hugetlbfs by allowing all -unused memory to be used as cache or other movable (or even unmovable -entities). It doesn't require reservation to prevent hugepage -allocation failures to be noticeable from userland. It allows paging -and all other advanced VM features to be available on the -hugepages. It requires no modifications for applications to take -advantage of it. - -Applications however can be further optimized to take advantage of -this feature, like for example they've been optimized before to avoid -a flood of mmap system calls for every malloc(4k). Optimizing userland -is by far not mandatory and khugepaged already can take care of long -lived page allocations even for hugepage unaware applications that -deals with large amounts of memory. - -In certain cases when hugepages are enabled system wide, application -may end up allocating more memory resources. An application may mmap a -large region but only touch 1 byte of it, in that case a 2M page might -be allocated instead of a 4k page for no good. This is why it's -possible to disable hugepages system-wide and to only have them inside -MADV_HUGEPAGE madvise regions. - -Embedded systems should enable hugepages only inside madvise regions -to eliminate any risk of wasting any precious byte of memory and to -only run faster. - -Applications that gets a lot of benefit from hugepages and that don't -risk to lose memory by using hugepages, should use -madvise(MADV_HUGEPAGE) on their critical mmapped regions. - -.. _thp_sysfs: - -sysfs -===== - -Global THP controls -------------------- - -Transparent Hugepage Support for anonymous memory can be entirely disabled -(mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE -regions (to avoid the risk of consuming more memory resources) or enabled -system wide. This can be achieved with one of:: - - echo always >/sys/kernel/mm/transparent_hugepage/enabled - echo madvise >/sys/kernel/mm/transparent_hugepage/enabled - echo never >/sys/kernel/mm/transparent_hugepage/enabled - -It's also possible to limit defrag efforts in the VM to generate -anonymous hugepages in case they're not immediately free to madvise -regions or to never try to defrag memory and simply fallback to regular -pages unless hugepages are immediately available. Clearly if we spend CPU -time to defrag memory, we would expect to gain even more by the fact we -use hugepages later instead of regular pages. This isn't always -guaranteed, but it may be more likely in case the allocation is for a -MADV_HUGEPAGE region. - -:: - - echo always >/sys/kernel/mm/transparent_hugepage/defrag - echo defer >/sys/kernel/mm/transparent_hugepage/defrag - echo defer+madvise >/sys/kernel/mm/transparent_hugepage/defrag - echo madvise >/sys/kernel/mm/transparent_hugepage/defrag - echo never >/sys/kernel/mm/transparent_hugepage/defrag - -always - means that an application requesting THP will stall on - allocation failure and directly reclaim pages and compact - memory in an effort to allocate a THP immediately. This may be - desirable for virtual machines that benefit heavily from THP - use and are willing to delay the VM start to utilise them. - -defer - means that an application will wake kswapd in the background - to reclaim pages and wake kcompactd to compact memory so that - THP is available in the near future. It's the responsibility - of khugepaged to then install the THP pages later. - -defer+madvise - will enter direct reclaim and compaction like ``always``, but - only for regions that have used madvise(MADV_HUGEPAGE); all - other regions will wake kswapd in the background to reclaim - pages and wake kcompactd to compact memory so that THP is - available in the near future. - -madvise - will enter direct reclaim like ``always`` but only for regions - that are have used madvise(MADV_HUGEPAGE). This is the default - behaviour. - -never - should be self-explanatory. - -By default kernel tries to use huge zero page on read page fault to -anonymous mapping. It's possible to disable huge zero page by writing 0 -or enable it back by writing 1:: - - echo 0 >/sys/kernel/mm/transparent_hugepage/use_zero_page - echo 1 >/sys/kernel/mm/transparent_hugepage/use_zero_page - -Some userspace (such as a test program, or an optimized memory allocation -library) may want to know the size (in bytes) of a transparent hugepage:: - - cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size - -khugepaged will be automatically started when -transparent_hugepage/enabled is set to "always" or "madvise, and it'll -be automatically shutdown if it's set to "never". - -Khugepaged controls -------------------- - -khugepaged runs usually at low frequency so while one may not want to -invoke defrag algorithms synchronously during the page faults, it -should be worth invoking defrag at least in khugepaged. However it's -also possible to disable defrag in khugepaged by writing 0 or enable -defrag in khugepaged by writing 1:: - - echo 0 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag - echo 1 >/sys/kernel/mm/transparent_hugepage/khugepaged/defrag - -You can also control how many pages khugepaged should scan at each -pass:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/pages_to_scan - -and how many milliseconds to wait in khugepaged between each pass (you -can set this to 0 to run khugepaged at 100% utilization of one core):: - - /sys/kernel/mm/transparent_hugepage/khugepaged/scan_sleep_millisecs - -and how many milliseconds to wait in khugepaged if there's an hugepage -allocation failure to throttle the next allocation attempt:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/alloc_sleep_millisecs - -The khugepaged progress can be seen in the number of pages collapsed:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed - -for each pass:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/full_scans - -``max_ptes_none`` specifies how many extra small pages (that are -not already mapped) can be allocated when collapsing a group -of small pages into one large page:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none - -A higher value leads to use additional memory for programs. -A lower value leads to gain less thp performance. Value of -max_ptes_none can waste cpu time very little, you can -ignore it. - -``max_ptes_swap`` specifies how many pages can be brought in from -swap when collapsing a group of pages into a transparent huge page:: - - /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_swap - -A higher value can cause excessive swap IO and waste -memory. A lower value can prevent THPs from being -collapsed, resulting fewer pages being collapsed into -THPs, and lower memory access performance. - -Boot parameter -============== - -You can change the sysfs boot time defaults of Transparent Hugepage -Support by passing the parameter ``transparent_hugepage=always`` or -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` -to the kernel command line. - -Hugepages in tmpfs/shmem -======================== - -You can control hugepage allocation policy in tmpfs with mount option -``huge=``. It can have following values: - -always - Attempt to allocate huge pages every time we need a new page; - -never - Do not allocate huge pages; - -within_size - Only allocate huge page if it will be fully within i_size. - Also respect fadvise()/madvise() hints; - -advise - Only allocate huge pages if requested with fadvise()/madvise(); - -The default policy is ``never``. - -``mount -o remount,huge= /mountpoint`` works fine after mount: remounting -``huge=never`` will not attempt to break up huge pages at all, just stop more -from being allocated. - -There's also sysfs knob to control hugepage allocation policy for internal -shmem mount: /sys/kernel/mm/transparent_hugepage/shmem_enabled. The mount -is used for SysV SHM, memfds, shared anonymous mmaps (of /dev/zero or -MAP_ANONYMOUS), GPU drivers' DRM objects, Ashmem. - -In addition to policies listed above, shmem_enabled allows two further -values: - -deny - For use in emergencies, to force the huge option off from - all mounts; -force - Force the huge option on for all - very useful for testing; - -Need of application restart -=========================== - -The transparent_hugepage/enabled values and tmpfs mount option only affect -future behavior. So to make them effective you need to restart any -application that could have been using hugepages. This also applies to the -regions registered in khugepaged. - -Monitoring usage -================ - -The number of anonymous transparent huge pages currently used by the -system is available by reading the AnonHugePages field in ``/proc/meminfo``. -To identify what applications are using anonymous transparent huge pages, -it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages fields -for each mapping. - -The number of file transparent huge pages mapped to userspace is available -by reading ShmemPmdMapped and ShmemHugePages fields in ``/proc/meminfo``. -To identify what applications are mapping file transparent huge pages, it -is necessary to read ``/proc/PID/smaps`` and count the FileHugeMapped fields -for each mapping. - -Note that reading the smaps file is expensive and reading it -frequently will incur overhead. - -There are a number of counters in ``/proc/vmstat`` that may be used to -monitor how successfully the system is providing huge pages for use. - -thp_fault_alloc - is incremented every time a huge page is successfully - allocated to handle a page fault. This applies to both the - first time a page is faulted and for COW faults. - -thp_collapse_alloc - is incremented by khugepaged when it has found - a range of pages to collapse into one huge page and has - successfully allocated a new huge page to store the data. - -thp_fault_fallback - is incremented if a page fault fails to allocate - a huge page and instead falls back to using small pages. - -thp_collapse_alloc_failed - is incremented if khugepaged found a range - of pages that should be collapsed into one huge page but failed - the allocation. - -thp_file_alloc - is incremented every time a file huge page is successfully - allocated. - -thp_file_mapped - is incremented every time a file huge page is mapped into - user address space. - -thp_split_page - is incremented every time a huge page is split into base - pages. This can happen for a variety of reasons but a common - reason is that a huge page is old and is being reclaimed. - This action implies splitting all PMD the page mapped with. - -thp_split_page_failed - is incremented if kernel fails to split huge - page. This can happen if the page was pinned by somebody. - -thp_deferred_split_page - is incremented when a huge page is put onto split - queue. This happens when a huge page is partially unmapped and - splitting it would free up some memory. Pages on split queue are - going to be split under memory pressure. - -thp_split_pmd - is incremented every time a PMD split into table of PTEs. - This can happen, for instance, when application calls mprotect() or - munmap() on part of huge page. It doesn't split huge page, only - page table entry. - -thp_zero_page_alloc - is incremented every time a huge zero page is - successfully allocated. It includes allocations which where - dropped due race with other allocation. Note, it doesn't count - every map of the huge zero page, only its allocation. - -thp_zero_page_alloc_failed - is incremented if kernel fails to allocate - huge zero page and falls back to using small pages. - -thp_swpout - is incremented every time a huge page is swapout in one - piece without splitting. - -thp_swpout_fallback - is incremented if a huge page has to be split before swapout. - Usually because failed to allocate some continuous swap space - for the huge page. - -As the system ages, allocating huge pages may be expensive as the -system uses memory compaction to copy data around memory to free a -huge page for use. There are some counters in ``/proc/vmstat`` to help -monitor this overhead. - -compact_stall - is incremented every time a process stalls to run - memory compaction so that a huge page is free for use. - -compact_success - is incremented if the system compacted memory and - freed a huge page for use. - -compact_fail - is incremented if the system tries to compact memory - but failed. - -compact_pages_moved - is incremented each time a page is moved. If - this value is increasing rapidly, it implies that the system - is copying a lot of data to satisfy the huge page allocation. - It is possible that the cost of copying exceeds any savings - from reduced TLB misses. - -compact_pagemigrate_failed - is incremented when the underlying mechanism - for moving a page failed. - -compact_blocks_moved - is incremented each time memory compaction examines - a huge page aligned range of pages. - -It is possible to establish how long the stalls were using the function -tracer to record how long was spent in __alloc_pages_nodemask and -using the mm_page_alloc tracepoint to identify which allocations were -for huge pages. - -Optimizing the applications -=========================== - -To be guaranteed that the kernel will map a 2M page immediately in any -memory region, the mmap region has to be hugepage naturally -aligned. posix_memalign() can provide that guarantee. - -Hugetlbfs -========= - -You can use hugetlbfs on a kernel that has transparent hugepage -support enabled just fine as always. No difference can be noted in -hugetlbfs other than there will be less overall fragmentation. All -usual features belonging to hugetlbfs are preserved and -unaffected. libhugetlbfs will also work fine as usual. +This document describes design principles Transparent Hugepage (THP) +Support and its interaction with other parts of the memory management. Design principles ================= |