docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge

			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>

   numa_memory_policy

   pagemap

   soft-dirty

   transhuge

   userfaultfd

.. _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.