docs/vm: transhuge.txt: convert to ReST format

= Transparent Hugepage Support =

.. _transhuge:

== Objective ==

============================

Transparent Hugepage Support

============================

Objective

=========

Performance critical computing applications dealing with large memory

working sets are already running on top of libhugetlbfs and in turn

one of the two is using hugepages just because of the fact the TLB

miss is going to run faster.

== Design ==

Design

======

- "graceful fallback": mm components which don't have transparent hugepage

  knowledge fall back to breaking huge pmd mapping into table of ptes and,

risk to lose memory by using hugepages, should use

madvise(MADV_HUGEPAGE) on their critical mmapped regions.

== sysfs ==

sysfs

=====

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:

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

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.

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:

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:

library) may want to know the size (in bytes) of a transparent hugepage::

	cat /sys/kernel/mm/transparent_hugepage/hpage_pmd_size

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:

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:

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):

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.

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:

The khugepaged progress can be seen in the number of pages collapsed::

	/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed

for each pass:

for each pass::

	/sys/kernel/mm/transparent_hugepage/khugepaged/full_scans

max_ptes_none specifies how many extra small pages (that are

``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.

of small pages into one large page::

	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none

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.

``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

collapsed, resulting fewer pages being collapsed into

THPs, and lower memory access performance.

== Boot parameter ==

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"

(without "") to the kernel command line.

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 ==

Hugepages in tmpfs/shmem

========================

You can control hugepage allocation policy in tmpfs with mount option

"huge=". It can have following values:

``huge=``. It can have following values:

  - "always":

always

    Attempt to allocate huge pages every time we need a new page;

  - "never":

never

    Do not allocate huge pages;

  - "within_size":

within_size

    Only allocate huge page if it will be fully within i_size.

    Also respect fadvise()/madvise() hints;

  - "advise:

advise

    Only allocate huge pages if requested with fadvise()/madvise();

The default policy is "never".

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

``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

In addition to policies listed above, shmem_enabled allows two further

values:

  - "deny":

deny

    For use in emergencies, to force the huge option off from

    all mounts;

  - "force":

force

    Force the huge option on for all - very useful for testing;

== Need of application restart ==

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 ==

Monitoring usage

================

The number of anonymous transparent huge pages currently used by the

system is available by reading the AnonHugePages field in /proc/meminfo.

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

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.

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

thp_zero_page_alloc_failed

	is incremented if kernel fails to allocate

	huge zero page and falls back to using small pages.

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

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

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

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

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

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

compact_pagemigrate_failed

	is incremented when the underlying mechanism

	for moving a page failed.

compact_blocks_moved is incremented each time memory compaction examines

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

using the mm_page_alloc tracepoint to identify which allocations were

for huge pages.

== get_user_pages and follow_page ==

get_user_pages and follow_page

==============================

get_user_pages and follow_page if run on a hugepage, will return the

head or tail pages as usual (exactly as they would do on

to check head page instead. Taking reference on any head/tail page would

prevent page from being split by anyone.

NOTE: these aren't new constraints to the GUP API, and they match the

.. note::

   these aren't new constraints to the GUP API, and they match the

   same constrains that applies to hugetlbfs too, so any driver capable

   of handling GUP on hugetlbfs will also work fine on transparent

   hugepage backed mappings.

page and pinning it during the copy but it pretends to migrate the

memory in regular page sizes and with regular pte/pmd mappings).

== Optimizing the applications ==

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 ==

Hugetlbfs

=========

You can use hugetlbfs on a kernel that has transparent hugepage

support enabled just fine as always. No difference can be noted in

usual features belonging to hugetlbfs are preserved and

unaffected. libhugetlbfs will also work fine as usual.

== Graceful fallback ==

Graceful fallback

=================

Code walking pagetables but unaware about huge pmds can simply call

split_huge_pmd(vma, pmd, addr) where the pmd is the one returned by

if the page is pinned and you must handle this correctly.

Example to make mremap.c transparent hugepage aware with a one liner

change:

change::

	diff --git a/mm/mremap.c b/mm/mremap.c

	--- a/mm/mremap.c

		if (pmd_none_or_clear_bad(pmd))

			return NULL;

== Locking in hugepage aware code ==

Locking in hugepage aware code

==============================

We want as much code as possible hugepage aware, as calling

split_huge_page() or split_huge_pmd() has a cost.

before. Otherwise you can proceed to process the huge pmd and the

hugepage natively. Once finished you can drop the page table lock.

== Refcounts and transparent huge pages ==

Refcounts and transparent huge pages

====================================

Refcounting on THP is mostly consistent with refcounting on other compound

pages:

Note that split_huge_pmd() doesn't have any limitation on refcounting:

pmd can be split at any point and never fails.

== Partial unmap and deferred_split_huge_page() ==

Partial unmap and deferred_split_huge_page()

============================================

Unmapping part of THP (with munmap() or other way) is not going to free

memory immediately. Instead, we detect that a subpage of THP is not in use