admin-guide/mm/transhuge.rst

.. _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 madvise(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.