* [PATCH 1/3] docs/vm: transhuge: change sections order
2018-05-14 8:13 [PATCH 0/3] docs/vm: transhuge: split userspace bits to admin-guide/mm Mike Rapoport
@ 2018-05-14 8:13 ` Mike Rapoport
2018-05-14 8:13 ` [PATCH 2/3] docs/vm: transhuge: minor updates Mike Rapoport
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: Mike Rapoport @ 2018-05-14 8:13 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: linux-doc, linux-mm, lkml, Mike Rapoport
so that userspace interface and implementation description will be grouped
together
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
Documentation/vm/transhuge.rst | 82 +++++++++++++++++++++---------------------
1 file changed, 41 insertions(+), 41 deletions(-)
diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst
index 2c6867f..56d04cbb 100644
--- a/Documentation/vm/transhuge.rst
+++ b/Documentation/vm/transhuge.rst
@@ -38,31 +38,6 @@ 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.
-Design
-======
-
-- "graceful fallback": mm components which don't have transparent hugepage
- knowledge fall back to breaking huge pmd mapping into table of ptes and,
- if necessary, split a transparent hugepage. Therefore these components
- can continue working on the regular pages or regular pte mappings.
-
-- if a hugepage allocation fails because of memory fragmentation,
- regular pages should be gracefully allocated instead and mixed in
- the same vma without any failure or significant delay and without
- userland noticing
-
-- if some task quits and more hugepages become available (either
- immediately in the buddy or through the VM), guest physical memory
- backed by regular pages should be relocated on hugepages
- automatically (with khugepaged)
-
-- it doesn't require memory reservation and in turn it uses hugepages
- whenever possible (the only possible reservation here is kernelcore=
- to avoid unmovable pages to fragment all the memory but such a tweak
- is not specific to transparent hugepage support and it's a generic
- feature that applies to all dynamic high order allocations in the
- kernel)
-
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
@@ -401,6 +376,47 @@ 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.
+
+Design principles
+=================
+
+- "graceful fallback": mm components which don't have transparent hugepage
+ knowledge fall back to breaking huge pmd mapping into table of ptes and,
+ if necessary, split a transparent hugepage. Therefore these components
+ can continue working on the regular pages or regular pte mappings.
+
+- if a hugepage allocation fails because of memory fragmentation,
+ regular pages should be gracefully allocated instead and mixed in
+ the same vma without any failure or significant delay and without
+ userland noticing
+
+- if some task quits and more hugepages become available (either
+ immediately in the buddy or through the VM), guest physical memory
+ backed by regular pages should be relocated on hugepages
+ automatically (with khugepaged)
+
+- it doesn't require memory reservation and in turn it uses hugepages
+ whenever possible (the only possible reservation here is kernelcore=
+ to avoid unmovable pages to fragment all the memory but such a tweak
+ is not specific to transparent hugepage support and it's a generic
+ feature that applies to all dynamic high order allocations in the
+ kernel)
+
get_user_pages and follow_page
==============================
@@ -432,22 +448,6 @@ hugepages being returned (as it's not only checking the pfn of the
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
-===========================
-
-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.
-
Graceful fallback
=================
--
2.7.4
^ permalink raw reply related [flat|nested] 5+ messages in thread* [PATCH 2/3] docs/vm: transhuge: minor updates
2018-05-14 8:13 [PATCH 0/3] docs/vm: transhuge: split userspace bits to admin-guide/mm Mike Rapoport
2018-05-14 8:13 ` [PATCH 1/3] docs/vm: transhuge: change sections order Mike Rapoport
@ 2018-05-14 8:13 ` Mike Rapoport
2018-05-14 8:13 ` [PATCH 3/3] docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge Mike Rapoport
2018-05-21 15:27 ` [PATCH 0/3] docs/vm: transhuge: split userspace bits to admin-guide/mm Jonathan Corbet
3 siblings, 0 replies; 5+ messages in thread
From: Mike Rapoport @ 2018-05-14 8:13 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: linux-doc, linux-mm, lkml, Mike Rapoport
Some formatting changes and addition of a sentence introducing khugepaged
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
Documentation/vm/transhuge.rst | 47 ++++++++++++++++++++++++++++++++----------
1 file changed, 36 insertions(+), 11 deletions(-)
diff --git a/Documentation/vm/transhuge.rst b/Documentation/vm/transhuge.rst
index 56d04cbb..47c7e47 100644
--- a/Documentation/vm/transhuge.rst
+++ b/Documentation/vm/transhuge.rst
@@ -9,14 +9,19 @@ 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 is an alternative means of
+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 it only works for anonymous memory mappings and tmpfs/shmem.
+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
@@ -28,15 +33,27 @@ 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) and 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.
+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
@@ -69,9 +86,14 @@ 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
@@ -142,6 +164,9 @@ 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
--
2.7.4
^ permalink raw reply related [flat|nested] 5+ messages in thread
* [PATCH 3/3] docs/vm: transhuge: split userspace bits to admin-guide/mm/transhuge
2018-05-14 8:13 [PATCH 0/3] docs/vm: transhuge: split userspace bits to admin-guide/mm Mike Rapoport
2018-05-14 8:13 ` [PATCH 1/3] docs/vm: transhuge: change sections order Mike Rapoport
2018-05-14 8:13 ` [PATCH 2/3] docs/vm: transhuge: minor updates Mike Rapoport
@ 2018-05-14 8:13 ` Mike Rapoport
2018-05-21 15:27 ` [PATCH 0/3] docs/vm: transhuge: split userspace bits to admin-guide/mm Jonathan Corbet
3 siblings, 0 replies; 5+ messages in thread
From: Mike Rapoport @ 2018-05-14 8:13 UTC (permalink / raw)
To: Jonathan Corbet; +Cc: linux-doc, linux-mm, lkml, Mike Rapoport
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +-
Documentation/admin-guide/mm/index.rst | 1 +
Documentation/admin-guide/mm/transhuge.rst | 418 ++++++++++++++++++++++++
Documentation/vm/transhuge.rst | 414 +----------------------
4 files changed, 423 insertions(+), 413 deletions(-)
create mode 100644 Documentation/admin-guide/mm/transhuge.rst
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 42f3e28..8d24270 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 a69aa69..8454be6 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 0000000..7ab93a8
--- /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 47c7e47..a8cf680 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
=================
--
2.7.4
^ permalink raw reply related [flat|nested] 5+ messages in thread