Hi,
These patches extend KSM documentation with high level design overview and
some details about reverse mappings and split the userspace interface
description to Documentation/admin-guide/mm.
The description of some KSM sysfs attributes is changed so that it won't
include implementation detail. The description of these implementation
details are moved to the new "Design" section.
The last patch in the series depends on the patchset that create
Documentation/admin-guide/mm [1], all the rest applies cleanly to the
current docs-next.
[1] https://lkml.org/lkml/2018/4/18/110
Mike Rapoport (7):
mm/ksm: docs: extend overview comment and make it "DOC:"
docs/vm: ksm: (mostly) formatting updates
docs/vm: ksm: add "Design" section
docs/vm: ksm: reshuffle text between "sysfs" and "design" sections
docs/vm: ksm: update stable_node_chains_prune_millisecs description
docs/vm: ksm: udpate description of stable_node_{dups,chains}
docs/vm: ksm: split userspace interface to admin-guide/mm/ksm.rst
Documentation/admin-guide/mm/index.rst | 1 +
Documentation/admin-guide/mm/ksm.rst | 189 ++++++++++++++++++++++++++
Documentation/vm/ksm.rst | 234 ++++++++++-----------------------
mm/ksm.c | 19 ++-
4 files changed, 277 insertions(+), 166 deletions(-)
create mode 100644 Documentation/admin-guide/mm/ksm.rst
--
2.7.4
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/admin-guide/mm/index.rst | 1 +
Documentation/admin-guide/mm/ksm.rst | 189 +++++++++++++++++++++++++++++++++
Documentation/vm/ksm.rst | 176 +-----------------------------
3 files changed, 191 insertions(+), 175 deletions(-)
create mode 100644 Documentation/admin-guide/mm/ksm.rst
diff --git a/Documentation/admin-guide/mm/index.rst b/Documentation/admin-guide/mm/index.rst
index 6c8b554..ad28644 100644
--- a/Documentation/admin-guide/mm/index.rst
+++ b/Documentation/admin-guide/mm/index.rst
@@ -23,6 +23,7 @@ the Linux memory management.
hugetlbpage
idle_page_tracking
+ ksm
pagemap
soft-dirty
userfaultfd
diff --git a/Documentation/admin-guide/mm/ksm.rst b/Documentation/admin-guide/mm/ksm.rst
new file mode 100644
index 0000000..9303786
--- /dev/null
+++ b/Documentation/admin-guide/mm/ksm.rst
@@ -0,0 +1,189 @@
+.. _admin_guide_ksm:
+
+=======================
+Kernel Samepage Merging
+=======================
+
+Overview
+========
+
+KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
+added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
+and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
+
+KSM was originally developed for use with KVM (where it was known as
+Kernel Shared Memory), to fit more virtual machines into physical memory,
+by sharing the data common between them. But it can be useful to any
+application which generates many instances of the same data.
+
+The KSM daemon ksmd periodically scans those areas of user memory
+which have been registered with it, looking for pages of identical
+content which can be replaced by a single write-protected page (which
+is automatically copied if a process later wants to update its
+content). The amount of pages that KSM daemon scans in a single pass
+and the time between the passes are configured using :ref:`sysfs
+intraface <ksm_sysfs>`
+
+KSM only merges anonymous (private) pages, never pagecache (file) pages.
+KSM's merged pages were originally locked into kernel memory, but can now
+be swapped out just like other user pages (but sharing is broken when they
+are swapped back in: ksmd must rediscover their identity and merge again).
+
+Controlling KSM with madvise
+============================
+
+KSM only operates on those areas of address space which an application
+has advised to be likely candidates for merging, by using the madvise(2)
+system call::
+
+ int madvise(addr, length, MADV_MERGEABLE)
+
+The app may call
+
+::
+
+ int madvise(addr, length, MADV_UNMERGEABLE)
+
+to cancel that advice and restore unshared pages: whereupon KSM
+unmerges whatever it merged in that range. Note: this unmerging call
+may suddenly require more memory than is available - possibly failing
+with EAGAIN, but more probably arousing the Out-Of-Memory killer.
+
+If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
+and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
+built with CONFIG_KSM=y, those calls will normally succeed: even if the
+the KSM daemon is not currently running, MADV_MERGEABLE still registers
+the range for whenever the KSM daemon is started; even if the range
+cannot contain any pages which KSM could actually merge; even if
+MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
+
+If a region of memory must be split into at least one new MADV_MERGEABLE
+or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
+will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
+
+Like other madvise calls, they are intended for use on mapped areas of
+the user address space: they will report ENOMEM if the specified range
+includes unmapped gaps (though working on the intervening mapped areas),
+and might fail with EAGAIN if not enough memory for internal structures.
+
+Applications should be considerate in their use of MADV_MERGEABLE,
+restricting its use to areas likely to benefit. KSM's scans may use a lot
+of processing power: some installations will disable KSM for that reason.
+
+.. _ksm_sysfs:
+
+KSM daemon sysfs interface
+==========================
+
+The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
+readable by all but writable only by root:
+
+pages_to_scan
+ how many pages to scan before ksmd goes to sleep
+ e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
+
+ Default: 100 (chosen for demonstration purposes)
+
+sleep_millisecs
+ how many milliseconds ksmd should sleep before next scan
+ e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
+
+ Default: 20 (chosen for demonstration purposes)
+
+merge_across_nodes
+ specifies if pages from different NUMA nodes can be merged.
+ When set to 0, ksm merges only pages which physically reside
+ in the memory area of same NUMA node. That brings lower
+ latency to access of shared pages. Systems with more nodes, at
+ significant NUMA distances, are likely to benefit from the
+ lower latency of setting 0. Smaller systems, which need to
+ minimize memory usage, are likely to benefit from the greater
+ sharing of setting 1 (default). You may wish to compare how
+ your system performs under each setting, before deciding on
+ which to use. ``merge_across_nodes`` setting can be changed only
+ when there are no ksm shared pages in the system: set run 2 to
+ unmerge pages first, then to 1 after changing
+ ``merge_across_nodes``, to remerge according to the new setting.
+
+ Default: 1 (merging across nodes as in earlier releases)
+
+run
+ * set to 0 to stop ksmd from running but keep merged pages,
+ * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
+ * set to 2 to stop ksmd and unmerge all pages currently merged, but
+ leave mergeable areas registered for next run.
+
+ Default: 0 (must be changed to 1 to activate KSM, except if
+ CONFIG_SYSFS is disabled)
+
+use_zero_pages
+ specifies whether empty pages (i.e. allocated pages that only
+ contain zeroes) should be treated specially. When set to 1,
+ empty pages are merged with the kernel zero page(s) instead of
+ with each other as it would happen normally. This can improve
+ the performance on architectures with coloured zero pages,
+ depending on the workload. Care should be taken when enabling
+ this setting, as it can potentially degrade the performance of
+ KSM for some workloads, for example if the checksums of pages
+ candidate for merging match the checksum of an empty
+ page. This setting can be changed at any time, it is only
+ effective for pages merged after the change.
+
+ Default: 0 (normal KSM behaviour as in earlier releases)
+
+max_page_sharing
+ Maximum sharing allowed for each KSM page. This enforces a
+ deduplication limit to avoid high latency for virtual memory
+ operations that involve traversal of the virtual mappings that
+ share the KSM page. The minimum value is 2 as a newly created
+ KSM page will have at least two sharers. The higher this value
+ the faster KSM will merge the memory and the higher the
+ deduplication factor will be, but the slower the worst case
+ virtual mappings traversal could be for any given KSM
+ page. Slowing down this traversal means there will be higher
+ latency for certain virtual memory operations happening during
+ swapping, compaction, NUMA balancing and page migration, in
+ turn decreasing responsiveness for the caller of those virtual
+ memory operations. The scheduler latency of other tasks not
+ involved with the VM operations doing the virtual mappings
+ traversal is not affected by this parameter as these
+ traversals are always schedule friendly themselves.
+
+stable_node_chains_prune_millisecs
+ specifies how frequently KSM checks the metadata of the pages
+ that hit the deduplication limit for stale information.
+ Smaller milllisecs values will free up the KSM metadata with
+ lower latency, but they will make ksmd use more CPU during the
+ scan. It's a noop if not a single KSM page hit the
+ ``max_page_sharing`` yet.
+
+The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
+
+pages_shared
+ how many shared pages are being used
+pages_sharing
+ how many more sites are sharing them i.e. how much saved
+pages_unshared
+ how many pages unique but repeatedly checked for merging
+pages_volatile
+ how many pages changing too fast to be placed in a tree
+full_scans
+ how many times all mergeable areas have been scanned
+stable_node_chains
+ the number of KSM pages that hit the ``max_page_sharing`` limit
+stable_node_dups
+ number of duplicated KSM pages
+
+A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
+sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
+indicates wasted effort. ``pages_volatile`` embraces several
+different kinds of activity, but a high proportion there would also
+indicate poor use of madvise MADV_MERGEABLE.
+
+The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
+``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
+be increased accordingly.
+
+--
+Izik Eidus,
+Hugh Dickins, 17 Nov 2009
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index afcf5a8..d32016d 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -4,185 +4,11 @@
Kernel Samepage Merging
=======================
-Overview
-========
-
KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
-KSM was originally developed for use with KVM (where it was known as
-Kernel Shared Memory), to fit more virtual machines into physical memory,
-by sharing the data common between them. But it can be useful to any
-application which generates many instances of the same data.
-
-The KSM daemon ksmd periodically scans those areas of user memory
-which have been registered with it, looking for pages of identical
-content which can be replaced by a single write-protected page (which
-is automatically copied if a process later wants to update its
-content). The amount of pages that KSM daemon scans in a single pass
-and the time between the passes are configured using :ref:`sysfs
-intraface <ksm_sysfs>`
-
-KSM only merges anonymous (private) pages, never pagecache (file) pages.
-KSM's merged pages were originally locked into kernel memory, but can now
-be swapped out just like other user pages (but sharing is broken when they
-are swapped back in: ksmd must rediscover their identity and merge again).
-
-Controlling KSM with madvise
-============================
-
-KSM only operates on those areas of address space which an application
-has advised to be likely candidates for merging, by using the madvise(2)
-system call::
-
- int madvise(addr, length, MADV_MERGEABLE)
-
-The app may call
-
-::
-
- int madvise(addr, length, MADV_UNMERGEABLE)
-
-to cancel that advice and restore unshared pages: whereupon KSM
-unmerges whatever it merged in that range. Note: this unmerging call
-may suddenly require more memory than is available - possibly failing
-with EAGAIN, but more probably arousing the Out-Of-Memory killer.
-
-If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
-and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
-built with CONFIG_KSM=y, those calls will normally succeed: even if the
-the KSM daemon is not currently running, MADV_MERGEABLE still registers
-the range for whenever the KSM daemon is started; even if the range
-cannot contain any pages which KSM could actually merge; even if
-MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
-
-If a region of memory must be split into at least one new MADV_MERGEABLE
-or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
-will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
-
-Like other madvise calls, they are intended for use on mapped areas of
-the user address space: they will report ENOMEM if the specified range
-includes unmapped gaps (though working on the intervening mapped areas),
-and might fail with EAGAIN if not enough memory for internal structures.
-
-Applications should be considerate in their use of MADV_MERGEABLE,
-restricting its use to areas likely to benefit. KSM's scans may use a lot
-of processing power: some installations will disable KSM for that reason.
-
-.. _ksm_sysfs:
-
-KSM daemon sysfs interface
-==========================
-
-The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
-readable by all but writable only by root:
-
-pages_to_scan
- how many pages to scan before ksmd goes to sleep
- e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
-
- Default: 100 (chosen for demonstration purposes)
-
-sleep_millisecs
- how many milliseconds ksmd should sleep before next scan
- e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
-
- Default: 20 (chosen for demonstration purposes)
-
-merge_across_nodes
- specifies if pages from different NUMA nodes can be merged.
- When set to 0, ksm merges only pages which physically reside
- in the memory area of same NUMA node. That brings lower
- latency to access of shared pages. Systems with more nodes, at
- significant NUMA distances, are likely to benefit from the
- lower latency of setting 0. Smaller systems, which need to
- minimize memory usage, are likely to benefit from the greater
- sharing of setting 1 (default). You may wish to compare how
- your system performs under each setting, before deciding on
- which to use. ``merge_across_nodes`` setting can be changed only
- when there are no ksm shared pages in the system: set run 2 to
- unmerge pages first, then to 1 after changing
- ``merge_across_nodes``, to remerge according to the new setting.
-
- Default: 1 (merging across nodes as in earlier releases)
-
-run
- * set to 0 to stop ksmd from running but keep merged pages,
- * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
- * set to 2 to stop ksmd and unmerge all pages currently merged, but
- leave mergeable areas registered for next run.
-
- Default: 0 (must be changed to 1 to activate KSM, except if
- CONFIG_SYSFS is disabled)
-
-use_zero_pages
- specifies whether empty pages (i.e. allocated pages that only
- contain zeroes) should be treated specially. When set to 1,
- empty pages are merged with the kernel zero page(s) instead of
- with each other as it would happen normally. This can improve
- the performance on architectures with coloured zero pages,
- depending on the workload. Care should be taken when enabling
- this setting, as it can potentially degrade the performance of
- KSM for some workloads, for example if the checksums of pages
- candidate for merging match the checksum of an empty
- page. This setting can be changed at any time, it is only
- effective for pages merged after the change.
-
- Default: 0 (normal KSM behaviour as in earlier releases)
-
-max_page_sharing
- Maximum sharing allowed for each KSM page. This enforces a
- deduplication limit to avoid high latency for virtual memory
- operations that involve traversal of the virtual mappings that
- share the KSM page. The minimum value is 2 as a newly created
- KSM page will have at least two sharers. The higher this value
- the faster KSM will merge the memory and the higher the
- deduplication factor will be, but the slower the worst case
- virtual mappings traversal could be for any given KSM
- page. Slowing down this traversal means there will be higher
- latency for certain virtual memory operations happening during
- swapping, compaction, NUMA balancing and page migration, in
- turn decreasing responsiveness for the caller of those virtual
- memory operations. The scheduler latency of other tasks not
- involved with the VM operations doing the virtual mappings
- traversal is not affected by this parameter as these
- traversals are always schedule friendly themselves.
-
-stable_node_chains_prune_millisecs
- specifies how frequently KSM checks the metadata of the pages
- that hit the deduplication limit for stale information.
- Smaller milllisecs values will free up the KSM metadata with
- lower latency, but they will make ksmd use more CPU during the
- scan. It's a noop if not a single KSM page hit the
- ``max_page_sharing`` yet.
-
-The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
-
-pages_shared
- how many shared pages are being used
-pages_sharing
- how many more sites are sharing them i.e. how much saved
-pages_unshared
- how many pages unique but repeatedly checked for merging
-pages_volatile
- how many pages changing too fast to be placed in a tree
-full_scans
- how many times all mergeable areas have been scanned
-stable_node_chains
- the number of KSM pages that hit the ``max_page_sharing`` limit
-stable_node_dups
- number of duplicated KSM pages
-
-A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
-sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
-indicates wasted effort. ``pages_volatile`` embraces several
-different kinds of activity, but a high proportion there would also
-indicate poor use of madvise MADV_MERGEABLE.
-
-The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
-``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
-be increased accordingly.
+The userspace interface of KSM is described in :ref:`Documentation/admin-guide/mm/ksm.rst <admin_guide_ksm>`
Design
======
--
2.7.4
Remove implementation details from sysfs parameter descriptions.
Also move the paragraph discussing fragmentation issues and their possible
solution to the "Design" section.
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/vm/ksm.rst | 21 ++++++++++-----------
1 file changed, 10 insertions(+), 11 deletions(-)
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index 18d7c71..afcf5a8 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -170,10 +170,9 @@ pages_volatile
full_scans
how many times all mergeable areas have been scanned
stable_node_chains
- number of stable node chains allocated, this is effectively
the number of KSM pages that hit the ``max_page_sharing`` limit
stable_node_dups
- number of stable node dups queued into the stable_node chains
+ number of duplicated KSM pages
A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
@@ -185,15 +184,6 @@ The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
be increased accordingly.
-The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
-``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
-in the stable_node dups, which could be solved by introducing
-fragmentation algorithms in ksmd which would refile rmap_items from
-one stable_node dup to another stable_node dup, in order to free up
-stable_node "dups" with few rmap_items in them, but that may increase
-the ksmd CPU usage and possibly slowdown the readonly computations on
-the KSM pages of the applications.
-
Design
======
@@ -247,6 +237,15 @@ deduplication factor at the expense of slower worst case for rmap
walks for any KSM page which can happen during swapping, compaction,
NUMA balancing and page migration.
+The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
+``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
+in the stable_node dups, which could be solved by introducing
+fragmentation algorithms in ksmd which would refile rmap_items from
+one stable_node dup to another stable_node dup, in order to free up
+stable_node "dups" with few rmap_items in them, but that may increase
+the ksmd CPU usage and possibly slowdown the readonly computations on
+the KSM pages of the applications.
+
The whole list of stable_node "dups" linked in the stable_node
"chains" is scanned periodically in order to prune stale stable_nodes.
The frequency of such scans is defined by
--
2.7.4
The description of "max_page_sharing" sysfs attribute includes lots of
implementation details that more naturally belong in the "Design"
section.
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/vm/ksm.rst | 51 ++++++++++++++++++++++++++++--------------------
1 file changed, 30 insertions(+), 21 deletions(-)
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index 0e5a085..00961b8 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -133,31 +133,21 @@ use_zero_pages
max_page_sharing
Maximum sharing allowed for each KSM page. This enforces a
- deduplication limit to avoid the virtual memory rmap lists to
- grow too large. The minimum value is 2 as a newly created KSM
- page will have at least two sharers. The rmap walk has O(N)
- complexity where N is the number of rmap_items (i.e. virtual
- mappings) that are sharing the page, which is in turn capped
- by ``max_page_sharing``. So this effectively spreads the linear
- O(N) computational complexity from rmap walk context over
- different KSM pages. The ksmd walk over the stable_node
- "chains" is also O(N), but N is the number of stable_node
- "dups", not the number of rmap_items, so it has not a
- significant impact on ksmd performance. In practice the best
- stable_node "dup" candidate will be kept and found at the head
- of the "dups" list. The higher this value the faster KSM will
- merge the memory (because there will be fewer stable_node dups
- queued into the stable_node chain->hlist to check for pruning)
- and the higher the deduplication factor will be, but the
- slowest the worst case rmap walk could be for any given KSM
- page. Slowing down the rmap_walk means there will be higher
+ deduplication limit to avoid high latency for virtual memory
+ operations that involve traversal of the virtual mappings that
+ share the KSM page. The minimum value is 2 as a newly created
+ KSM page will have at least two sharers. The higher this value
+ the faster KSM will merge the memory and the higher the
+ deduplication factor will be, but the slower the worst case
+ virtual mappings traversal could be for any given KSM
+ page. Slowing down this traversal means there will be higher
latency for certain virtual memory operations happening during
swapping, compaction, NUMA balancing and page migration, in
turn decreasing responsiveness for the caller of those virtual
memory operations. The scheduler latency of other tasks not
- involved with the VM operations doing the rmap walk is not
- affected by this parameter as the rmap walks are always
- schedule friendly themselves.
+ involved with the VM operations doing the virtual mappings
+ traversal is not affected by this parameter as these
+ traversals are always schedule friendly themselves.
stable_node_chains_prune_millisecs
How frequently to walk the whole list of stable_node "dups"
@@ -240,6 +230,25 @@ if compared to an unlimited list of reverse mappings. It is still
enforced that there cannot be KSM page content duplicates in the
stable tree itself.
+The deduplication limit enforced by ``max_page_sharing`` is required
+to avoid the virtual memory rmap lists to grow too large. The rmap
+walk has O(N) complexity where N is the number of rmap_items
+(i.e. virtual mappings) that are sharing the page, which is in turn
+capped by ``max_page_sharing``. So this effectively spreads the linear
+O(N) computational complexity from rmap walk context over different
+KSM pages. The ksmd walk over the stable_node "chains" is also O(N),
+but N is the number of stable_node "dups", not the number of
+rmap_items, so it has not a significant impact on ksmd performance. In
+practice the best stable_node "dup" candidate will be kept and found
+at the head of the "dups" list.
+
+High values of ``max_page_sharing`` result in faster memory merging
+(because there will be fewer stable_node dups queued into the
+stable_node chain->hlist to check for pruning) and higher
+deduplication factor at the expense of slower worst case for rmap
+walks for any KSM page which can happen during swapping, compaction,
+NUMA balancing and page migration.
+
Reference
---------
.. kernel-doc:: mm/ksm.c
--
2.7.4
The existing comment provides a good overview of KSM implementation. Let's
update it to reflect recent additions of "chain" and "dup" variants of the
stable tree nodes and mark it as "DOC:" for inclusion into the KSM
documentation.
Signed-off-by: Mike Rapoport <[email protected]>
---
mm/ksm.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 58c2741..54155b1 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -51,7 +51,9 @@
#define DO_NUMA(x) do { } while (0)
#endif
-/*
+/**
+ * DOC: Overview
+ *
* A few notes about the KSM scanning process,
* to make it easier to understand the data structures below:
*
@@ -67,6 +69,21 @@
* this tree is fully assured to be working (except when pages are unmapped),
* and therefore this tree is called the stable tree.
*
+ * The stable tree node includes information required for reverse
+ * mapping from a KSM page to virtual addresses that map this page.
+ *
+ * In order to avoid large latencies of the rmap walks on KSM pages,
+ * KSM maintains two types of nodes in the stable tree:
+ *
+ * * the regular nodes that keep the reverse mapping structures in a
+ * linked list
+ * * the "chains" that link nodes ("dups") that represent the same
+ * write protected memory content, but each "dup" corresponds to a
+ * different KSM page copy of that content
+ *
+ * Internally, the regular nodes, "dups" and "chains" are represented
+ * using the same :c:type:`struct stable_node` structure.
+ *
* In addition to the stable tree, KSM uses a second data structure called the
* unstable tree: this tree holds pointers to pages which have been found to
* be "unchanged for a period of time". The unstable tree sorts these pages
--
2.7.4
Aside from the formatting:
* fixed typos
* added section and sub-section headers
* moved ksmd overview after the description of KSM origins
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/vm/ksm.rst | 110 +++++++++++++++++++++++++++++------------------
1 file changed, 69 insertions(+), 41 deletions(-)
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index 87e7eef..786d460 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -4,34 +4,50 @@
Kernel Samepage Merging
=======================
+Overview
+========
+
KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
-The KSM daemon ksmd periodically scans those areas of user memory which
-have been registered with it, looking for pages of identical content which
-can be replaced by a single write-protected page (which is automatically
-copied if a process later wants to update its content).
-
KSM was originally developed for use with KVM (where it was known as
Kernel Shared Memory), to fit more virtual machines into physical memory,
by sharing the data common between them. But it can be useful to any
application which generates many instances of the same data.
+The KSM daemon ksmd periodically scans those areas of user memory
+which have been registered with it, looking for pages of identical
+content which can be replaced by a single write-protected page (which
+is automatically copied if a process later wants to update its
+content). The amount of pages that KSM daemon scans in a single pass
+and the time between the passes are configured using :ref:`sysfs
+intraface <ksm_sysfs>`
+
KSM only merges anonymous (private) pages, never pagecache (file) pages.
KSM's merged pages were originally locked into kernel memory, but can now
be swapped out just like other user pages (but sharing is broken when they
are swapped back in: ksmd must rediscover their identity and merge again).
+Controlling KSM with madvise
+============================
+
KSM only operates on those areas of address space which an application
has advised to be likely candidates for merging, by using the madvise(2)
-system call: int madvise(addr, length, MADV_MERGEABLE).
+system call::
+
+ int madvise(addr, length, MADV_MERGEABLE)
+
+The app may call
+
+::
+
+ int madvise(addr, length, MADV_UNMERGEABLE)
-The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
-that advice and restore unshared pages: whereupon KSM unmerges whatever
-it merged in that range. Note: this unmerging call may suddenly require
-more memory than is available - possibly failing with EAGAIN, but more
-probably arousing the Out-Of-Memory killer.
+to cancel that advice and restore unshared pages: whereupon KSM
+unmerges whatever it merged in that range. Note: this unmerging call
+may suddenly require more memory than is available - possibly failing
+with EAGAIN, but more probably arousing the Out-Of-Memory killer.
If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
@@ -43,7 +59,7 @@ MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
If a region of memory must be split into at least one new MADV_MERGEABLE
or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
-will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).
+will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
Like other madvise calls, they are intended for use on mapped areas of
the user address space: they will report ENOMEM if the specified range
@@ -54,21 +70,28 @@ Applications should be considerate in their use of MADV_MERGEABLE,
restricting its use to areas likely to benefit. KSM's scans may use a lot
of processing power: some installations will disable KSM for that reason.
+.. _ksm_sysfs:
+
+KSM daemon sysfs interface
+==========================
+
The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
readable by all but writable only by root:
pages_to_scan
- how many present pages to scan before ksmd goes to sleep
- e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan`` Default: 100
- (chosen for demonstration purposes)
+ how many pages to scan before ksmd goes to sleep
+ e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
+
+ Default: 100 (chosen for demonstration purposes)
sleep_millisecs
how many milliseconds ksmd should sleep before next scan
- e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs`` Default: 20
- (chosen for demonstration purposes)
+ e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
+
+ Default: 20 (chosen for demonstration purposes)
merge_across_nodes
- specifies if pages from different numa nodes can be merged.
+ specifies if pages from different NUMA nodes can be merged.
When set to 0, ksm merges only pages which physically reside
in the memory area of same NUMA node. That brings lower
latency to access of shared pages. Systems with more nodes, at
@@ -77,19 +100,21 @@ merge_across_nodes
minimize memory usage, are likely to benefit from the greater
sharing of setting 1 (default). You may wish to compare how
your system performs under each setting, before deciding on
- which to use. merge_across_nodes setting can be changed only
- when there are no ksm shared pages in system: set run 2 to
+ which to use. ``merge_across_nodes`` setting can be changed only
+ when there are no ksm shared pages in the system: set run 2 to
unmerge pages first, then to 1 after changing
- merge_across_nodes, to remerge according to the new setting.
+ ``merge_across_nodes``, to remerge according to the new setting.
+
Default: 1 (merging across nodes as in earlier releases)
run
- set 0 to stop ksmd from running but keep merged pages,
- set 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
- set 2 to stop ksmd and unmerge all pages currently merged, but
- leave mergeable areas registered for next run Default: 0 (must
- be changed to 1 to activate KSM, except if CONFIG_SYSFS is
- disabled)
+ * set to 0 to stop ksmd from running but keep merged pages,
+ * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
+ * set to 2 to stop ksmd and unmerge all pages currently merged, but
+ leave mergeable areas registered for next run.
+
+ Default: 0 (must be changed to 1 to activate KSM, except if
+ CONFIG_SYSFS is disabled)
use_zero_pages
specifies whether empty pages (i.e. allocated pages that only
@@ -102,8 +127,9 @@ use_zero_pages
KSM for some workloads, for example if the checksums of pages
candidate for merging match the checksum of an empty
page. This setting can be changed at any time, it is only
- effective for pages merged after the change. Default: 0
- (normal KSM behaviour as in earlier releases)
+ effective for pages merged after the change.
+
+ Default: 0 (normal KSM behaviour as in earlier releases)
max_page_sharing
Maximum sharing allowed for each KSM page. This enforces a
@@ -112,7 +138,7 @@ max_page_sharing
page will have at least two sharers. The rmap walk has O(N)
complexity where N is the number of rmap_items (i.e. virtual
mappings) that are sharing the page, which is in turn capped
- by max_page_sharing. So this effectively spread the the linear
+ by ``max_page_sharing``. So this effectively spreads the linear
O(N) computational complexity from rmap walk context over
different KSM pages. The ksmd walk over the stable_node
"chains" is also O(N), but N is the number of stable_node
@@ -140,7 +166,7 @@ stable_node_chains_prune_millisecs
metadata with lower latency, but they will make ksmd use more
CPU during the scan. This only applies to the stable_node
chains so it's a noop if not a single KSM page hit the
- max_page_sharing yet (there would be no stable_node chains in
+ ``max_page_sharing`` yet (there would be no stable_node chains in
such case).
The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
@@ -157,27 +183,29 @@ full_scans
how many times all mergeable areas have been scanned
stable_node_chains
number of stable node chains allocated, this is effectively
- the number of KSM pages that hit the max_page_sharing limit
+ the number of KSM pages that hit the ``max_page_sharing`` limit
stable_node_dups
number of stable node dups queued into the stable_node chains
-A high ratio of pages_sharing to pages_shared indicates good sharing, but
-a high ratio of pages_unshared to pages_sharing indicates wasted effort.
-pages_volatile embraces several different kinds of activity, but a high
-proportion there would also indicate poor use of madvise MADV_MERGEABLE.
+A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
+sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
+indicates wasted effort. ``pages_volatile`` embraces several
+different kinds of activity, but a high proportion there would also
+indicate poor use of madvise MADV_MERGEABLE.
-The maximum possible page_sharing/page_shared ratio is limited by the
-max_page_sharing tunable. To increase the ratio max_page_sharing must
+The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
+``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
be increased accordingly.
-The stable_node_dups/stable_node_chains ratio is also affected by the
-max_page_sharing tunable, and an high ratio may indicate fragmentation
+The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
+``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
in the stable_node dups, which could be solved by introducing
fragmentation algorithms in ksmd which would refile rmap_items from
-one stable_node dup to another stable_node dup, in order to freeup
+one stable_node dup to another stable_node dup, in order to free up
stable_node "dups" with few rmap_items in them, but that may increase
the ksmd CPU usage and possibly slowdown the readonly computations on
the KSM pages of the applications.
+--
Izik Eidus,
Hugh Dickins, 17 Nov 2009
--
2.7.4
Make the description of stable_node_chains_prune_millisecs sysfs parameter
less implementation aware and add a few words about this parameter in the
"Design" section.
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/vm/ksm.rst | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index 00961b8..18d7c71 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -150,14 +150,12 @@ max_page_sharing
traversals are always schedule friendly themselves.
stable_node_chains_prune_millisecs
- How frequently to walk the whole list of stable_node "dups"
- linked in the stable_node "chains" in order to prune stale
- stable_nodes. Smaller milllisecs values will free up the KSM
- metadata with lower latency, but they will make ksmd use more
- CPU during the scan. This only applies to the stable_node
- chains so it's a noop if not a single KSM page hit the
- ``max_page_sharing`` yet (there would be no stable_node chains in
- such case).
+ specifies how frequently KSM checks the metadata of the pages
+ that hit the deduplication limit for stale information.
+ Smaller milllisecs values will free up the KSM metadata with
+ lower latency, but they will make ksmd use more CPU during the
+ scan. It's a noop if not a single KSM page hit the
+ ``max_page_sharing`` yet.
The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
@@ -249,6 +247,11 @@ deduplication factor at the expense of slower worst case for rmap
walks for any KSM page which can happen during swapping, compaction,
NUMA balancing and page migration.
+The whole list of stable_node "dups" linked in the stable_node
+"chains" is scanned periodically in order to prune stale stable_nodes.
+The frequency of such scans is defined by
+``stable_node_chains_prune_millisecs`` sysfs tunable.
+
Reference
---------
.. kernel-doc:: mm/ksm.c
--
2.7.4
Include the KSM description from the source code comment, add a subsection
about reverse mapping and include kernel-doc references for KSM data
structures.
Signed-off-by: Mike Rapoport <[email protected]>
---
Documentation/vm/ksm.rst | 39 +++++++++++++++++++++++++++++++++++++++
1 file changed, 39 insertions(+)
diff --git a/Documentation/vm/ksm.rst b/Documentation/vm/ksm.rst
index 786d460..0e5a085 100644
--- a/Documentation/vm/ksm.rst
+++ b/Documentation/vm/ksm.rst
@@ -206,6 +206,45 @@ stable_node "dups" with few rmap_items in them, but that may increase
the ksmd CPU usage and possibly slowdown the readonly computations on
the KSM pages of the applications.
+Design
+======
+
+Overview
+--------
+
+.. kernel-doc:: mm/ksm.c
+ :DOC: Overview
+
+Reverse mapping
+---------------
+KSM maintains reverse mapping information for KSM pages in the stable
+tree.
+
+If a KSM page is shared between less than ``max_page_sharing`` VMAs,
+the node of the stable tree that represents such KSM page points to a
+list of :c:type:`struct rmap_item` and the ``page->mapping`` of the
+KSM page points to the stable tree node.
+
+When the sharing passes this threshold, KSM adds a second dimension to
+the stable tree. The tree node becomes a "chain" that links one or
+more "dups". Each "dup" keeps reverse mapping information for a KSM
+page with ``page->mapping`` pointing to that "dup".
+
+Every "chain" and all "dups" linked into a "chain" enforce the
+invariant that they represent the same write protected memory content,
+even if each "dup" will be pointed by a different KSM page copy of
+that content.
+
+This way the stable tree lookup computational complexity is unaffected
+if compared to an unlimited list of reverse mappings. It is still
+enforced that there cannot be KSM page content duplicates in the
+stable tree itself.
+
+Reference
+---------
+.. kernel-doc:: mm/ksm.c
+ :functions: mm_slot ksm_scan stable_node rmap_item
+
--
Izik Eidus,
Hugh Dickins, 17 Nov 2009
--
2.7.4
On Tue, 24 Apr 2018 09:40:21 +0300
Mike Rapoport <[email protected]> wrote:
> These patches extend KSM documentation with high level design overview and
> some details about reverse mappings and split the userspace interface
> description to Documentation/admin-guide/mm.
>
> The description of some KSM sysfs attributes is changed so that it won't
> include implementation detail. The description of these implementation
> details are moved to the new "Design" section.
>
> The last patch in the series depends on the patchset that create
> Documentation/admin-guide/mm [1], all the rest applies cleanly to the
> current docs-next.
I've applied the set, thanks.
jon