numaperf
pagemap
soft-dirty
+ swap_numa
transhuge
userfaultfd
+ zswap
--- /dev/null
+.. _swap_numa:
+
+===========================================
+Automatically bind swap device to numa node
+===========================================
+
+If the system has more than one swap device and swap device has the node
+information, we can make use of this information to decide which swap
+device to use in get_swap_pages() to get better performance.
+
+
+How to use this feature
+=======================
+
+Swap device has priority and that decides the order of it to be used. To make
+use of automatically binding, there is no need to manipulate priority settings
+for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and
+swapB, with swapA attached to node 0 and swapB attached to node 1, are going
+to be swapped on. Simply swapping them on by doing::
+
+ # swapon /dev/swapA
+ # swapon /dev/swapB
+
+Then node 0 will use the two swap devices in the order of swapA then swapB and
+node 1 will use the two swap devices in the order of swapB then swapA. Note
+that the order of them being swapped on doesn't matter.
+
+A more complex example on a 4 node machine. Assume 6 swap devices are going to
+be swapped on: swapA and swapB are attached to node 0, swapC is attached to
+node 1, swapD and swapE are attached to node 2 and swapF is attached to node3.
+The way to swap them on is the same as above::
+
+ # swapon /dev/swapA
+ # swapon /dev/swapB
+ # swapon /dev/swapC
+ # swapon /dev/swapD
+ # swapon /dev/swapE
+ # swapon /dev/swapF
+
+Then node 0 will use them in the order of::
+
+ swapA/swapB -> swapC -> swapD -> swapE -> swapF
+
+swapA and swapB will be used in a round robin mode before any other swap device.
+
+node 1 will use them in the order of::
+
+ swapC -> swapA -> swapB -> swapD -> swapE -> swapF
+
+node 2 will use them in the order of::
+
+ swapD/swapE -> swapA -> swapB -> swapC -> swapF
+
+Similaly, swapD and swapE will be used in a round robin mode before any
+other swap devices.
+
+node 3 will use them in the order of::
+
+ swapF -> swapA -> swapB -> swapC -> swapD -> swapE
+
+
+Implementation details
+======================
+
+The current code uses a priority based list, swap_avail_list, to decide
+which swap device to use and if multiple swap devices share the same
+priority, they are used round robin. This change here replaces the single
+global swap_avail_list with a per-numa-node list, i.e. for each numa node,
+it sees its own priority based list of available swap devices. Swap
+device's priority can be promoted on its matching node's swap_avail_list.
+
+The current swap device's priority is set as: user can set a >=0 value,
+or the system will pick one starting from -1 then downwards. The priority
+value in the swap_avail_list is the negated value of the swap device's
+due to plist being sorted from low to high. The new policy doesn't change
+the semantics for priority >=0 cases, the previous starting from -1 then
+downwards now becomes starting from -2 then downwards and -1 is reserved
+as the promoted value. So if multiple swap devices are attached to the same
+node, they will all be promoted to priority -1 on that node's plist and will
+be used round robin before any other swap devices.
--- /dev/null
+.. _zswap:
+
+=====
+zswap
+=====
+
+Overview
+========
+
+Zswap is a lightweight compressed cache for swap pages. It takes pages that are
+in the process of being swapped out and attempts to compress them into a
+dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles
+for potentially reduced swap I/O. This trade-off can also result in a
+significant performance improvement if reads from the compressed cache are
+faster than reads from a swap device.
+
+.. note::
+ Zswap is a new feature as of v3.11 and interacts heavily with memory
+ reclaim. This interaction has not been fully explored on the large set of
+ potential configurations and workloads that exist. For this reason, zswap
+ is a work in progress and should be considered experimental.
+
+ Some potential benefits:
+
+* Desktop/laptop users with limited RAM capacities can mitigate the
+ performance impact of swapping.
+* Overcommitted guests that share a common I/O resource can
+ dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
+ throttling by the hypervisor. This allows more work to get done with less
+ impact to the guest workload and guests sharing the I/O subsystem
+* Users with SSDs as swap devices can extend the life of the device by
+ drastically reducing life-shortening writes.
+
+Zswap evicts pages from compressed cache on an LRU basis to the backing swap
+device when the compressed pool reaches its size limit. This requirement had
+been identified in prior community discussions.
+
+Whether Zswap is enabled at the boot time depends on whether
+the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not.
+This setting can then be overridden by providing the kernel command line
+``zswap.enabled=`` option, for example ``zswap.enabled=0``.
+Zswap can also be enabled and disabled at runtime using the sysfs interface.
+An example command to enable zswap at runtime, assuming sysfs is mounted
+at ``/sys``, is::
+
+ echo 1 > /sys/module/zswap/parameters/enabled
+
+When zswap is disabled at runtime it will stop storing pages that are
+being swapped out. However, it will _not_ immediately write out or fault
+back into memory all of the pages stored in the compressed pool. The
+pages stored in zswap will remain in the compressed pool until they are
+either invalidated or faulted back into memory. In order to force all
+pages out of the compressed pool, a swapoff on the swap device(s) will
+fault back into memory all swapped out pages, including those in the
+compressed pool.
+
+Design
+======
+
+Zswap receives pages for compression through the Frontswap API and is able to
+evict pages from its own compressed pool on an LRU basis and write them back to
+the backing swap device in the case that the compressed pool is full.
+
+Zswap makes use of zpool for the managing the compressed memory pool. Each
+allocation in zpool is not directly accessible by address. Rather, a handle is
+returned by the allocation routine and that handle must be mapped before being
+accessed. The compressed memory pool grows on demand and shrinks as compressed
+pages are freed. The pool is not preallocated. By default, a zpool
+of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created,
+but it can be overridden at boot time by setting the ``zpool`` attribute,
+e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs
+``zpool`` attribute, e.g.::
+
+ echo zbud > /sys/module/zswap/parameters/zpool
+
+The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
+means the compression ratio will always be 2:1 or worse (because of half-full
+zbud pages). The zsmalloc type zpool has a more complex compressed page
+storage method, and it can achieve greater storage densities. However,
+zsmalloc does not implement compressed page eviction, so once zswap fills it
+cannot evict the oldest page, it can only reject new pages.
+
+When a swap page is passed from frontswap to zswap, zswap maintains a mapping
+of the swap entry, a combination of the swap type and swap offset, to the zpool
+handle that references that compressed swap page. This mapping is achieved
+with a red-black tree per swap type. The swap offset is the search key for the
+tree nodes.
+
+During a page fault on a PTE that is a swap entry, frontswap calls the zswap
+load function to decompress the page into the page allocated by the page fault
+handler.
+
+Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
+in the swap_map goes to 0) the swap code calls the zswap invalidate function,
+via frontswap, to free the compressed entry.
+
+Zswap seeks to be simple in its policies. Sysfs attributes allow for one user
+controlled policy:
+
+* max_pool_percent - The maximum percentage of memory that the compressed
+ pool can occupy.
+
+The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT``
+Kconfig option, but it can be overridden at boot time by setting the
+``compressor`` attribute, e.g. ``zswap.compressor=lzo``.
+It can also be changed at runtime using the sysfs "compressor"
+attribute, e.g.::
+
+ echo lzo > /sys/module/zswap/parameters/compressor
+
+When the zpool and/or compressor parameter is changed at runtime, any existing
+compressed pages are not modified; they are left in their own zpool. When a
+request is made for a page in an old zpool, it is uncompressed using its
+original compressor. Once all pages are removed from an old zpool, the zpool
+and its compressor are freed.
+
+Some of the pages in zswap are same-value filled pages (i.e. contents of the
+page have same value or repetitive pattern). These pages include zero-filled
+pages and they are handled differently. During store operation, a page is
+checked if it is a same-value filled page before compressing it. If true, the
+compressed length of the page is set to zero and the pattern or same-filled
+value is stored.
+
+Same-value filled pages identification feature is enabled by default and can be
+disabled at boot time by setting the ``same_filled_pages_enabled`` attribute
+to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and
+disabled at runtime using the sysfs ``same_filled_pages_enabled``
+attribute, e.g.::
+
+ echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
+
+When zswap same-filled page identification is disabled at runtime, it will stop
+checking for the same-value filled pages during store operation. However, the
+existing pages which are marked as same-value filled pages remain stored
+unchanged in zswap until they are either loaded or invalidated.
+
+To prevent zswap from shrinking pool when zswap is full and there's a high
+pressure on swap (this will result in flipping pages in and out zswap pool
+without any real benefit but with a performance drop for the system), a
+special parameter has been introduced to implement a sort of hysteresis to
+refuse taking pages into zswap pool until it has sufficient space if the limit
+has been hit. To set the threshold at which zswap would start accepting pages
+again after it became full, use the sysfs ``accept_threshold_percent``
+attribute, e. g.::
+
+ echo 80 > /sys/module/zswap/parameters/accept_threshold_percent
+
+Setting this parameter to 100 will disable the hysteresis.
+
+A debugfs interface is provided for various statistic about pool size, number
+of pages stored, same-value filled pages and various counters for the reasons
+pages are rejected.
=====================================
This is a collection of documents about the Linux memory management (mm)
-subsystem. If you are looking for advice on simply allocating memory,
-see the :ref:`memory_allocation`.
-
-User guides for MM features
-===========================
-
-The following documents provide guides for controlling and tuning
-various features of the Linux memory management
-
-.. toctree::
- :maxdepth: 1
-
- swap_numa
- zswap
-
-Kernel developers MM documentation
-==================================
-
-The below documents describe MM internals with different level of
-details ranging from notes and mailing list responses to elaborate
-descriptions of data structures and algorithms.
+subsystem internals with different level of details ranging from notes and
+mailing list responses for elaborating descriptions of data structures and
+algorithms. If you are looking for advice on simply allocating memory, see the
+:ref:`memory_allocation`. For controlling and tuning guides, see the
+:doc:`admin guide <../admin-guide/mm/index>`.
.. toctree::
:maxdepth: 1
+++ /dev/null
-.. _swap_numa:
-
-===========================================
-Automatically bind swap device to numa node
-===========================================
-
-If the system has more than one swap device and swap device has the node
-information, we can make use of this information to decide which swap
-device to use in get_swap_pages() to get better performance.
-
-
-How to use this feature
-=======================
-
-Swap device has priority and that decides the order of it to be used. To make
-use of automatically binding, there is no need to manipulate priority settings
-for swap devices. e.g. on a 2 node machine, assume 2 swap devices swapA and
-swapB, with swapA attached to node 0 and swapB attached to node 1, are going
-to be swapped on. Simply swapping them on by doing::
-
- # swapon /dev/swapA
- # swapon /dev/swapB
-
-Then node 0 will use the two swap devices in the order of swapA then swapB and
-node 1 will use the two swap devices in the order of swapB then swapA. Note
-that the order of them being swapped on doesn't matter.
-
-A more complex example on a 4 node machine. Assume 6 swap devices are going to
-be swapped on: swapA and swapB are attached to node 0, swapC is attached to
-node 1, swapD and swapE are attached to node 2 and swapF is attached to node3.
-The way to swap them on is the same as above::
-
- # swapon /dev/swapA
- # swapon /dev/swapB
- # swapon /dev/swapC
- # swapon /dev/swapD
- # swapon /dev/swapE
- # swapon /dev/swapF
-
-Then node 0 will use them in the order of::
-
- swapA/swapB -> swapC -> swapD -> swapE -> swapF
-
-swapA and swapB will be used in a round robin mode before any other swap device.
-
-node 1 will use them in the order of::
-
- swapC -> swapA -> swapB -> swapD -> swapE -> swapF
-
-node 2 will use them in the order of::
-
- swapD/swapE -> swapA -> swapB -> swapC -> swapF
-
-Similaly, swapD and swapE will be used in a round robin mode before any
-other swap devices.
-
-node 3 will use them in the order of::
-
- swapF -> swapA -> swapB -> swapC -> swapD -> swapE
-
-
-Implementation details
-======================
-
-The current code uses a priority based list, swap_avail_list, to decide
-which swap device to use and if multiple swap devices share the same
-priority, they are used round robin. This change here replaces the single
-global swap_avail_list with a per-numa-node list, i.e. for each numa node,
-it sees its own priority based list of available swap devices. Swap
-device's priority can be promoted on its matching node's swap_avail_list.
-
-The current swap device's priority is set as: user can set a >=0 value,
-or the system will pick one starting from -1 then downwards. The priority
-value in the swap_avail_list is the negated value of the swap device's
-due to plist being sorted from low to high. The new policy doesn't change
-the semantics for priority >=0 cases, the previous starting from -1 then
-downwards now becomes starting from -2 then downwards and -1 is reserved
-as the promoted value. So if multiple swap devices are attached to the same
-node, they will all be promoted to priority -1 on that node's plist and will
-be used round robin before any other swap devices.
+++ /dev/null
-.. _zswap:
-
-=====
-zswap
-=====
-
-Overview
-========
-
-Zswap is a lightweight compressed cache for swap pages. It takes pages that are
-in the process of being swapped out and attempts to compress them into a
-dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles
-for potentially reduced swap I/O. This trade-off can also result in a
-significant performance improvement if reads from the compressed cache are
-faster than reads from a swap device.
-
-.. note::
- Zswap is a new feature as of v3.11 and interacts heavily with memory
- reclaim. This interaction has not been fully explored on the large set of
- potential configurations and workloads that exist. For this reason, zswap
- is a work in progress and should be considered experimental.
-
- Some potential benefits:
-
-* Desktop/laptop users with limited RAM capacities can mitigate the
- performance impact of swapping.
-* Overcommitted guests that share a common I/O resource can
- dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
- throttling by the hypervisor. This allows more work to get done with less
- impact to the guest workload and guests sharing the I/O subsystem
-* Users with SSDs as swap devices can extend the life of the device by
- drastically reducing life-shortening writes.
-
-Zswap evicts pages from compressed cache on an LRU basis to the backing swap
-device when the compressed pool reaches its size limit. This requirement had
-been identified in prior community discussions.
-
-Whether Zswap is enabled at the boot time depends on whether
-the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not.
-This setting can then be overridden by providing the kernel command line
-``zswap.enabled=`` option, for example ``zswap.enabled=0``.
-Zswap can also be enabled and disabled at runtime using the sysfs interface.
-An example command to enable zswap at runtime, assuming sysfs is mounted
-at ``/sys``, is::
-
- echo 1 > /sys/module/zswap/parameters/enabled
-
-When zswap is disabled at runtime it will stop storing pages that are
-being swapped out. However, it will _not_ immediately write out or fault
-back into memory all of the pages stored in the compressed pool. The
-pages stored in zswap will remain in the compressed pool until they are
-either invalidated or faulted back into memory. In order to force all
-pages out of the compressed pool, a swapoff on the swap device(s) will
-fault back into memory all swapped out pages, including those in the
-compressed pool.
-
-Design
-======
-
-Zswap receives pages for compression through the Frontswap API and is able to
-evict pages from its own compressed pool on an LRU basis and write them back to
-the backing swap device in the case that the compressed pool is full.
-
-Zswap makes use of zpool for the managing the compressed memory pool. Each
-allocation in zpool is not directly accessible by address. Rather, a handle is
-returned by the allocation routine and that handle must be mapped before being
-accessed. The compressed memory pool grows on demand and shrinks as compressed
-pages are freed. The pool is not preallocated. By default, a zpool
-of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created,
-but it can be overridden at boot time by setting the ``zpool`` attribute,
-e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs
-``zpool`` attribute, e.g.::
-
- echo zbud > /sys/module/zswap/parameters/zpool
-
-The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
-means the compression ratio will always be 2:1 or worse (because of half-full
-zbud pages). The zsmalloc type zpool has a more complex compressed page
-storage method, and it can achieve greater storage densities. However,
-zsmalloc does not implement compressed page eviction, so once zswap fills it
-cannot evict the oldest page, it can only reject new pages.
-
-When a swap page is passed from frontswap to zswap, zswap maintains a mapping
-of the swap entry, a combination of the swap type and swap offset, to the zpool
-handle that references that compressed swap page. This mapping is achieved
-with a red-black tree per swap type. The swap offset is the search key for the
-tree nodes.
-
-During a page fault on a PTE that is a swap entry, frontswap calls the zswap
-load function to decompress the page into the page allocated by the page fault
-handler.
-
-Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
-in the swap_map goes to 0) the swap code calls the zswap invalidate function,
-via frontswap, to free the compressed entry.
-
-Zswap seeks to be simple in its policies. Sysfs attributes allow for one user
-controlled policy:
-
-* max_pool_percent - The maximum percentage of memory that the compressed
- pool can occupy.
-
-The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT``
-Kconfig option, but it can be overridden at boot time by setting the
-``compressor`` attribute, e.g. ``zswap.compressor=lzo``.
-It can also be changed at runtime using the sysfs "compressor"
-attribute, e.g.::
-
- echo lzo > /sys/module/zswap/parameters/compressor
-
-When the zpool and/or compressor parameter is changed at runtime, any existing
-compressed pages are not modified; they are left in their own zpool. When a
-request is made for a page in an old zpool, it is uncompressed using its
-original compressor. Once all pages are removed from an old zpool, the zpool
-and its compressor are freed.
-
-Some of the pages in zswap are same-value filled pages (i.e. contents of the
-page have same value or repetitive pattern). These pages include zero-filled
-pages and they are handled differently. During store operation, a page is
-checked if it is a same-value filled page before compressing it. If true, the
-compressed length of the page is set to zero and the pattern or same-filled
-value is stored.
-
-Same-value filled pages identification feature is enabled by default and can be
-disabled at boot time by setting the ``same_filled_pages_enabled`` attribute
-to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and
-disabled at runtime using the sysfs ``same_filled_pages_enabled``
-attribute, e.g.::
-
- echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
-
-When zswap same-filled page identification is disabled at runtime, it will stop
-checking for the same-value filled pages during store operation. However, the
-existing pages which are marked as same-value filled pages remain stored
-unchanged in zswap until they are either loaded or invalidated.
-
-To prevent zswap from shrinking pool when zswap is full and there's a high
-pressure on swap (this will result in flipping pages in and out zswap pool
-without any real benefit but with a performance drop for the system), a
-special parameter has been introduced to implement a sort of hysteresis to
-refuse taking pages into zswap pool until it has sufficient space if the limit
-has been hit. To set the threshold at which zswap would start accepting pages
-again after it became full, use the sysfs ``accept_threshold_percent``
-attribute, e. g.::
-
- echo 80 > /sys/module/zswap/parameters/accept_threshold_percent
-
-Setting this parameter to 100 will disable the hysteresis.
-
-A debugfs interface is provided for various statistic about pool size, number
-of pages stored, same-value filled pages and various counters for the reasons
-pages are rejected.
projects / linux-2.6-microblaze.git / commitdiff