docs/vm: idle_page_tracking.txt: convert to ReST format
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mike Rapoport
Jonathan Corbet
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1 changed files with 36 additions and 19 deletions
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@ -1,4 +1,11 @@
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MOTIVATION
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.. _idle_page_tracking:
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==================
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Idle Page Tracking
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==================
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Motivation
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==========
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The idle page tracking feature allows to track which memory pages are being
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accessed by a workload and which are idle. This information can be useful for
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@ -8,10 +15,14 @@ or deciding where to place the workload within a compute cluster.
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It is enabled by CONFIG_IDLE_PAGE_TRACKING=y.
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USER API
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.. _user_api:
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The idle page tracking API is located at /sys/kernel/mm/page_idle. Currently,
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it consists of the only read-write file, /sys/kernel/mm/page_idle/bitmap.
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User API
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========
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The idle page tracking API is located at ``/sys/kernel/mm/page_idle``.
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Currently, it consists of the only read-write file,
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``/sys/kernel/mm/page_idle/bitmap``.
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The file implements a bitmap where each bit corresponds to a memory page. The
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bitmap is represented by an array of 8-byte integers, and the page at PFN #i is
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@ -19,8 +30,9 @@ mapped to bit #i%64 of array element #i/64, byte order is native. When a bit is
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set, the corresponding page is idle.
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A page is considered idle if it has not been accessed since it was marked idle
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(for more details on what "accessed" actually means see the IMPLEMENTATION
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DETAILS section). To mark a page idle one has to set the bit corresponding to
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(for more details on what "accessed" actually means see the :ref:`Implementation
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Details <impl_details>` section).
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To mark a page idle one has to set the bit corresponding to
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the page by writing to the file. A value written to the file is OR-ed with the
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current bitmap value.
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@ -30,9 +42,9 @@ page types (e.g. SLAB pages) an attempt to mark a page idle is silently ignored,
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and hence such pages are never reported idle.
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For huge pages the idle flag is set only on the head page, so one has to read
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/proc/kpageflags in order to correctly count idle huge pages.
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``/proc/kpageflags`` in order to correctly count idle huge pages.
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Reading from or writing to /sys/kernel/mm/page_idle/bitmap will return
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Reading from or writing to ``/sys/kernel/mm/page_idle/bitmap`` will return
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-EINVAL if you are not starting the read/write on an 8-byte boundary, or
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if the size of the read/write is not a multiple of 8 bytes. Writing to
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this file beyond max PFN will return -ENXIO.
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@ -41,21 +53,25 @@ That said, in order to estimate the amount of pages that are not used by a
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workload one should:
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1. Mark all the workload's pages as idle by setting corresponding bits in
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/sys/kernel/mm/page_idle/bitmap. The pages can be found by reading
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/proc/pid/pagemap if the workload is represented by a process, or by
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filtering out alien pages using /proc/kpagecgroup in case the workload is
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placed in a memory cgroup.
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``/sys/kernel/mm/page_idle/bitmap``. The pages can be found by reading
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``/proc/pid/pagemap`` if the workload is represented by a process, or by
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filtering out alien pages using ``/proc/kpagecgroup`` in case the workload
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is placed in a memory cgroup.
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2. Wait until the workload accesses its working set.
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3. Read /sys/kernel/mm/page_idle/bitmap and count the number of bits set. If
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one wants to ignore certain types of pages, e.g. mlocked pages since they
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are not reclaimable, he or she can filter them out using /proc/kpageflags.
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3. Read ``/sys/kernel/mm/page_idle/bitmap`` and count the number of bits set.
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If one wants to ignore certain types of pages, e.g. mlocked pages since they
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are not reclaimable, he or she can filter them out using
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``/proc/kpageflags``.
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See Documentation/vm/pagemap.txt for more information about /proc/pid/pagemap,
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/proc/kpageflags, and /proc/kpagecgroup.
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See Documentation/vm/pagemap.txt for more information about
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``/proc/pid/pagemap``, ``/proc/kpageflags``, and ``/proc/kpagecgroup``.
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IMPLEMENTATION DETAILS
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.. _impl_details:
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Implementation Details
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======================
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The kernel internally keeps track of accesses to user memory pages in order to
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reclaim unreferenced pages first on memory shortage conditions. A page is
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@ -77,7 +93,8 @@ When a dirty page is written to swap or disk as a result of memory reclaim or
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exceeding the dirty memory limit, it is not marked referenced.
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The idle memory tracking feature adds a new page flag, the Idle flag. This flag
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is set manually, by writing to /sys/kernel/mm/page_idle/bitmap (see the USER API
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is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` (see the
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:ref:`User API <user_api>`
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section), and cleared automatically whenever a page is referenced as defined
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above.
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