1 .. _idle_page_tracking:
10 The idle page tracking feature allows to track which memory pages are being
11 accessed by a workload and which are idle. This information can be useful for
12 estimating the workload's working set size, which, in turn, can be taken into
13 account when configuring the workload parameters, setting memory cgroup limits,
14 or deciding where to place the workload within a compute cluster.
16 It is enabled by CONFIG_IDLE_PAGE_TRACKING=y.
23 The idle page tracking API is located at ``/sys/kernel/mm/page_idle``.
24 Currently, it consists of the only read-write file,
25 ``/sys/kernel/mm/page_idle/bitmap``.
27 The file implements a bitmap where each bit corresponds to a memory page. The
28 bitmap is represented by an array of 8-byte integers, and the page at PFN #i is
29 mapped to bit #i%64 of array element #i/64, byte order is native. When a bit is
30 set, the corresponding page is idle.
32 A page is considered idle if it has not been accessed since it was marked idle
33 (for more details on what "accessed" actually means see the :ref:`Implementation
34 Details <impl_details>` section).
35 To mark a page idle one has to set the bit corresponding to
36 the page by writing to the file. A value written to the file is OR-ed with the
39 Only accesses to user memory pages are tracked. These are pages mapped to a
40 process address space, page cache and buffer pages, swap cache pages. For other
41 page types (e.g. SLAB pages) an attempt to mark a page idle is silently ignored,
42 and hence such pages are never reported idle.
44 For huge pages the idle flag is set only on the head page, so one has to read
45 ``/proc/kpageflags`` in order to correctly count idle huge pages.
47 Reading from or writing to ``/sys/kernel/mm/page_idle/bitmap`` will return
48 -EINVAL if you are not starting the read/write on an 8-byte boundary, or
49 if the size of the read/write is not a multiple of 8 bytes. Writing to
50 this file beyond max PFN will return -ENXIO.
52 That said, in order to estimate the amount of pages that are not used by a
55 1. Mark all the workload's pages as idle by setting corresponding bits in
56 ``/sys/kernel/mm/page_idle/bitmap``. The pages can be found by reading
57 ``/proc/pid/pagemap`` if the workload is represented by a process, or by
58 filtering out alien pages using ``/proc/kpagecgroup`` in case the workload
59 is placed in a memory cgroup.
61 2. Wait until the workload accesses its working set.
63 3. Read ``/sys/kernel/mm/page_idle/bitmap`` and count the number of bits set.
64 If one wants to ignore certain types of pages, e.g. mlocked pages since they
65 are not reclaimable, he or she can filter them out using
68 See :ref:`Documentation/admin-guide/mm/pagemap.rst <pagemap>` for more
69 information about ``/proc/pid/pagemap``, ``/proc/kpageflags``, and
70 ``/proc/kpagecgroup``.
74 Implementation Details
75 ======================
77 The kernel internally keeps track of accesses to user memory pages in order to
78 reclaim unreferenced pages first on memory shortage conditions. A page is
79 considered referenced if it has been recently accessed via a process address
80 space, in which case one or more PTEs it is mapped to will have the Accessed bit
81 set, or marked accessed explicitly by the kernel (see mark_page_accessed()). The
84 - a userspace process reads or writes a page using a system call (e.g. read(2)
87 - a page that is used for storing filesystem buffers is read or written,
88 because a process needs filesystem metadata stored in it (e.g. lists a
91 - a page is accessed by a device driver using get_user_pages()
93 When a dirty page is written to swap or disk as a result of memory reclaim or
94 exceeding the dirty memory limit, it is not marked referenced.
96 The idle memory tracking feature adds a new page flag, the Idle flag. This flag
97 is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` (see the
98 :ref:`User API <user_api>`
99 section), and cleared automatically whenever a page is referenced as defined
102 When a page is marked idle, the Accessed bit must be cleared in all PTEs it is
103 mapped to, otherwise we will not be able to detect accesses to the page coming
104 from a process address space. To avoid interference with the reclaimer, which,
105 as noted above, uses the Accessed bit to promote actively referenced pages, one
106 more page flag is introduced, the Young flag. When the PTE Accessed bit is
107 cleared as a result of setting or updating a page's Idle flag, the Young flag
108 is set on the page. The reclaimer treats the Young flag as an extra PTE
109 Accessed bit and therefore will consider such a page as referenced.
111 Since the idle memory tracking feature is based on the memory reclaimer logic,
112 it only works with pages that are on an LRU list, other pages are silently
113 ignored. That means it will ignore a user memory page if it is isolated, but
114 since there are usually not many of them, it should not affect the overall
115 result noticeably. In order not to stall scanning of the idle page bitmap,
116 locked pages may be skipped too.