117 lines
5.1 KiB
ReStructuredText
117 lines
5.1 KiB
ReStructuredText
.. _idle_page_tracking:
|
|
|
|
==================
|
|
Idle Page Tracking
|
|
==================
|
|
|
|
Motivation
|
|
==========
|
|
|
|
The idle page tracking feature allows to track which memory pages are being
|
|
accessed by a workload and which are idle. This information can be useful for
|
|
estimating the workload's working set size, which, in turn, can be taken into
|
|
account when configuring the workload parameters, setting memory cgroup limits,
|
|
or deciding where to place the workload within a compute cluster.
|
|
|
|
It is enabled by CONFIG_IDLE_PAGE_TRACKING=y.
|
|
|
|
.. _user_api:
|
|
|
|
User API
|
|
========
|
|
|
|
The idle page tracking API is located at ``/sys/kernel/mm/page_idle``.
|
|
Currently, it consists of the only read-write file,
|
|
``/sys/kernel/mm/page_idle/bitmap``.
|
|
|
|
The file implements a bitmap where each bit corresponds to a memory page. The
|
|
bitmap is represented by an array of 8-byte integers, and the page at PFN #i is
|
|
mapped to bit #i%64 of array element #i/64, byte order is native. When a bit is
|
|
set, the corresponding page is idle.
|
|
|
|
A page is considered idle if it has not been accessed since it was marked idle
|
|
(for more details on what "accessed" actually means see the :ref:`Implementation
|
|
Details <impl_details>` section).
|
|
To mark a page idle one has to set the bit corresponding to
|
|
the page by writing to the file. A value written to the file is OR-ed with the
|
|
current bitmap value.
|
|
|
|
Only accesses to user memory pages are tracked. These are pages mapped to a
|
|
process address space, page cache and buffer pages, swap cache pages. For other
|
|
page types (e.g. SLAB pages) an attempt to mark a page idle is silently ignored,
|
|
and hence such pages are never reported idle.
|
|
|
|
For huge pages the idle flag is set only on the head page, so one has to read
|
|
``/proc/kpageflags`` in order to correctly count idle huge pages.
|
|
|
|
Reading from or writing to ``/sys/kernel/mm/page_idle/bitmap`` will return
|
|
-EINVAL if you are not starting the read/write on an 8-byte boundary, or
|
|
if the size of the read/write is not a multiple of 8 bytes. Writing to
|
|
this file beyond max PFN will return -ENXIO.
|
|
|
|
That said, in order to estimate the amount of pages that are not used by a
|
|
workload one should:
|
|
|
|
1. Mark all the workload's pages as idle by setting corresponding bits in
|
|
``/sys/kernel/mm/page_idle/bitmap``. The pages can be found by reading
|
|
``/proc/pid/pagemap`` if the workload is represented by a process, or by
|
|
filtering out alien pages using ``/proc/kpagecgroup`` in case the workload
|
|
is placed in a memory cgroup.
|
|
|
|
2. Wait until the workload accesses its working set.
|
|
|
|
3. Read ``/sys/kernel/mm/page_idle/bitmap`` and count the number of bits set.
|
|
If one wants to ignore certain types of pages, e.g. mlocked pages since they
|
|
are not reclaimable, he or she can filter them out using
|
|
``/proc/kpageflags``.
|
|
|
|
See :ref:`Documentation/admin-guide/mm/pagemap.rst <pagemap>` for more
|
|
information about ``/proc/pid/pagemap``, ``/proc/kpageflags``, and
|
|
``/proc/kpagecgroup``.
|
|
|
|
.. _impl_details:
|
|
|
|
Implementation Details
|
|
======================
|
|
|
|
The kernel internally keeps track of accesses to user memory pages in order to
|
|
reclaim unreferenced pages first on memory shortage conditions. A page is
|
|
considered referenced if it has been recently accessed via a process address
|
|
space, in which case one or more PTEs it is mapped to will have the Accessed bit
|
|
set, or marked accessed explicitly by the kernel (see mark_page_accessed()). The
|
|
latter happens when:
|
|
|
|
- a userspace process reads or writes a page using a system call (e.g. read(2)
|
|
or write(2))
|
|
|
|
- a page that is used for storing filesystem buffers is read or written,
|
|
because a process needs filesystem metadata stored in it (e.g. lists a
|
|
directory tree)
|
|
|
|
- a page is accessed by a device driver using get_user_pages()
|
|
|
|
When a dirty page is written to swap or disk as a result of memory reclaim or
|
|
exceeding the dirty memory limit, it is not marked referenced.
|
|
|
|
The idle memory tracking feature adds a new page flag, the Idle flag. This flag
|
|
is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` (see the
|
|
:ref:`User API <user_api>`
|
|
section), and cleared automatically whenever a page is referenced as defined
|
|
above.
|
|
|
|
When a page is marked idle, the Accessed bit must be cleared in all PTEs it is
|
|
mapped to, otherwise we will not be able to detect accesses to the page coming
|
|
from a process address space. To avoid interference with the reclaimer, which,
|
|
as noted above, uses the Accessed bit to promote actively referenced pages, one
|
|
more page flag is introduced, the Young flag. When the PTE Accessed bit is
|
|
cleared as a result of setting or updating a page's Idle flag, the Young flag
|
|
is set on the page. The reclaimer treats the Young flag as an extra PTE
|
|
Accessed bit and therefore will consider such a page as referenced.
|
|
|
|
Since the idle memory tracking feature is based on the memory reclaimer logic,
|
|
it only works with pages that are on an LRU list, other pages are silently
|
|
ignored. That means it will ignore a user memory page if it is isolated, but
|
|
since there are usually not many of them, it should not affect the overall
|
|
result noticeably. In order not to stall scanning of the idle page bitmap,
|
|
locked pages may be skipped too.
|