This patch updates the documentation with the new page_idle tracking feature which uses virtual address indexing. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- .../admin-guide/mm/idle_page_tracking.rst | 43 ++++++++++++++++--- 1 file changed, 36 insertions(+), 7 deletions(-) diff --git a/Documentation/admin-guide/mm/idle_page_tracking.rst b/Documentation/admin-guide/mm/idle_page_tracking.rst index df9394fb39c2..1eeac78c94a7 100644 --- a/Documentation/admin-guide/mm/idle_page_tracking.rst +++ b/Documentation/admin-guide/mm/idle_page_tracking.rst @@ -19,10 +19,14 @@ It is enabled by CONFIG_IDLE_PAGE_TRACKING=y. User API ======== +There are 2 ways to access the idle page tracking API. One uses physical +address indexing, another uses a simpler virtual address indexing scheme. -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``. +Physical address indexing +------------------------- +The idle page tracking API for physical address indexing using page frame +numbers (PFN) 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 @@ -74,6 +78,31 @@ See :ref:`Documentation/admin-guide/mm/pagemap.rst <pagemap>` for more information about ``/proc/pid/pagemap``, ``/proc/kpageflags``, and ``/proc/kpagecgroup``. +Virtual address indexing +------------------------ +The idle page tracking API for virtual address indexing using virtual page +frame numbers (VFN) is located at ``/proc/<pid>/page_idle``. It is a bitmap +that follows the same semantics as ``/sys/kernel/mm/page_idle/bitmap`` +except that it uses virtual instead of physical frame numbers. + +This idle page tracking API does not need deal with PFN so it does not require +prior lookups of ``pagemap`` in order to find if page is idle or not. This is +an advantage on some systems where looking up PFN is considered a security +issue. Also in some cases, this interface could be slightly more reliable to +use than physical address indexing, since in physical address indexing, address +space changes can occur between reading the ``pagemap`` and reading the +``bitmap``, while in virtual address indexing, the process's ``mmap_sem`` is +held for the duration of the access. + +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 + ``/proc/<pid>/page_idle``. + + 2. Wait until the workload accesses its working set. + + 3. Read ``/proc/<pid>/page_idle`` and count the number of bits set. + .. _impl_details: Implementation Details @@ -99,10 +128,10 @@ 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. +is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` for physical +addressing or by writing to ``/proc/<pid>/page_idle`` for virtual +addressing (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 -- 2.22.0.709.g102302147b-goog
