doc/kernel/memory_management/demand_paging.rst
.. _memory_management_api_demand_paging:
Demand Paging #############
Demand paging provides a mechanism where data is only brought into physical memory as required by current execution context. The physical memory is conceptually divided in page-sized page frames as regions to hold data.
When the processor tries to access data and the data page exists in one of the page frames, the execution continues without any interruptions.
When the processor tries to access the data page that does not exist in any page frames, a page fault occurs. The paging code then brings in the corresponding data page from backing store into physical memory if there is a free page frame. If there is no more free page frames, the eviction algorithm is invoked to select a data page to be paged out, thus freeing up a page frame for new data to be paged in. If this data page has been modified after it is first paged in, the data will be written back into the backing store. If no modifications is done or after written back into backing store, the data page is now considered paged out and the corresponding page frame is now free. The paging code then invokes the backing store to page in the data page corresponding to the location of the requested data. The backing store copies that data page into the free page frame. Now the data page is in physical memory and execution can continue.
There are functions where paging in and out can be invoked manually
using :c:func:k_mem_page_in() and :c:func:k_mem_page_out().
:c:func:k_mem_page_in() can be used to page in data pages
in anticipation that they are required in the near future. This is used to
minimize number of page faults as these data pages are already in physical
memory, and thus minimizing latency. :c:func:k_mem_page_out() can be
used to page out data pages where they are not going to be accessed for
a considerable amount of time. This frees up page frames so that the next
page in can be executed faster as the paging code does not need to invoke
the eviction algorithm.
Terminology
Data Page
A data page is a page-sized region of data. It may exist in a page frame,
or be paged out to some backing store. Its location can always be looked
up in the CPU's page tables (or equivalent) by virtual address.
The data type will always be void * or in some cases uint8_t *
when doing pointer arithmetic.
Page Frame
A page frame is a page-sized physical memory region in RAM. It is a
container where a data page may be placed. It is always referred to by
physical address. Zephyr has a convention of using uintptr_t for physical
addresses. For every page frame, a struct k_mem_page_frame is instantiated to
store metadata. Flags for each page frame:
K_MEM_PAGE_FRAME_FREE indicates a page frame is unused and on the list of
free page frames. When this flag is set, none of the other flags are
meaningful and they must not be modified.
K_MEM_PAGE_FRAME_PINNED indicates a page frame is pinned in memory
and should never be paged out.
K_MEM_PAGE_FRAME_RESERVED indicates a physical page reserved by hardware
and should not be used at all.
K_MEM_PAGE_FRAME_MAPPED is set when a physical page is mapped to
virtual memory address.
K_MEM_PAGE_FRAME_BUSY indicates a page frame is currently involved in
a page-in/out operation.
K_MEM_PAGE_FRAME_BACKED indicates a page frame has a clean copy
in the backing store.
K_MEM_SCRATCH_PAGE The virtual address of a special page provided to the backing store to:
k_MEM_SCRATCH_PAGE to the specified location; or,K_MEM_SCRATCH_PAGE.
This is used as an intermediate page for page in/out operations. This
scratch needs to be mapped read/write for backing store code to access.
However the data page itself may only be mapped as read-only in virtual
address space. If this page is provided as-is to backing store,
the data page must be re-mapped as read/write which has security
implications as the data page is no longer read-only to other parts of
the application.Paging Statistics
Paging statistics can be obtained via various function calls when
:kconfig:option:CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM_NUM_BINS is enabled:
Overall statistics via :c:func:k_mem_paging_stats_get()
Per-thread statistics via :c:func:k_mem_paging_thread_stats_get()
if :kconfig:option:CONFIG_DEMAND_PAGING_THREAD_STATS is enabled
Execution time histogram can be obtained when
:kconfig:option:CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM is enabled, and
:kconfig:option:CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM_NUM_BINS is defined.
Note that the timing is highly dependent on the architecture,
SoC or board. It is highly recommended that
k_mem_paging_eviction_histogram_bounds[] and
k_mem_paging_backing_store_histogram_bounds[]
be defined for a particular application.
Execution time histogram of eviction algorithm via
:c:func:k_mem_paging_histogram_eviction_get()
Execution time histogram of backing store doing page-in via
:c:func:k_mem_paging_histogram_backing_store_page_in_get()
Execution time histogram of backing store doing page-out via
:c:func:k_mem_paging_histogram_backing_store_page_out_get()
Eviction Algorithm
The eviction algorithm is used to determine which data page and its corresponding page frame can be paged out to free up a page frame for the next page in operation. There are four functions which are called from the kernel paging code:
:c:func:k_mem_paging_eviction_init() is called to initialize
the eviction algorithm. This is called at POST_KERNEL.
:c:func:k_mem_paging_eviction_add() is called each time a data page becomes
eligible for future eviction.
:c:func:k_mem_paging_eviction_remove() is called when a data page is no
longer eligible for eviction. This may happen if the given data page becomes
pinned, gets unmapped or is about to be evicted.
:c:func:k_mem_paging_eviction_select() is called to select
a data page to evict. A function argument dirty is written to
signal the caller whether the selected data page has been modified
since it is first paged in. If the dirty bit is returned
as set, the paging code signals to the backing store to write
the data page back into storage (thus updating its content).
The function returns a pointer to the page frame corresponding to
the selected data page.
There is one additional function which is called by the architecture's memory
management code to flag data pages when they trigger an access fault:
:c:func:k_mem_paging_eviction_accessed(). This is used by the LRU algorithm
to requeue "used" pages.
Two eviction algorithms are currently available:
An NRU (Not-Recently-Used) eviction algorithm has been implemented as a sample. This is a very simple algorithm which ranks data pages on whether they have been accessed and modified. The selection is based on this ranking.
An LRU (Least-Recently-Used) eviction algorithm is also available. It is based on a sorted queue of data pages. The LRU code is more complex compared to the NRU code but also considerably more efficient. This is recommended for production use.
To implement a new eviction algorithm, :c:func:k_mem_paging_eviction_init()
and :c:func:k_mem_paging_eviction_select() must be implemented.
If :kconfig:option:CONFIG_EVICTION_TRACKING is enabled for an algorithm,
these additional functions must also be implemented,
:c:func:k_mem_paging_eviction_add(), :c:func:k_mem_paging_eviction_remove(),
:c:func:k_mem_paging_eviction_accessed().
Backing Store
Backing store is responsible for paging in/out data page between their corresponding page frames and storage. These are the functions which must be implemented:
:c:func:k_mem_paging_backing_store_init() is called to
initialized the backing store at POST_KERNEL.
:c:func:k_mem_paging_backing_store_location_get() is called to
reserve a backing store location so a data page can be paged out.
This location token is passed to
:c:func:k_mem_paging_backing_store_page_out() to perform actual
page out operation.
:c:func:k_mem_paging_backing_store_location_free() is called to
free a backing store location (the location token) which can
then be used for subsequent page out operation.
:c:func:k_mem_paging_backing_store_location_query() is called to obtain
the location token corresponding to storage content to be virtually
mapped and paged-in on demand. Most useful with
:kconfig:option:CONFIG_DEMAND_MAPPING.
:c:func:k_mem_paging_backing_store_page_in() copies a data page
from the backing store location associated with the provided
location token to the page pointed by K_MEM_SCRATCH_PAGE.
:c:func:k_mem_paging_backing_store_page_out() copies a data page
from K_MEM_SCRATCH_PAGE to the backing store location associated
with the provided location token.
:c:func:k_mem_paging_backing_store_page_finalize() is invoked after
:c:func:k_mem_paging_backing_store_page_in() so that the page frame
struct may be updated for internal accounting. This can be
a no-op.
To implement a new backing store, the functions mentioned above
must be implemented.
:c:func:k_mem_paging_backing_store_page_finalize() can be an empty
function if so desired.
API Reference
.. doxygengroup:: mem-demand-paging
.. doxygengroup:: mem-demand-paging-eviction
.. doxygengroup:: mem-demand-paging-backing-store