admin-guide/mm/concepts.rst

=================
Concepts overview
=================

The memory management in Linux is a complex system that evolved over the
years and included more and more functionality to support a variety of
systems from MMU-less microcontrollers to supercomputers. The memory
management for systems without an MMU is called ``nommu`` and it
definitely deserves a dedicated document, which hopefully will be
eventually written. Yet, although some of the concepts are the same,
here we assume that an MMU is available and a CPU can translate a virtual
address to a physical address.

.. contents:: :local:

Virtual Memory Primer
=====================

The physical memory in a computer system is a limited resource and
even for systems that support memory hotplug there is a hard limit on
the amount of memory that can be installed. The physical memory is not
necessarily contiguous; it might be accessible as a set of distinct
address ranges. Besides, different CPU architectures, and even
different implementations of the same architecture have different views
of how these address ranges are defined.

All this makes dealing directly with physical memory quite complex and
to avoid this complexity a concept of virtual memory was developed.

The virtual memory abstracts the details of physical memory from the
application software, allows to keep only needed information in the
physical memory (demand paging) and provides a mechanism for the
protection and controlled sharing of data between processes.

With virtual memory, each and every memory access uses a virtual
address. When the CPU decodes an instruction that reads (or
writes) from (or to) the system memory, it translates the `virtual`
address encoded in that instruction to a `physical` address that the
memory controller can understand.

The physical system memory is divided into page frames, or pages. The
size of each page is architecture specific. Some architectures allow
selection of the page size from several supported values; this
selection is performed at the kernel build time by setting an
appropriate kernel configuration option.

Each physical memory page can be mapped as one or more virtual
pages. These mappings are described by page tables that allow
translation from a virtual address used by programs to the physical
memory address. The page tables are organized hierarchically.

The tables at the lowest level of the hierarchy contain physical
addresses of actual pages used by the software. The tables at higher
levels contain physical addresses of the pages belonging to the lower
levels. The pointer to the top level page table resides in a
register. When the CPU performs the address translation, it uses this
register to access the top level page table. The high bits of the
virtual address are used to index an entry in the top level page
table. That entry is then used to access the next level in the
hierarchy with the next bits of the virtual address as the index to
that level page table. The lowest bits in the virtual address define
the offset inside the actual page.

Huge Pages
==========

The address translation requires several memory accesses and memory
accesses are slow relatively to CPU speed. To avoid spending precious
processor cycles on the address translation, CPUs maintain a cache of
such translations called Translation Lookaside Buffer (or
TLB). Usually TLB is pretty scarce resource and applications with
large memory working set will experience performance hit because of
TLB misses.

Many modern CPU architectures allow mapping of the memory pages
directly by the higher levels in the page table. For instance, on x86,
it is possible to map 2M and even 1G pages using entries in the second
and the third level page tables. In Linux such pages are called
`huge`. Usage of huge pages significantly reduces pressure on TLB,
improves TLB hit-rate and thus improves overall system performance.

There are two mechanisms in Linux that enable mapping of the physical
memory with the huge pages. The first one is `HugeTLB filesystem`, or
hugetlbfs. It is a pseudo filesystem that uses RAM as its backing
store. For the files created in this filesystem the data resides in
the memory and mapped using huge pages. The hugetlbfs is described at
Documentation/admin-guide/mm/hugetlbpage.rst.

Another, more recent, mechanism that enables use of the huge pages is
called `Transparent HugePages`, or THP. Unlike the hugetlbfs that
requires users and/or system administrators to configure what parts of
the system memory should and can be mapped by the huge pages, THP
manages such mappings transparently to the user and hence the
name. See Documentation/admin-guide/mm/transhuge.rst for more details
about THP.

Zones
=====

Often hardware poses restrictions on how different physical memory
ranges can be accessed. In some cases, devices cannot perform DMA to
all the addressable memory. In other cases, the size of the physical
memory exceeds the maximal addressable size of virtual memory and
special actions are required to access portions of the memory. Linux
groups memory pages into `zones` according to their possible
usage. For example, ZONE_DMA will contain memory that can be used by
devices for DMA, ZONE_HIGHMEM will contain memory that is not
permanently mapped into kernel's address space and ZONE_NORMAL will
contain normally addressed pages.

The actual layout of the memory zones is hardware dependent as not all
architectures define all zones, and requirements for DMA are different
for different platforms.

Nodes
=====

Many multi-processor machines are NUMA - Non-Uniform Memory Access -
systems. In such systems the memory is arranged into banks that have
different access latency depending on the "distance" from the
processor. Each bank is referred to as a `node` and for each node Linux
constructs an independent memory management subsystem. A node has its
own set of zones, lists of free and used pages and various statistics
counters. You can find more details about NUMA in
Documentation/mm/numa.rst` and in
Documentation/admin-guide/mm/numa_memory_policy.rst.

Page cache
==========

The physical memory is volatile and the common case for getting data
into the memory is to read it from files. Whenever a file is read, the
data is put into the `page cache` to avoid expensive disk access on
the subsequent reads. Similarly, when one writes to a file, the data
is placed in the page cache and eventually gets into the backing
storage device. The written pages are marked as `dirty` and when Linux
decides to reuse them for other purposes, it makes sure to synchronize
the file contents on the device with the updated data.

Anonymous Memory
================

The `anonymous memory` or `anonymous mappings` represent memory that
is not backed by a filesystem. Such mappings are implicitly created
for program's stack and heap or by explicit calls to mmap(2) system
call. Usually, the anonymous mappings only define virtual memory areas
that the program is allowed to access. The read accesses will result
in creation of a page table entry that references a special physical
page filled with zeroes. When the program performs a write, a regular
physical page will be allocated to hold the written data. The page
will be marked dirty and if the kernel decides to repurpose it,
the dirty page will be swapped out.

Reclaim
=======

Throughout the system lifetime, a physical page can be used for storing
different types of data. It can be kernel internal data structures,
DMA'able buffers for device drivers use, data read from a filesystem,
memory allocated by user space processes etc.

Depending on the page usage it is treated differently by the Linux
memory management. The pages that can be freed at any time, either
because they cache the data available elsewhere, for instance, on a
hard disk, or because they can be swapped out, again, to the hard
disk, are called `reclaimable`. The most notable categories of the
reclaimable pages are page cache and anonymous memory.

In most cases, the pages holding internal kernel data and used as DMA
buffers cannot be repurposed, and they remain pinned until freed by
their user. Such pages are called `unreclaimable`. However, in certain
circumstances, even pages occupied with kernel data structures can be
reclaimed. For instance, in-memory caches of filesystem metadata can
be re-read from the storage device and therefore it is possible to
discard them from the main memory when system is under memory
pressure.

The process of freeing the reclaimable physical memory pages and
repurposing them is called (surprise!) `reclaim`. Linux can reclaim
pages either asynchronously or synchronously, depending on the state
of the system. When the system is not loaded, most of the memory is free
and allocation requests will be satisfied immediately from the free
pages supply. As the load increases, the amount of the free pages goes
down and when it reaches a certain threshold (low watermark), an
allocation request will awaken the ``kswapd`` daemon. It will
asynchronously scan memory pages and either just free them if the data
they contain is available elsewhere, or evict to the backing storage
device (remember those dirty pages?). As memory usage increases even
more and reaches another threshold - min watermark - an allocation
will trigger `direct reclaim`. In this case allocation is stalled
until enough memory pages are reclaimed to satisfy the request.

Compaction
==========

As the system runs, tasks allocate and free the memory and it becomes
fragmented. Although with virtual memory it is possible to present
scattered physical pages as virtually contiguous range, sometimes it is
necessary to allocate large physically contiguous memory areas. Such
need may arise, for instance, when a device driver requires a large
buffer for DMA, or when THP allocates a huge page. Memory `compaction`
addresses the fragmentation issue. This mechanism moves occupied pages
from the lower part of a memory zone to free pages in the upper part
of the zone. When a compaction scan is finished free pages are grouped
together at the beginning of the zone and allocations of large
physically contiguous areas become possible.

Like reclaim, the compaction may happen asynchronously in the ``kcompactd``
daemon or synchronously as a result of a memory allocation request.

OOM killer
==========

It is possible that on a loaded machine memory will be exhausted and the
kernel will be unable to reclaim enough memory to continue to operate. In
order to save the rest of the system, it invokes the `OOM killer`.

The `OOM killer` selects a task to sacrifice for the sake of the overall
system health. The selected task is killed in a hope that after it exits
enough memory will be freed to continue normal operation.