Documentation/vm/damon/design.rst

.. SPDX-License-Identifier: GPL-2.0

======
Design
======

Configurable Layers
===================

DAMON provides data access monitoring functionality while making the accuracy
and the overhead controllable.  The fundamental access monitorings require
primitives that dependent on and optimized for the target address space.  On
the other hand, the accuracy and overhead tradeoff mechanism, which is the core
of DAMON, is in the pure logic space.  DAMON separates the two parts in
different layers and defines its interface to allow various low level
primitives implementations configurable with the core logic.

Due to this separated design and the configurable interface, users can extend
DAMON for any address space by configuring the core logics with appropriate low
level primitive implementations.  If appropriate one is not provided, users can
implement the primitives on their own.

For example, physical memory, virtual memory, swap space, those for specific
processes, NUMA nodes, files, and backing memory devices would be supportable.
Also, if some architectures or devices support special optimized access check
primitives, those will be easily configurable.


Reference Implementations of Address Space Specific Primitives
==============================================================

The low level primitives for the fundamental access monitoring are defined in
two parts:

1. Identification of the monitoring target address range for the address space.
2. Access check of specific address range in the target space.

DAMON currently provides the implementations of the primitives for the physical
and virtual address spaces. Below two subsections describe how those work.


VMA-based Target Address Range Construction
-------------------------------------------

This is only for the virtual address space primitives implementation.  That for
the physical address space simply asks users to manually set the monitoring
target address ranges.

Only small parts in the super-huge virtual address space of the processes are
mapped to the physical memory and accessed.  Thus, tracking the unmapped
address regions is just wasteful.  However, because DAMON can deal with some
level of noise using the adaptive regions adjustment mechanism, tracking every
mapping is not strictly required but could even incur a high overhead in some
cases.  That said, too huge unmapped areas inside the monitoring target should
be removed to not take the time for the adaptive mechanism.

For the reason, this implementation converts the complex mappings to three
distinct regions that cover every mapped area of the address space.  The two
gaps between the three regions are the two biggest unmapped areas in the given
address space.  The two biggest unmapped areas would be the gap between the
heap and the uppermost mmap()-ed region, and the gap between the lowermost
mmap()-ed region and the stack in most of the cases.  Because these gaps are
exceptionally huge in usual address spaces, excluding these will be sufficient
to make a reasonable trade-off.  Below shows this in detail::

    <heap>
    <BIG UNMAPPED REGION 1>
    <uppermost mmap()-ed region>
    (small mmap()-ed regions and munmap()-ed regions)
    <lowermost mmap()-ed region>
    <BIG UNMAPPED REGION 2>
    <stack>


PTE Accessed-bit Based Access Check
-----------------------------------

Both of the implementations for physical and virtual address spaces use PTE
Accessed-bit for basic access checks.  Only one difference is the way of
finding the relevant PTE Accessed bit(s) from the address.  While the
implementation for the virtual address walks the page table for the target task
of the address, the implementation for the physical address walks every page
table having a mapping to the address.  In this way, the implementations find
and clear the bit(s) for next sampling target address and checks whether the
bit(s) set again after one sampling period.  This could disturb other kernel
subsystems using the Accessed bits, namely Idle page tracking and the reclaim
logic.  To avoid such disturbances, DAMON makes it mutually exclusive with Idle
page tracking and uses ``PG_idle`` and ``PG_young`` page flags to solve the
conflict with the reclaim logic, as Idle page tracking does.


Address Space Independent Core Mechanisms
=========================================

Below four sections describe each of the DAMON core mechanisms and the five
monitoring attributes, ``sampling interval``, ``aggregation interval``,
``regions update interval``, ``minimum number of regions``, and ``maximum
number of regions``.


Access Frequency Monitoring
---------------------------

The output of DAMON says what pages are how frequently accessed for a given
duration.  The resolution of the access frequency is controlled by setting
``sampling interval`` and ``aggregation interval``.  In detail, DAMON checks
access to each page per ``sampling interval`` and aggregates the results.  In
other words, counts the number of the accesses to each page.  After each
``aggregation interval`` passes, DAMON calls callback functions that previously
registered by users so that users can read the aggregated results and then
clears the results.  This can be described in below simple pseudo-code::

    while monitoring_on:
        for page in monitoring_target:
            if accessed(page):
                nr_accesses[page] += 1
        if time() % aggregation_interval == 0:
            for callback in user_registered_callbacks:
                callback(monitoring_target, nr_accesses)
            for page in monitoring_target:
                nr_accesses[page] = 0
        sleep(sampling interval)

The monitoring overhead of this mechanism will arbitrarily increase as the
size of the target workload grows.


Region Based Sampling
---------------------

To avoid the unbounded increase of the overhead, DAMON groups adjacent pages
that assumed to have the same access frequencies into a region.  As long as the
assumption (pages in a region have the same access frequencies) is kept, only
one page in the region is required to be checked.  Thus, for each ``sampling
interval``, DAMON randomly picks one page in each region, waits for one
``sampling interval``, checks whether the page is accessed meanwhile, and
increases the access frequency of the region if so.  Therefore, the monitoring
overhead is controllable by setting the number of regions.  DAMON allows users
to set the minimum and the maximum number of regions for the trade-off.

This scheme, however, cannot preserve the quality of the output if the
assumption is not guaranteed.


Adaptive Regions Adjustment
---------------------------

Even somehow the initial monitoring target regions are well constructed to
fulfill the assumption (pages in same region have similar access frequencies),
the data access pattern can be dynamically changed.  This will result in low
monitoring quality.  To keep the assumption as much as possible, DAMON
adaptively merges and splits each region based on their access frequency.

For each ``aggregation interval``, it compares the access frequencies of
adjacent regions and merges those if the frequency difference is small.  Then,
after it reports and clears the aggregated access frequency of each region, it
splits each region into two or three regions if the total number of regions
will not exceed the user-specified maximum number of regions after the split.

In this way, DAMON provides its best-effort quality and minimal overhead while
keeping the bounds users set for their trade-off.


Dynamic Target Space Updates Handling
-------------------------------------

The monitoring target address range could dynamically changed.  For example,
virtual memory could be dynamically mapped and unmapped.  Physical memory could
be hot-plugged.

As the changes could be quite frequent in some cases, DAMON checks the dynamic
memory mapping changes and applies it to the abstracted target area only for
each of a user-specified time interval (``regions update interval``).