[PATCH v3 0/4] mm/ksm: Add ksm advisor

Stefan Roesch
2023-12-04T23:49:35Z

What is the KSM advisor?
=========================
The ksm advisor automatically manages the pages_to_scan setting to
achieve a target scan time. The target scan time defines how many seconds
it should take to scan all the candidate KSM pages. In other words the
pages_to_scan rate is changed by the advisor to achieve the target scan
time.

Why do we need a KSM advisor?
==============================
The number of candidate pages for KSM is dynamic. It can often be observed
that during the startup of an application more candidate pages need to be
processed. Without an advisor the pages_to_scan parameter needs to be
sized for the maximum number of candidate pages. With the scan time
advisor the pages_to_scan parameter based can be changed based on demand.

Algorithm
==========
The algorithm calculates the change value based on the target scan time
and the previous scan time. To avoid pertubations an exponentially
weighted moving average is applied.

The algorithm has a max and min
value to:
- guarantee responsiveness to changes
- to limit CPU resource consumption

Parameters to influence the KSM scan advisor
=============================================
The respective parameters are:
- ksm_advisor_mode
0: None (default), 1: scan time advisor
- ksm_advisor_target_scan_time
how many seconds a scan should of all candidate pages take
- ksm_advisor_max_cpu
upper limit for the cpu usage in percent of the ksmd background thread

The initial value and the max value for the pages_to_scan parameter can
be limited with:
- ksm_advisor_min_pages
minimum value for pages_to_scan per batch
- ksm_advisor_max_pages
maximum value for pages_to_scan per batch

The default settings for the above two parameters should be suitable for
most workloads.

The parameters are exposed as knobs in /sys/kernel/mm/ksm. By default the
scan time advisor is disabled.

Currently there are two advisors:
- none and
- scan-time.

Resource savings
=================
Tests with various workloads have shown considerable CPU savings. Most
of the workloads I have investigated have more candidate pages during
startup. Once the workload is stable in terms of memory, the number of
candidate pages is reduced. Without the advisor, the pages_to_scan needs
to be sized for the maximum number of candidate pages. So having this
advisor definitely helps in reducing CPU consumption.

For the instagram workload, the advisor achieves a 25% CPU reduction.
Once the memory is stable, the pages_to_scan parameter gets reduced to
about 40% of its max value.

The new advisor works especially well if the smart scan feature is also
enabled.

How is defining a target scan time better?
===========================================
For an administrator it is more logical to set a target scan time.. The
administrator can determine how many pages are scanned on each scan.
Therefore setting a target scan time makes more sense.

In addition the administrator might have a good idea about the memory
sizing of its respective workloads.

Setting cpu limits is easier than setting The pages_to_scan parameter. The
pages_to_scan parameter is per batch. For the administrator it is difficult
to set the pages_to_scan parameter.

Tracing
=======
A new tracing event has been added for the scan time advisor. The new
trace event is called ksm_advisor. It reports the scan time, the new
pages_to_scan setting and the cpu usage of the ksmd background thread.

Other approaches
=================

Approach 1: Adapt pages_to_scan after processing each batch. If KSM
merges pages, increase the scan rate, if less KSM pages, reduce the
the pages_to_scan rate. This doesn't work too well. While it increases
the pages_to_scan for a short period, but generally it ends up with a
too low pages_to_scan rate.

Approach 2: Adapt pages_to_scan after each scan. The problem with that
approach is that the calculated scan rate tends to be high. The more
aggressive KSM scans, the more pages it can de-duplicate.

There have been earlier attempts at an advisor:
propose auto-run mode of ksm and its tests
(https://marc.info/?l=linux-mm&m=166029880214485&w=2)

Changes:
========
V3:
- Use string parameters for advisor mode
- Removed min cpu load sysfs knob
- dropped unused enums in ksm_advisor_type
- renamed KSM_ADVISOR_LAST to KSM_ADVISOR_COUNT
- init_advisor() is needed but changed how it is initialized
- don't allow to change pages_to_scan parameter when scan-time advisor
is enabled
- add ksm_advisor_start_scan() and ksm_advisor_stop_scan() functions
to calculate scan time
- removed scan time parameter to scan_time_advisor() function

V2:
- Use functions for long long calculations to support 32 bit platforms
- Use cpu min and cpu max settings for the advisor instead of the pages
min and max parameters.
- pages min and max values are now used for the initial and max values.
Generally they are not required to be changed.
- Add cpu percent usage value to tracepoint definition
- Update documentation for cpu min and cpu max values
- Update commit messages for the above changes

*** BLURB HERE ***

Stefan Roesch (4):
mm/ksm: add ksm advisor
mm/ksm: add sysfs knobs for advisor
mm/ksm: add tracepoint for ksm advisor
mm/ksm: document ksm advisor and its sysfs knobs

Documentation/admin-guide/mm/ksm.rst | 54 +++++
include/trace/events/ksm.h | 33 +++
mm/ksm.c | 303 ++++++++++++++++++++++++++-
3 files changed, 389 insertions(+), 1 deletion(-)

base-commit: 12d04a7bf0da67321229d2bc8b1a7074d65415a9
--
2.39.3