[PATCH v1 0/4] mm/ksm: Add ksm advisor

[Stefan Roesch]

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 avoid to spend too much CPU

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_min_pages
  minimum value for pages_to_scan per batch
- ksm_advisor_max_pages
  maximum value for pages_to_scan per batch

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.

The pages_to_scan parameter is per batch. But the administrator has
no insight how many batches are needed per scan.

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 and the new
pages_to_scan setting.

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)



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 |  45 +++++
 include/trace/events/ksm.h           |  28 ++++
 mm/ksm.c                             | 242 ++++++++++++++++++++++++++-
 3 files changed, 314 insertions(+), 1 deletion(-)


base-commit: 12d04a7bf0da67321229d2bc8b1a7074d65415a9
-- 
2.39.3