[v4,0/4] Smart scanning mode for KSM

Message

Stefan Roesch Sept. 27, 2023, 4:22 p.m. UTC

This patch series adds "smart scanning" for KSM.

What is smart scanning?
=======================
KSM evaluates all the candidate pages for each scan. It does not use historic
information from previous scans. This has the effect that candidate pages that
couldn't be used for KSM de-duplication continue to be evaluated for each scan.

The idea of "smart scanning" is to keep historic information. With the historic
information we can temporarily skip the candidate page for one or several scans.

Details:
========
"Smart scanning" is to keep two small counters to store if the page has been
used for KSM. One counter stores how often we already tried to use the page for
KSM and the other counter stores how often we skip a page.

How often we skip the candidate page depends how often a page failed KSM
de-duplication. The code skips a maximum of 8 times. During testing this has
shown to be a good compromise for different workloads.

New sysfs knob:
===============
Smart scanning is enabled by default. With /sys/kernel/mm/ksm/smart_scan
smart scanning can be disabled.

Monitoring:
===========
To monitor how effective smart scanning is a new sysfs knob has been introduced.
/sys/kernel/mm/pages_skipped report how many pages have been skipped by smart
scanning.

Results:
========
- Various workloads have shown a 20% - 25% reduction in page scans
  For the instagram workload for instance, the number of pages scanned has been
  reduced from over 20M pages per scan to less than 15M pages.
- Less pages scans also resulted in an overall higher de-duplication rate as
  some shorter lived pages could be de-duplicated additionally
- Less pages scanned allows to reduce the pages_to_scan parameter
  and this resulted in  a 25% reduction in terms of CPU.
- The improvements have been observed for workloads that enable KSM with
  madvise as well as prctl


Changes:
  - V4:
    - Changed default from disabled to enabled
    - Changed commit message for first patch to mention it is enabled
    - Changed documentation to mention it is enabled by default
    - Changed cover letter description to mention it is enabled by default
  - V3:
    - Renamed field skip_age to remaining_skips
    - Moved fields after old_checksum
    - Changed should_skip_rmap_item to use remaining_skips field
  - V2:
    - Renamed function inc_skip_age() to skip_age()
    - Added comment to skip_age() function
    - Renamed function skip_rmap_item() to should_skip_rmap_item()
    - Added more comments to should_skip_rmap_item function
    - Added explicit modification of age with overflow check


Stefan Roesch (4):
  mm/ksm: add "smart" page scanning mode
  mm/ksm: add pages_skipped metric
  mm/ksm: document smart scan mode
  mm/ksm: document pages_skipped sysfs knob

 Documentation/admin-guide/mm/ksm.rst |  11 +++
 mm/ksm.c                             | 115 +++++++++++++++++++++++++++
 2 files changed, 126 insertions(+)


base-commit: 15bcc9730fcd7526a3b92eff105d6701767a53bb