Re: [RFC PATCH 0/2] mm: multi-gen LRU: working set extensions

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

Yuanchu Xie <yuanchu@google.com> writes:

> Introduce a way of monitoring the working set of a workload, per page
> type and per NUMA node, with granularity in minutes. It has page-level
> granularity and minimal memory overhead by building on the
> Multi-generational LRU framework, which already has most of the
> infrastructure and is just missing a useful interface.
>
> MGLRU organizes pages in generations, where an older generation contains
> colder pages, and aging promotes the recently used pages into the young
> generation and creates a new one. The working set size is how much
> memory an application needs to keep working, the amount of "hot" memory
> that's frequently used. The only missing pieces between MGLRU
> generations and working set estimation are a consistent aging cadence
> and an interface; we introduce the two additions.

So with kold kthread do we need aging in reclaim ? Should we switch reciam
to wakeup up kold kthread to do aging instead of doing try_to_inc_max_seq?
This would also help us to try different aging mechanism which can run
better in a kthread. 


>
> Periodic aging
> ======
> MGLRU Aging is currently driven by reclaim, so the amount of time
> between generations is non-deterministic. With memcgs being aged
> regularly, MGLRU generations become time-based working set information.
>
> - memory.periodic_aging: a new root-level only file in cgroupfs
> Writing to memory.periodic_aging sets the aging interval and opts into
> periodic aging.
> - kold: a new kthread that ages memcgs based on the set aging interval.
>
> Page idle age stats
> ======
> - memory.page_idle_age: we group pages into idle age ranges, and present
>   the number of pages per node per pagetype in each range. This
>   aggregates the time information from MGLRU generations hierarchically.
>
> Use case: proactive reclaimer
> ======
> The proactive reclaimer sets the aging interval, and periodically reads
> the page idle age stats, forming a working set estimation, which it then
> calculates an amount to write to memory.reclaim.
>
> With the page idle age stats, a proactive reclaimer could calculate a
> precise amount of memory to reclaim without continuously probing and
> inducing reclaim.
>
> A proactive reclaimer that uses a similar interface is used in the
> Google data centers.
>
> Use case: workload introspection
> ======
> A workload may use the working set estimates to adjust application
> behavior as needed, e.g. preemptively killing some of its workers to
> avoid its working set thrashing, or dropping caches to fit within a
> limit.
> It can also be valuable to application developers, who can benefit from
> an out-of-the-box overview of the application's usage behaviors.
>
> TODO List
> ======
> - selftests
> - a userspace demonstrator combining periodic aging, page idle age
>   stats, memory.reclaim, and/or PSI
>
> Open questions
> ======
> - MGLRU aging mechanism has a flag called force_scan. With
>   force_scan=false, invoking MGLRU aging when an lruvec has a maximum
>   number of generations does not actually perform aging.
>   However, with force_scan=true, MGLRU moves the pages in the oldest
>   generation to the second oldest generation. The force_scan=true flag
>   also disables some optimizations in MGLRU's page table walks.
>   The current patch sets force_scan=true, so that periodic aging would
>   work without a proactive reclaimer evicting the oldest generation.
>
> - The page idle age format uses a fixed set of time ranges in seconds.
>   I have considered having it be based on the aging interval, or just
>   compiling the raw timestamps.
>   With the age ranges based on the aging interval, a memcg that's
>   undergoing memcg reclaim might have its generations in the 10
>   seconds range, and a much longer aging interval would obscure this
>   fact.
>   The raw timestamps from MGLRU could lead to a very large file when
>   aggregated hierarchically.
>
> Yuanchu Xie (2):
>   mm: multi-gen LRU: periodic aging
>   mm: multi-gen LRU: cgroup working set stats
>
>  include/linux/kold.h   |  44 ++++++++++
>  include/linux/mmzone.h |   4 +-
>  mm/Makefile            |   3 +
>  mm/kold.c              | 150 ++++++++++++++++++++++++++++++++
>  mm/memcontrol.c        | 188 +++++++++++++++++++++++++++++++++++++++++
>  mm/vmscan.c            |  35 +++++++-
>  6 files changed, 422 insertions(+), 2 deletions(-)
>  create mode 100644 include/linux/kold.h
>  create mode 100644 mm/kold.c
>
> -- 
> 2.39.0.314.g84b9a713c41-goog