Re: [PATCH v5 1/4] memcg: introduce per-memcg reclaim interface

Davidlohr Bueso
2022-05-04T00:25:27Z

On Mon, 25 Apr 2022, Yosry Ahmed wrote:

>From: Shakeel Butt <shakeelb@google.com>
>
>Introduce a memcg interface to trigger memory reclaim on a memory cgroup.
>
>Use case: Proactive Reclaim
>---------------------------
>
>A userspace proactive reclaimer can continuously probe the memcg to
>reclaim a small amount of memory. This gives more accurate and
>up-to-date workingset estimation as the LRUs are continuously
>sorted and can potentially provide more deterministic memory
>overcommit behavior. The memory overcommit controller can provide
>more proactive response to the changing behavior of the running
>applications instead of being reactive.
>
>A userspace reclaimer's purpose in this case is not a complete replacement
>for kswapd or direct reclaim, it is to proactively identify memory savings
>opportunities and reclaim some amount of cold pages set by the policy
>to free up the memory for more demanding jobs or scheduling new jobs.
>
>A user space proactive reclaimer is used in Google data centers.
>Additionally, Meta's TMO paper recently referenced a very similar
>interface used for user space proactive reclaim:
>https://dl.acm.org/doi/pdf/10.1145/3503222.3507731
>
>Benefits of a user space reclaimer:
>-----------------------------------
>
>1) More flexible on who should be charged for the cpu of the memory
>reclaim. For proactive reclaim, it makes more sense to be centralized.
>
>2) More flexible on dedicating the resources (like cpu). The memory
>overcommit controller can balance the cost between the cpu usage and
>the memory reclaimed.
>
>3) Provides a way to the applications to keep their LRUs sorted, so,
>under memory pressure better reclaim candidates are selected. This also
>gives more accurate and uptodate notion of working set for an
>application.
>
>Why memory.high is not enough?
>------------------------------
>
>- memory.high can be used to trigger reclaim in a memcg and can
> potentially be used for proactive reclaim.
> However there is a big downside in using memory.high. It can potentially
> introduce high reclaim stalls in the target application as the
> allocations from the processes or the threads of the application can hit
> the temporary memory.high limit.
>
>- Userspace proactive reclaimers usually use feedback loops to decide
> how much memory to proactively reclaim from a workload. The metrics
> used for this are usually either refaults or PSI, and these metrics
> will become messy if the application gets throttled by hitting the
> high limit.
>
>- memory.high is a stateful interface, if the userspace proactive
> reclaimer crashes for any reason while triggering reclaim it can leave
> the application in a bad state.
>
>- If a workload is rapidly expanding, setting memory.high to proactively
> reclaim memory can result in actually reclaiming more memory than
> intended.
>
>The benefits of such interface and shortcomings of existing interface
>were further discussed in this RFC thread:
>https://lore.kernel.org/linux-mm/5df21376-7dd1-bf81-8414-32a73cea45dd@google.com/
>
>Interface:
>----------
>
>Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to
>trigger reclaim in the target memory cgroup.
>
>The interface is introduced as a nested-keyed file to allow for future
>optional arguments to be easily added to configure the behavior of
>reclaim.
>
>Possible Extensions:
>--------------------
>
>- This interface can be extended with an additional parameter or flags
> to allow specifying one or more types of memory to reclaim from (e.g.
> file, anon, ..).
>
>- The interface can also be extended with a node mask to reclaim from
> specific nodes. This has use cases for reclaim-based demotion in memory
> tiering systens.
>
>- A similar per-node interface can also be added to support proactive
> reclaim and reclaim-based demotion in systems without memcg.
>
>- Add a timeout parameter to make it easier for user space to call the
> interface without worrying about being blocked for an undefined amount
> of time.
>
>For now, let's keep things simple by adding the basic functionality.
>
>[yosryahmed@google.com: worked on versions v2 onwards, refreshed to
>current master, updated commit message based on recent
>discussions and use cases]
>
>Signed-off-by: Shakeel Butt <shakeelb@google.com>
>Co-developed-by: Yosry Ahmed <yosryahmed@google.com>
>Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>Acked-by: Michal Hocko <mhocko@suse.com>
>Acked-by: Wei Xu <weixugc@google.com>
>Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>