On Fri, Apr 1, 2022 at 6:54 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Thu 31-03-22 08:41:51, Yosry Ahmed wrote: > > From: Shakeel Butt <shakeelb@xxxxxxxxxx> > > > > Introduce an 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@xxxxxxxxxx/ > > > > Interface: > > ---------- > > > > Introducing a very simple memcg interface 'echo 10M > memory.reclaim' to > > trigger reclaim in the target memory cgroup. > > > > > > 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. > > > > For now, let's keep things simple by adding the basic functionality. > > Yes, I am for the simplicity and this really looks like a bare minumum > interface. But it is not really clear who do you want to add flags on > top of it? > > I am not really sure we really need a node aware interface for memcg. > The global reclaim interface will likely need a different node because > we do not want to make this CONFIG_MEMCG constrained. A nodemask argument for memory.reclaim can be useful for memory tiering between NUMA nodes with different performance. Similar to proactive reclaim, it can allow a userspace daemon to drive memcg-based proactive demotion via the reclaim-based demotion mechanism in the kernel. > > [yosryahmed@xxxxxxxxxx: refreshed to current master, updated commit > > message based on recent discussions and use cases] > > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > All that being said. I haven't been a great fan for explicit reclaim > triggered from the userspace but I do recognize that limitations of the > existing interfaces is just too restrictive. > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > > Thanks! > -- > Michal Hocko > SUSE Labs
