On Mon, Apr 04, 2022 at 10:13:03AM -0700, Yosry Ahmed wrote: > On Fri, Apr 1, 2022 at 11:39 AM Roman Gushchin <roman.gushchin@xxxxxxxxx> wrote: > > > > On Fri, Apr 01, 2022 at 02:11:51AM -0700, Yosry Ahmed wrote: > > > On Thu, Mar 31, 2022 at 10:25 AM Roman Gushchin > > > <roman.gushchin@xxxxxxxxx> wrote: > > > > > > > > On Thu, Mar 31, 2022 at 08:41:51AM +0000, 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/ > > > > > > > > Hello! > > > > > > > > I'm totally up for the proposed feature! It makes total sense and is proved > > > > to be useful, let's add it. > > > > > > > > > > > > > > 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. > > > > > > > > Maybe an option to specify a timeout? That might simplify the userspace part. > > > > Also, please please add a test to selftests/cgroup/memcg tests. > > > > It will also provide an example on how the userspace can use the feature. > > > > > > > > > > Hi Roman, thanks for taking the time to review this! > > > > > > A timeout can be a good extension, I will add it to the commit message > > > in the next version in possible extensions. > > > > > > I will add a test in v2, thanks! > > > > Great, thank you! > > > > > > > > > > > > > > > > > > > For now, let's keep things simple by adding the basic functionality. > > > > > > > > What I'm worried about is how we gonna extend it? How do you see the interface > > > > with 2-3 extensions from the list above? All these extensions look very > > > > reasonable to me, so we'll likely have to implement them soon. So let's think > > > > about the extensibility now. > > > > > > > > > > My idea is to have these extensions as optional positional arguments > > > (like Wei suggested), so that the interface does not get too > > > complicated for users who don't care about tuning these options. If > > > this is the case then I think there is nothing to worry about. > > > Otherwise, if you think some of these options make sense to be a > > > required argument instead, we can rethink the initial interface. > > > > The interface you're proposing is not really extensible, so we'll likely need to > > introduce a new interface like memory.reclaim_ext very soon. Why not create > > an extensible API from scratch? > > > > I'm looking at cgroup v2 documentation which describes various interface files > > formats and it seems like given the number of potential optional arguments > > the best option is nested keyed (please, refer to the Interface Files section). > > > > E.g. the format can be: > > echo "1G type=file nodemask=1-2 timeout=30s" > memory.reclaim > > > > We can say that now we don't support any keyed arguments, but they can be > > added in the future. > > > > Basically you don't even need to change any code, only document the interface > > properly, so we can extend it later without breaking the API. > > > > Thanks a lot for suggesting this, it indeed looks very much cleaner. > > I will make sure to document the interface properly as suggested in v2. > > > > > > > > I wonder if it makes more sense to introduce a sys_reclaim() syscall instead? > > > > In the end, such a feature might make sense on the system level too. > > > > Yes, there is the drop_caches sysctl, but it's too radical for many cases. > > > > > > > > > > I think in the RFC discussion there was consensus to add both a > > > per-memcg knob, as well as per-node / per-system knobs (through sysfs > > > or syscalls) later. Wei also points out that it's not common for a > > > syscall to have a cgroup argument. > > > > Actually there are examples (e.g. sys_bpf), but my only point is to make > > the API extensible, so maybe syscall is not the best idea. > > > > I'd add the root level interface from scratch: the code change is simple > > and it makes sense as a feature. Then likely we don't really need another > > system-level interface at all. > > > > I think we would still need a system-level interface anyway for > systems with no memcg that wish to make use of proactive memory > reclaim. We can still make the memory.reclaim interface available for > root as well if you think this is desirable. Yes, I think it's a good idea. !memcg systems is a different story, we can handle them separately. Thanks!
