On Wed, Oct 13, 2021 at 12:23 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote: > > Below is a proposal for deterministic charging of shared memory. > Please take a look and let me know if there are any major concerns: > Friendly ping on the proposal below. If there are any issues you see that I can address in the v1 I send for review, I would love to know. And if the proposal seems fine as is I would also love to know. Thanks! Mina > Problem: > Currently shared memory is charged to the memcg of the allocating > process. This makes memory usage of processes accessing shared memory > a bit unpredictable since whichever process accesses the memory first > will get charged. We have a number of use cases where our userspace > would like deterministic charging of shared memory: > > 1. System services allocating memory for client jobs: > We have services (namely a network access service[1]) that provide > functionality for clients running on the machine and allocate memory > to carry out these services. The memory usage of these services > depends on the number of jobs running on the machine and the nature of > the requests made to the service, which makes the memory usage of > these services hard to predict and thus hard to limit via memory.max. > These system services would like a way to allocate memory and instruct > the kernel to charge this memory to the client’s memcg. > > 2. Shared filesystem between subtasks of a large job > Our infrastructure has large meta jobs such as kubernetes which spawn > multiple subtasks which share a tmpfs mount. These jobs and its > subtasks use that tmpfs mount for various purposes such as data > sharing or persistent data between the subtask restarts. In kubernetes > terminology, the meta job is similar to pods and subtasks are > containers under pods. We want the shared memory to be > deterministically charged to the kubernetes's pod and independent to > the lifetime of containers under the pod. > > 3. Shared libraries and language runtimes shared between independent jobs. > We’d like to optimize memory usage on the machine by sharing libraries > and language runtimes of many of the processes running on our machines > in separate memcgs. This produces a side effect that one job may be > unlucky to be the first to access many of the libraries and may get > oom killed as all the cached files get charged to it. > > Design: > My rough proposal to solve this problem is to simply add a > ‘memcg=/path/to/memcg’ mount option for filesystems (namely tmpfs): > directing all the memory of the file system to be ‘remote charged’ to > cgroup provided by that memcg= option. > > Caveats: > 1. One complication to address is the behavior when the target memcg > hits its memory.max limit because of remote charging. In this case the > oom-killer will be invoked, but the oom-killer may not find anything > to kill in the target memcg being charged. In this case, I propose > simply failing the remote charge which will cause the process > executing the remote charge to get an ENOMEM This will be documented > behavior of remote charging. > 2. I would like to provide an initial implementation that adds this > support for tmpfs, while leaving the implementation generic enough for > myself or others to extend to more filesystems where they find the > feature useful. > 3. I would like to implement this for both cgroups v2 _and_ cgroups > v1, as we still have cgroup v1 users. If this is unacceptable I can > provide the v2 implementation only, and maintain a local patch for the > v1 support. > > If this proposal sounds good in principle. I have an experimental > implementation that I can make ready for review. Please let me know of > any concerns you may have. Thank you very much in advance! > Mina Almasry > > [1] https://research.google/pubs/pub48630/
