On Tue, Jul 21, 2020 at 11:51 AM Roman Gushchin <guro@xxxxxx> wrote: > > On Tue, Jul 21, 2020 at 01:41:26PM -0400, Johannes Weiner wrote: > > On Tue, Jul 21, 2020 at 11:19:52AM +0000, jingrui wrote: > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> ; Michal Hocko <mhocko@xxxxxxxxxx>; Vladimir Davydov <vdavydov.dev@xxxxxxxxx> > > > > > > Thanks. > > > > > > --- > > > PROBLEM: cgroup cost too much memory when transfer small files to tmpfs. > > > > > > keywords: cgroup PERCPU/memory cost too much. > > > > > > description: > > > > > > We send small files from node-A to node-B tmpfs /tmp directory using sftp. On > > > node-B the systemd configured with pam on like below. > > > > > > cat /etc/pam.d/password-auth | grep systemd > > > -session optional pam_systemd.so > > > > > > So when transfer a file, a systemd session is created, that means a cgroup is > > > created, then file saved at /tmp will associated with a cgroup object. After > > > file transferred, session and cgroup-dir will be removed, but the file in /tmp > > > still associated with the cgroup object. The PERCPU memory in cgroup/css object > > > cost a lot(about 0.5MB/per-cgroup-object) on 200/cpus machine. > > > > CC Roman who had a patch series to free all this extended (percpu) > > memory upon cgroup deletion: > > > > https://lore.kernel.org/patchwork/cover/1050508/ > > > > It looks like it never got merged for some reason. > > The mentioned patchset can make the problem less noticeable, but can't solve it completely. > It has never been merged, because the dying cgroup problem was mostly solved by other methods: > slab memory reparenting and various reclaim fixes. So there was no more reason to complicate > the code to release the memcg memory early. > > The overhead of creating and destroying a new memory cgroup for a transfer of a small > file will be noticeable anyway. So IMO the solution is to use a single cgroup for all > transfers. I don't know if systemd supports such mode out of the box, but it shouldn't > be hard to add it. > > But also I wonder if we need a special tmpfs mount option, something like "noaccount". > Not only for this specific case, but also for the case when tmpfs is extensively > shared between multiple cgroups or if it's used to pass some data from one cgroup > to another, or if we care about the performance more than about the accounting; > in other words for cases where the accounting makes more harm than good. > Internally we actually have an tmpfs mount option "memcg=" which charges all the memory of the tmpfs files on that mount to the given memcg and the motivation is the shared tmpfs files between multiple cgroups. One concrete use-case is the shared memory used for communication between the application and the user space network driver [1]. The "memcg=root" can be used as a "noaccount" option. [1] https://sosp19.rcs.uwaterloo.ca/slides/marty.pdf
