On Thu, Apr 12, 2012 at 09:56:49AM +0900, KAMEZAWA Hiroyuki wrote: > (2012/04/12 3:57), Frederic Weisbecker wrote: > > > Hi, > > > > While talking with Tejun about targetting the cgroup task counter subsystem > > for the next merge window, he suggested to check if this could be merged into > > the memcg subsystem rather than creating a new one cgroup subsystem just > > for task count limit purpose. > > > > So I'm pinging you guys to seek your insight. > > > > I assume not everybody in the Cc list knows what the task counter subsystem > > is all about. So here is a summary: this is a cgroup subsystem (latest version > > in https://lwn.net/Articles/478631/) that keeps track of the number of tasks > > present in a cgroup. Hooks are set in task fork/exit and cgroup migration to > > maintain this accounting visible to a special tasks.usage file. The user can > > set a limit on the number of tasks by writing on the tasks.limit file. > > Further forks or cgroup migration are then rejected if the limit is exceeded. > > > > This feature is especially useful to protect against forkbombs in containers. > > Or more generally to limit the resources on the number of tasks on a cgroup > > as it involves some kernel memory allocation. > > > > Now the dilemna is how to implement it? > > > > 1) As a standalone subsystem, as it stands currently (https://lwn.net/Articles/478631/) > > > > 2) As a feature in memcg, part of the memory.kmem.* files. This makes sense > > because this is about kernel memory allocation limitation. We could have a > > memory.kmem.tasks.count > > > > My personal opinion is that the task counter brings some overhead: a charge > > across the whole hierarchy at every fork, and the mirrored uncharge on task exit. > > And this overhead happens even in the off-case (when the task counter susbsystem > > is mounted but the limit is the default: ULLONG_MAX). > > > > So if we choose the second solution, this overhead will be added unconditionally > > to memcg. > > But I don't expect every users of memcg will need the task counter. So perhaps > > the overhead should be kept in its own separate subsystem. > > > > OTOH memory.kmem.* interface would have be a good fit. > > > > What do you think? > > > Sounds interesting to me. Hm, does your 'overhead' of task accounting is > enough large to be visible to users ? How performance regression is big ? I haven't measured. But on every fork, we do a res_counter_charge() that walks through css_set and all its css_set ancestors, take a spinlock and increment something to every level. In terms of cache trashing and algorithm complexity, I believe the issue is real. > BTW, now, all memcg's limit interfaces use 'bytes' as an unit of accounting. > It's a small concern to me to have mixture of bytes and numbers of objects > for accounting. Indeed, this can be confusing for users. > But I think increasing number of subsystem is not very good.... If the result is a better granularity on the overhead, I believe this can be a good thing. _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
