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Message-Id: <1201811862.32654.29.camel@lappy> Date: Thu, 31 Jan 2008 21:37:42 +0100 From: Peter Zijlstra <a.p.zijlstra@...llo.nl> To: balbir@...ux.vnet.ibm.com Cc: vatsa@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>, menage@...gle.com, containers@...ts.osdl.org, dhaval@...ux.vnet.ibm.com, Balbir Singh <balbir@...ibm.com>, pj@....com Subject: Re: [RFC] Default child of a cgroup On Thu, 2008-01-31 at 23:39 +0530, Balbir Singh wrote: > Srivatsa Vaddagiri wrote: > > Hi, > > As we were implementing multiple-hierarchy support for CPU > > controller, we hit some oddities in its implementation, partly related > > to current cgroups implementation. Peter and I have been debating on the > > exact solution and I thought of bringing that discussion to lkml. > > > > Consider the cgroup filesystem structure for managing cpu resource. > > > > # mount -t cgroup -ocpu,cpuacct none /cgroup > > # mkdir /cgroup/A > > # mkdir /cgroup/B > > # mkdir /cgroup/A/a1 > > > > will result in: > > > > /cgroup > > |------<tasks> > > |------<cpuacct.usage> > > |------<cpu.shares> > > | > > |----[A] > > | |----<tasks> > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | |---[a1] > > | |----<tasks> > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | > > |----[B] > > | |----<tasks> > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | > > > > > > Here are some questions that arise in this picture: > > > > 1. What is the relationship of the task-group in A/tasks with the > > task-group in A/a1/tasks? In otherwords do they form siblings > > of the same parent A? > > > > I consider them to be the same relationship between directories and files. > A/tasks are siblings of A/a1 and A/other children, *but* the entities of > interest are A and A/a1. > > > 2. Somewhat related to the above question, how much resource should the > > task-group A/a1/tasks get in relation to A/tasks? Is it 1/2 of parent > > A's share or 1/(1 + N) of parent A's share (where N = number of tasks > > in A/tasks)? > > > > I propose that it gets 1/2 of the bandwidth, here is why > > 1. Assume that a task in A/tasks forks 1000 children, what happens to the > bandwidth of A/a1's tasks then? We have no control over how many tasks can be > created on A/tasks as a consequence of moving one task to A/tasks. Doing it the > other way would mean, that A/a1/tasks will get 1/1001 of the bandwidth (sounds > very unfair and prone to Denial of Service/Fairness) And I oppose this, it means not all siblings are treated equal. Also, I miss the story of the 'hidden' group here. The biggest objection is this hidden group with no direct controls. My proposal is to make it a hard constraint, either a group has task children or a group has group children, but not mixed. That keeps the interface explicit and doesn't hide the tricks we play. > > 3. What should A/cpuacct.usage reflect? CPU usage of A/tasks? Or CPU usage > > of all siblings put together? It can reflect only one, in which case > > user has to manually derive the other component of the statistics. > > > > It should reflect the accumulated usage of A's children and the tasks in A. A's children includes tasks in this context. See where the confusion is? > > It seems to me that tasks in A/tasks form what can be called the > > "default" child group of A, in which case: > > > > 4. Modifications to A/cpu.shares should affect the parent or its default > > child group (A/tasks)? > > > > To avoid these ambiguities, it may be good if cgroup create this > > "default child group" automatically whenever a cgroup is created? > > Something like below (not the absence of tasks file in some directories > > now): > > > > I think the concept makes sense, but creating a default child is going to be > confusing, as it is not really a child of A. Quite so. I really hate this hidden group. > > > > /cgroup > > | > > |------<cpuacct.usage> > > |------<cpu.shares> > > | > > |---[def_child] > > | |----<tasks> > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | > > |----[A] > > | | > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | |---[def_child] > > | | |----<tasks> > > | | |----<cpuacct.usage> > > | | |----<cpu.shares> > > | | | > > | | > > | |---[a1] > > | | > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | |---[def_child] > > | | |---<tasks> > > | | |---<cpuacct.usage> > > | | |---<cpu.shares> > > | | | > > | > > |----[B] > > | | > > | |----<cpuacct.usage> > > | |----<cpu.shares> > > | | > > | |---[def_child] > > | | |----<tasks> > > | | |----<cpuacct.usage> > > | | |----<cpu.shares> > > | | | > > > > Note that user cannot create subdirectories under def_child with this > > scheme! I am also not sure what impact this will have on other resources > > like cpusets .. > > > > Which means we'll need special logic in the cgroup filesystem to handle > def_child. Not a very good idea. agreed. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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