Messages in this thread | | | Subject | Re: [RFC 3/5] sched: Add CPU rate hard caps | From | Matt Helsley <> | Date | Fri, 02 Jun 2006 04:23:04 -0700 |
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On Fri, 2006-06-02 at 11:33 +1000, Peter Williams wrote: > Al Boldi wrote: > > Chandra Seetharaman wrote: > >> On Thu, 2006-06-01 at 14:04 +0530, Balbir Singh wrote: > >>> Kirill Korotaev wrote: > >>>>> Do you have any documented requirements for container resource > >>>>> management? > >>>>> Is there a minimum list of features and nice to have features for > >>>>> containers > >>>>> as far as resource management is concerned? > >>>> Sure! You can check OpenVZ project (http://openvz.org) for example of > >>>> required resource management. BTW, I must agree with other people here > >>>> who noticed that per-process resource management is really useless and > >>>> hard to use :( > >> I totally agree. > >> > >>> I'll take a look at the references. I agree with you that it will be > >>> useful to have resource management for a group of tasks. > > > > For Resource Management to be useful it must depend on Resource Control. > > Resource Control depends on per-process accounting. Per-process accounting, > > when abstracted sufficiently, may enable higher level routines, preferrably > > in userland, to extend functionality at will. All efforts should really go > > into the successful abstraction of per-process accounting. > > I couldn't agree more. All that's needed in the kernel is low level per > task control and statistics gathering. The rest can be done in user space.
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I'm assuming by "The rest can be done in user space" you mean that tasks can be grouped, accounting information updated (% CPU), and various knobs (nice) can be turned to keep task resource (CPU) usage under control.
If I seem to be describing your suggestion then I don't think it will work. Below you'll find the reasons I've come to this conclusion. Am I oversimplifying or misunderstanding something critical?
Groups are needed to prevent processes from consuming unlimited resources using clone/fork. However, since our accounting sources and control knobs are per-task we must adjust per-task knobs within a group every time accounting indicates a change in resource usage.
Let us suppose we have a UP system with 3 tasks -- group X: X1, X2; and Z. By adjusting nice values of X1 and X2 Z is responsible for ensuring that group X does not exceed its limit of 50% CPU. Further suppose that X1 and X2 are each using 25% of the CPU. In order to prevent X1 + X2 from exceeding 50% each must be limited to 25% by an appropriate nice value. [Note the hand wave: I'm assuming nice can be mapped to a predictable percentage of CPU on a UP system.]
When accounting data indicates X2 has dropped to 15% of the CPU, Z may raise X1's limit (to 35% at most) and it must lower X2's limit (down to as little as 15%). Z must raise X1's limit by some amount (delta) otherwise X1 could never increase its CPU usage. Z must decrease X2 to 25 - delta, otherwise the sum could exceed 50%. [Aside: In fact, if we have N tasks in group X then it seems Z ought to adjust N nice values by a total of delta. How delta gets distributed limits the rate at which CPU usage may increase and would ideally depend on future changes in usage.]
There are two problems as I see it:
1) If X1 grows to use 35% then X2's usage can't grow back from 15% until X1 relents. This is seems unpleasantly like cooperative scheduling within group X because if we take this to its limit X2 gets 0% and X1 gets 50% -- effectively starving X2. What little I know about nice suggests this wouldn't really happen. However I think may highlight one case where fiddling with nice can't effectively control CPU usage.
2) Suppose we add group Y with tasks Y1-YM, Y's CPU usage is limited to 49%, each task of Y uses its limit of (M/49)% CPU, and the remaining 1% is left for Z (i.e. the single CPU is being used heavily). Z must use this 1% to read accounting information and adjust nice values as described above. If X1 spawns X3 we're likely in trouble -- Z might not get to run for a while but X3 has inheritted X1's nice value. If we return to our initial assumption that X1 and X2 are each using their limit of 25% then X3 will get limited to 25% too. The sum of Xi can now exceed 50% until Z is scheduled next. This only gets worse if there is an imbalance between X1 and X2 as described earlier. In that case group X could use 100% CPU until Z is scheduled! It also probably gets worse as load increases and the number of scheduling opportunities for Z decrease.
I don't see how task Z could solve the second problem. As with UP, in SMP I think it depends on when Z (or one Z fixed to each CPU) is scheduled.
I think these are simple scenarios that demonstrate the problem with splitting resource management into accounting and control with userspace in between.
Cheers, -Matt Helsley
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