Matt Helsley wrote: > 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. > > <snip> > > 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.
You're trying to do it all with nice. I said it could be done with nice plus the CPU capping functionality my patch provides. Plus the stats of course. Peter -- Peter Williams [EMAIL PROTECTED] "Learning, n. The kind of ignorance distinguishing the studious." -- Ambrose Bierce _______________________________________________ ckrm-tech mailing list https://lists.sourceforge.net/lists/listinfo/ckrm-tech
