On Mon, Dec 17, 2012 at 01:17:38PM -0800, Tejun Heo wrote: > Hello, > > On Mon, Dec 17, 2012 at 03:53:18PM -0500, Vivek Goyal wrote: > > On Fri, Dec 14, 2012 at 02:41:20PM -0800, Tejun Heo wrote: > > > Currently, cfqg charges are scaled directly according to cfqg->weight. > > > Regardless of the number of active cfqgs or the amount of active > > > weights, a given weight value always scales charge the same way. This > > > works fine as long as all cfqgs are treated equally regardless of > > > their positions in the hierarchy, which is what cfq currently > > > implements. It can't work in hierarchical settings because the > > > interpretation of a given weight value depends on where the weight is > > > located in the hierarchy. > > > > I did not understand this. Why the current scheme will not work with > > hierarchy? > > Because the meaning of a weight changes depending on where the weight > exists in the hierarchy? > > > While we calculate the vdisktime, this is calculated with the help > > of CFQ_DEFAULT_WEIGHT and cfqg->weight. So we scale used time slice > > in proportion to CFQ_DEFAULT_WEIGTH/cfqg->weight. So higher the weight > > lesser the charge and cfqg gets scheduled again faster and lower the > > weight, higher the vdisktime and cfqg gets scheduled less frequently. > > > > As every cfqg does the same thing on service tree, they automatically > > get fair share w.r.t their weight. > > > > And this mechanism should not be impacted by the hierarchy because we > > have a separate service tree at separate level. This will not work > > only if you come up with one compressed tree and then weights will > > have to be adjusted. If we have a separate service tree in each group > > then it should work just fine. > > Why would you create N service trees when you can almost trivially use > one by calcualting the effective weight? You would have to be > adjusting all trees above whenever something changes in a child. One of the reasons I can think is accuracy. If a task/group is added to a service tree, it mostly does not change the fraction of parent and does not change the fraction of parent's sibling. By making everything flat any addition/removal of an entity changes fraction of everything on the tree. Not that I am bothered about it because we do not focus that strictly on fairness. So I would not care about it. What I do care about is atleast being able to read and understand the code easily. Right now, it is hard to understand. I am still struggling to wrap my head around it. For example, while adding a group to service tree we calculate cfqg->vfaction as follows. vfr = vfr * pos->leaf_weight / pos->level_weight; and then vfr = vfr * pos->weight / parent->level_weight; cfqg->vfraction = max_t(unsigned, vfr, 1) If cfqg->vfraction is about cfqg then why should we take into account leaf_weight and level_weight. We should be just worried about pos->weight and parent->level_weight and that should determine vfaction of cfqg. Thanks Vivek _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
