On Fri, Dec 28, 2012 at 12:35:35PM -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. > > This patch reimplements cfqg charge scaling so that it can be used to > support hierarchy properly. The scheme is fairly simple and > light-weight. > > * When a cfqg is added to the service tree, v(disktime)weight is > calculated. It walks up the tree to root calculating the fraction > it has in the hierarchy. At each level, the fraction can be > calculated as > > cfqg->weight / parent->level_weight > > By compounding these, the global fraction of vdisktime the cfqg has > claim to - vfraction - can be determined. > > * When the cfqg needs to be charged, the charge is scaled inversely > proportionally to the vfraction. > > The new scaling scheme uses the same CFQ_SERVICE_SHIFT for fixed point > representation as before; however, the smallest scaling factor is now > 1 (ie. 1 << CFQ_SERVICE_SHIFT). This is different from before where 1 > was for CFQ_WEIGHT_DEFAULT and higher weight would result in smaller > scaling factor. > > While this shifts the global scale of vdisktime a bit, it doesn't > change the relative relationships among cfqgs and the scheduling > result isn't different. > > cfq_group_notify_queue_add uses fixed CFQ_IDLE_DELAY when appending > new cfqg to the service tree. The specific value of CFQ_IDLE_DELAY > didn't have any relevance to vdisktime before and is unlikely to cause > any visible behavior difference now especially as the scale shift > isn't that large. > > As the new scheme now makes proper distinction between cfqg->weight > and ->leaf_weight, reverse the weight aliasing for root cfqgs. For > root, both weights are now mapped to ->leaf_weight instead of the > other way around. > > Because we're still using cfqg_flat_parent(), this patch shouldn't > change the scheduling behavior in any noticeable way. > > v2: Beefed up comments on vfraction as requested by Vivek. > > Signed-off-by: Tejun Heo <tj@xxxxxxxxxx> > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx> Looks good to me. Acked-by: Vivek Goyal <vgoyal@xxxxxxxxxx> Vivek _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
