performace although this can cause the latency of some I/O to increase due
to more number of requests.
+CFQ Group scheduling
+====================
+
+CFQ supports blkio cgroup and has "blkio." prefixed files in each
+blkio cgroup directory. It is weight-based and there are four knobs
+for configuration - weight[_device] and leaf_weight[_device].
+Internal cgroup nodes (the ones with children) can also have tasks in
+them, so the former two configure how much proportion the cgroup as a
+whole is entitled to at its parent's level while the latter two
+configure how much proportion the tasks in the cgroup have compared to
+its direct children.
+
+Another way to think about it is assuming that each internal node has
+an implicit leaf child node which hosts all the tasks whose weight is
+configured by leaf_weight[_device]. Let's assume a blkio hierarchy
+composed of five cgroups - root, A, B, AA and AB - with the following
+weights where the names represent the hierarchy.
+
+ weight leaf_weight
+ root : 125 125
+ A : 500 750
+ B : 250 500
+ AA : 500 500
+ AB : 1000 500
+
+root never has a parent making its weight is meaningless. For backward
+compatibility, weight is always kept in sync with leaf_weight. B, AA
+and AB have no child and thus its tasks have no children cgroup to
+compete with. They always get 100% of what the cgroup won at the
+parent level. Considering only the weights which matter, the hierarchy
+looks like the following.
+
+ root
+ / | \
+ A B leaf
+ 500 250 125
+ / | \
+ AA AB leaf
+ 500 1000 750
+
+If all cgroups have active IOs and competing with each other, disk
+time will be distributed like the following.
+
+Distribution below root. The total active weight at this level is
+A:500 + B:250 + C:125 = 875.
+
+ root-leaf : 125 / 875 =~ 14%
+ A : 500 / 875 =~ 57%
+ B(-leaf) : 250 / 875 =~ 28%
+
+A has children and further distributes its 57% among the children and
+the implicit leaf node. The total active weight at this level is
+AA:500 + AB:1000 + A-leaf:750 = 2250.
+
+ A-leaf : ( 750 / 2250) * A =~ 19%
+ AA(-leaf) : ( 500 / 2250) * A =~ 12%
+ AB(-leaf) : (1000 / 2250) * A =~ 25%
+
CFQ IOPS Mode for group scheduling
===================================
Basic CFQ design is to provide priority based time slices. Higher priority
Hierarchical Cgroups
====================
-- Currently none of the IO control policy supports hierarchical groups. But
- cgroup interface does allow creation of hierarchical cgroups and internally
- IO policies treat them as flat hierarchy.
+- Currently only CFQ supports hierarchical groups. For throttling,
+ cgroup interface does allow creation of hierarchical cgroups and
+ internally it treats them as flat hierarchy.
- So this patch will allow creation of cgroup hierarchcy but at the backend
- everything will be treated as flat. So if somebody created a hierarchy like
- as follows.
+ If somebody created a hierarchy like as follows.
root
/ \
|
test3
- CFQ and throttling will practically treat all groups at same level.
+ CFQ will handle the hierarchy correctly but and throttling will
+ practically treat all groups at same level. For details on CFQ
+ hierarchy support, refer to Documentation/block/cfq-iosched.txt.
+ Throttling will treat the hierarchy as if it looks like the
+ following.
pivot
/ / \ \
root test1 test2 test3
- Down the line we can implement hierarchical accounting/control support
- and also introduce a new cgroup file "use_hierarchy" which will control
- whether cgroup hierarchy is viewed as flat or hierarchical by the policy..
- This is how memory controller also has implemented the things.
+ Nesting cgroups, while allowed, isn't officially supported and blkio
+ genereates warning when cgroups nest. Once throttling implements
+ hierarchy support, hierarchy will be supported and the warning will
+ be removed.
Various user visible config options
===================================
dev weight
8:16 300
+- blkio.leaf_weight[_device]
+ - Equivalents of blkio.weight[_device] for the purpose of
+ deciding how much weight tasks in the given cgroup has while
+ competing with the cgroup's child cgroups. For details,
+ please refer to Documentation/block/cfq-iosched.txt.
+
- blkio.time
- disk time allocated to cgroup per device in milliseconds. First
two fields specify the major and minor number of the device and
and minor number of the device and third field specifies the number
of times a group was dequeued from a particular device.
+- blkio.*_recursive
+ - Recursive version of various stats. These files show the
+ same information as their non-recursive counterparts but
+ include stats from all the descendant cgroups.
+
Throttling/Upper limit policy files
-----------------------------------
- blkio.throttle.read_bps_device
return pd_to_cfqg(blkg_to_pd(blkg, &blkcg_policy_cfq));
}
-/*
- * Determine the parent cfqg for weight calculation. Currently, cfqg
- * scheduling is flat and the root is the parent of everyone else.
- */
-static inline struct cfq_group *cfqg_flat_parent(struct cfq_group *cfqg)
+static inline struct cfq_group *cfqg_parent(struct cfq_group *cfqg)
{
- struct blkcg_gq *blkg = cfqg_to_blkg(cfqg);
- struct cfq_group *root;
-
- while (blkg->parent)
- blkg = blkg->parent;
- root = blkg_to_cfqg(blkg);
+ struct blkcg_gq *pblkg = cfqg_to_blkg(cfqg)->parent;
- return root != cfqg ? root : NULL;
+ return pblkg ? blkg_to_cfqg(pblkg) : NULL;
}
static inline void cfqg_get(struct cfq_group *cfqg)
#else /* CONFIG_CFQ_GROUP_IOSCHED */
-static inline struct cfq_group *cfqg_flat_parent(struct cfq_group *cfqg) { return NULL; }
+static inline struct cfq_group *cfqg_parent(struct cfq_group *cfqg) { return NULL; }
static inline void cfqg_get(struct cfq_group *cfqg) { }
static inline void cfqg_put(struct cfq_group *cfqg) { }
* stops once an already activated node is met. vfraction
* calculation should always continue to the root.
*/
- while ((parent = cfqg_flat_parent(pos))) {
+ while ((parent = cfqg_parent(pos))) {
if (propagate) {
propagate = !parent->nr_active++;
parent->children_weight += pos->weight;
pos->children_weight -= pos->leaf_weight;
while (propagate) {
- struct cfq_group *parent = cfqg_flat_parent(pos);
+ struct cfq_group *parent = cfqg_parent(pos);
/* @pos has 0 nr_active at this point */
WARN_ON_ONCE(pos->children_weight);
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