Diffstat (limited to 'Documentation')
2 files changed, 82 insertions, 11 deletions
diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt
index d89b4fe724d..a5eb7d19a65 100644
@@ -102,6 +102,64 @@ processing of request. Therefore, increasing the value can imporve the
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.
+ / | \
+ 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
diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index a794ce91a2d..da272c8f44e 100644
@@ -94,13 +94,11 @@ Throttling/Upper Limit policy
-- 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.
@@ -108,16 +106,20 @@ Hierarchical Cgroups
- 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
/ / \ \
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
@@ -172,6 +174,12 @@ Proportional weight policy files
+ - 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.
- disk time allocated to cgroup per device in milliseconds. First
two fields specify the major and minor number of the device and
@@ -279,6 +287,11 @@ Proportional weight policy files
and minor number of the device and third field specifies the number
of times a group was dequeued from a particular device.
+ - 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