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cfq-iosched: enable full blkcg hierarchy support

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With the previous two patches, all cfqg scheduling decisions are based
on vfraction and ready for hierarchy support.  The only thing which
keeps the behavior flat is cfqg_flat_parent() which makes vfraction
calculation consider all non-root cfqgs children of the root cfqg.

Replace it with cfqg_parent() which returns the real parent.  This
enables full blkcg hierarchy support for cfq-iosched.  For example,
consider the following hierarchy.

        root
      /      \
   A:500      B:250
  /     \
 AA:500  AB:1000

For simplicity, let's say all the leaf nodes have active tasks and are
on service tree.  For each leaf node, vfraction would be

 AA: (500  / 1500) * (500 / 750) =~ 0.2222
 AB: (1000 / 1500) * (500 / 750) =~ 0.4444
  B:                 (250 / 750) =~ 0.3333

and vdisktime will be distributed accordingly.  For more detail,
please refer to Documentation/block/cfq-iosched.txt.

v2: cfq-iosched.txt updated to describe group scheduling as suggested
    by Vivek.

v3: blkio-controller.txt updated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
This commit is contained in:
Tejun Heo 2013-01-09 08:05:11 -08:00
parent 41cad6ab2c
commit d02f7aa8dc
3 changed files with 88 additions and 26 deletions
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58
Documentation/block/cfq-iosched.txt
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@ -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.
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