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    Subject[patch 1/4] io controller: documentation

    Signed-off-by: Vivek Goyal <>

    Index: linux17/Documentation/controllers/io-controller.txt
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux17/Documentation/controllers/io-controller.txt 2008-11-06 09:12:44.000000000 -0500
    @@ -0,0 +1,172 @@
    + IO Controller
    + ============
    +This patchset implements a basic version of proportional weight IO controller.
    +It is heavily derived from dm-ioband IO controller with one key difference
    +and that is, there is no separate device mapper driver and there is no
    +need to create a dm-ioband device on top of every block device which needs
    +to do the IO control. In this implementation, all the control logic has
    +been internalized and has been made per request queue. Enabling or disabling
    +IO control on a block device is just a matter of writing a 0 or 1 in
    +appropriate sysfs file.
    +This is a proportional weight controller and that means various cgroups
    +are assigned shares and tasks in those cgroups get to dispatch the bio
    +in proportion to their cgroup share.
    +All the contending cgroups are assigned tokens proportionate to their
    +weights. One token is charged for one sector of IO. Once all the contending
    +cgroups have consumed their tokens, fresh token allocation takes place and
    +this is how disk bandwidth allocation proportion to weight is achieved.
    +The bigger picture is that all the bios being submitted to a block device
    +are first inspected by IO controller logic (bio_group_controller()), only if
    +IO controller has been enabled on that device. The cgroup of the bio is
    +determined and controller checks if this cgroup has sufficient tokens to
    +dispatch the bio. If sufficient tokens are there, bio submitting thread
    +continues to dispatch the bio through normal path otherwise IO controller
    +buffers the bio and submitting thread returns back. These buffered bios
    +are dispatched to lower layers later once the associate group (bio group)
    +has sufficient tokens to dispatch the bios. This delayed dispatching is
    +done with the help of a worker thread (biogroup).
    +IO control can be enabled/disabled dynamically on any of the block device
    +through sysfs file system. For example, to enable IO control on a device
    +do following.
    +echo 1 > /sys/block/sda/biogroup
    +To disable IO control write 0.
    +echo 0 > /sys/block/sda/biogroup
    +This should be doable for any of the block device in the stack. Currently this
    +patch places the hooks only for device mapper driver and still need to tweak
    +For example, assume there are two cgroups A and B with weights 1024 and 2048
    +in the system. Tasks in two cgroups A and B are doing IO to two disks sda and
    +sdb in the system. A user has enabled IO control on both sda and sdb. Now on
    +both sda and sdb, tasks in cgroup B will get to use 2/3 of disk BW and
    +tasks in cgroup A will get to use 1/3 of disk bandwidth, only in case of
    +contention. If tasks in any of the groups stop doing IO to a particular disk,
    +task in other group will get to use full disk BW for that duration.
    +- Enable cgroup, memory controller and block IO controller in kernel config
    + file.
    +- Boot into the kernel and mount io controller.
    + mount -t cgroup -o bio none /cgroup/bio/
    +- Create two cgroups test1 and test2
    + cd /cgroup/bio
    + mkdir test1 test2
    +- Allocate weight 4096 to test1 and weight 2048 to test2
    + echo 4096 > /cgroup/bio/test1/bio.shares
    + echo 2048 > /cgroup/bio/test1/bio.shares
    +- Launch "dd" operations in cgroup test1 and test2.
    + echo $$ > /cgroup/bio/test1/tasks
    + dd if=/somefile1 of=/dev/null
    + echo $$ > /cgroup/bio/test2/tasks
    + dd if=/somefile2 of=/dev/null
    +Job in cgroup test1 should finish before job in cgroup test2. To verify
    +that "dd" in cgroup test1 got to dispatch more bio as compared to "dd" in
    +test2, look at "bio.aggregate_tokens" in both the cgroup (At same time). At
    +any point of time when both the dd's are running, aggregate_tokens in cgroup
    +test1 should be approximately double of aggregate_tokens in cgroup test2
    +(Because weight of cgroup test1 is double of weight of cgroup test2).
    +Some Tunables
    +Some tunables appear in cgroup file system and in sysfs for respective
    +device for debug and for configuration. Here is a brief description.
    +Cgroup Files
    + Specifies the weight of the cgroup.
    + Specifies total number of tokens dispatched by this cgroup. One token
    + represents one sector of IO.
    + What was the jiffies values when last bio from this cgroup was released.
    + How many times this cgroup got the token allocation done from token
    + slice. We kind of create a token slice and every contending cgroup
    + gets the pie out of the slice based on the share.
    + How many times this bio group went off the list of contending groups.
    + We maintain an rb-tree of biogroups contending for IO and token
    + allocation takes place to these groups regularly. If some group stops
    + doing IO then it is considered to be idle and removed from the tree
    + and added back later when group has IO to perform. This file just
    + counts how many times this bio group went off the tree.
    +Sysfs Tunabels
    +/sys/block/{deice name}/biogroup
    + Whether IO controller (bio groups) are active on this device or not.
    +/sys/block/{deice name}/deftoken
    + Default number of tokens which are given to a bio group upon start
    + if all the bio groups were of same weight. token slice is of dynamic
    + length. So if there are 3 cgroups contending and deftoken is 100 then
    + token slice lenght will be 100*3 = 300 and now out of this slice
    + three groups will get the tokens based on their weights.
    +/sys/block/{deice name}/idletime
    + The time after which if a bio group does not generate the bio, it is
    + considered idle and removed from the rb-tree. Currently by default it
    + is 8ms.
    +/sys/block/{deice name}/newslice_count
    + How many times new token allocation took place on this queue.
    +- Do extensive testing in various scenarios and do performance optimization
    + and fix the things where broken.
    +- IO schedulers derive context information from "current". This assumption
    + will be broken if bios are being submitted by a worker thread (biogroup).
    + Probably we need to put io context pointer in bio itself to get rid of
    + this dependency.
    +- Allocating tokens for per sector of IO is crude approximation and will lead
    + to unfair bandwidth allocation in case task in cgroup is doing sequential IO
    + and task in other group is doing random IO. Rik Van Riel, suggested that
    + probably we should switch to time based scheme. Keep a track of average time
    + it takes to complete IO from a cgroup and do the allocation accordingly.
    +- Currently this controller is dependent on memory controller being enabled.
    + Try to reduce this coupling.
    +- IO controller can buffer the bios if suffcient tokens were not available
    + at the time of bio submission. Once the tokens are available, these bios
    + are dispatched to elevator/lower layers in first come first serve manner.
    + And this has potential to break CFQ where a RT tasks should be able to
    + dispatch the bio first or a high priority task should be able to release
    + more bio as compared to low priority task in same cgroup.
    + Not sure how to fix it. May be we need to maintain another rb-tree and
    + keep track of RT tasks and tasks priorities and dispatch accordingly. This
    + is equivalent of duplicating lots of CFQ logic and not sure how would it
    + impact AS behaviour.

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