[PATCH 01/20] io-controller: Documentation

From: Vivek Goyal
Date: Fri Jun 19 2009 - 16:42:45 EST

o Documentation for io-controller.

Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Documentation/block/00-INDEX | 2 +
Documentation/block/io-controller.txt | 360 +++++++++++++++++++++++++++++++++
2 files changed, 362 insertions(+), 0 deletions(-)
create mode 100644 Documentation/block/io-controller.txt

diff --git a/Documentation/block/00-INDEX b/Documentation/block/00-INDEX
index 961a051..dc8bf95 100644
--- a/Documentation/block/00-INDEX
+++ b/Documentation/block/00-INDEX
@@ -10,6 +10,8 @@ capability.txt
- Generic Block Device Capability (/sys/block/<disk>/capability)
- Deadline IO scheduler tunables
+ - IO controller for provding hierarchical IO scheduling
- Block io priorities (in CFQ scheduler)
diff --git a/Documentation/block/io-controller.txt b/Documentation/block/io-controller.txt
new file mode 100644
index 0000000..bf95bf7
--- /dev/null
+++ b/Documentation/block/io-controller.txt
@@ -0,0 +1,360 @@
+ IO Controller
+ =============
+This patchset implements a proportional weight IO controller. That is one
+can create cgroups and assign prio/weights to those cgroups and task group
+will get access to disk proportionate to the weight of the group.
+These patches modify elevator layer and individual IO schedulers to do
+IO control hence this io controller works only on block devices which use
+one of the standard io schedulers can not be used with any xyz logical block
+The assumption/thought behind modifying IO scheduler is that resource control
+is needed only on leaf nodes where the actual contention for resources is
+present and not on intertermediate logical block devices.
+Consider following hypothetical scenario. Lets say there are three physical
+disks, namely sda, sdb and sdc. Two logical volumes (lv0 and lv1) have been
+created on top of these. Some part of sdb is in lv0 and some part is in lv1.
+ lv0 lv1
+ / \ / \
+ sda sdb sdc
+Also consider following cgroup hierarchy
+ root
+ / \
+ A B
+ / \ / \
+ T1 T2 T3 T4
+A and B are two cgroups and T1, T2, T3 and T4 are tasks with-in those cgroups.
+Assuming T1, T2, T3 and T4 are doing IO on lv0 and lv1. These tasks should
+get their fair share of bandwidth on disks sda, sdb and sdc. There is no
+IO control on intermediate logical block nodes (lv0, lv1).
+So if tasks T1 and T2 are doing IO on lv0 and T3 and T4 are doing IO on lv1
+only, there will not be any contetion for resources between group A and B if
+IO is going to sda or sdc. But if actual IO gets translated to disk sdb, then
+IO scheduler associated with the sdb will distribute disk bandwidth to
+group A and B proportionate to their weight.
+CFQ already has the notion of fairness and it provides differential disk
+access based on priority and class of the task. Just that it is flat and
+with cgroup stuff, it needs to be made hierarchical to achive a good
+hierarchical control on IO.
+Rest of the IO schedulers (noop, deadline and AS) don't have any notion
+of fairness among various threads. They maintain only one queue where all
+the IO gets queued (internally this queue is split in read and write queue
+for deadline and AS). With this patchset, now we maintain one queue per
+cgropu per device and then try to do fair queuing among those queues.
+One of the concerns raised with modifying IO schedulers was that we don't
+want to replicate the code in all the IO schedulers. These patches share
+the fair queuing code which has been moved to a common layer (elevator
+layer). Hence we don't end up replicating code across IO schedulers. Following
+diagram depicts the concept.
+ --------------------------------
+ | Elevator Layer + Fair Queuing |
+ --------------------------------
+ | | | |
+This patchset primarily uses BFQ (Budget Fair Queuing) code to provide
+fairness among different IO queues. Fabio and Paolo implemented BFQ which uses
+B-WF2Q+ algorithm for fair queuing.
+Why BFQ?
+- Not sure if weighted round robin logic of CFQ can be easily extended for
+ hierarchical mode. One of the things is that we can not keep dividing
+ the time slice of parent group among childrens. Deeper we go in hierarchy
+ time slice will get smaller.
+ One of the ways to implement hierarchical support could be to keep track
+ of virtual time and service provided to queue/group and select a queue/group
+ for service based on any of the various available algoriths.
+ BFQ already had support for hierarchical scheduling, taking those patches
+ was easier.
+- BFQ was designed to provide tighter bounds/delay w.r.t service provided
+ to a queue. Delay/Jitter with BFQ is O(1).
+ Note: BFQ originally used amount of IO done (number of sectors) as notion
+ of service provided. IOW, it tried to provide fairness in terms of
+ actual IO done and not in terms of actual time disk access was
+ given to a queue.
+ This patcheset modified BFQ to provide fairness in time domain because
+ that's what CFQ does. So idea was try not to deviate too much from
+ the CFQ behavior initially.
+ Providing fairness in time domain makes accounting trciky because
+ due to command queueing, at one time there might be multiple requests
+ from different queues and there is no easy way to find out how much
+ disk time actually was consumed by the requests of a particular
+ queue. More about this in comments in source code.
+We have taken BFQ code as starting point for providing fairness among groups
+because it already contained lots of features which we required to implement
+hierarhical IO scheduling. With this patch set, I am not trying to ensure O(1)
+delay here as my goal is to provide fairness among groups. Most likely that
+will mean that latencies are not worse than what cfq currently provides (if
+not improved ones). Once fairness is ensured, one can look into more in
+ensuring O(1) latencies.
+From data structure point of view, one can think of a tree per device, where
+io groups and io queues are hanging and are being scheduled using B-WF2Q+
+algorithm. io_queue, is end queue where requests are actually stored and
+dispatched from (like cfqq).
+These io queues are primarily created by and managed by end io schedulers
+depending on its semantics. For example, noop, deadline and AS ioschedulers
+keep one io queues per cgroup and cfqq keeps one io queue per io_context in
+a cgroup (apart from async queues).
+A request is mapped to an io group by elevator layer and which io queue it
+is mapped to with in group depends on ioscheduler. Currently "current" task
+is used to determine the cgroup (hence io group) of the request. Down the
+line we need to make use of bio-cgroup patches to map delayed writes to
+right group.
+Going back to old behavior
+In new scheme of things essentially we are creating hierarchical fair
+queuing logic in elevator layer and chaning IO schedulers to make use of
+that logic so that end IO schedulers start supporting hierarchical scheduling.
+Elevator layer continues to support the old interfaces. So even if fair queuing
+is enabled at elevator layer, one can have both new hierchical scheduler as
+well as old non-hierarchical scheduler operating.
+Also noop, deadline and AS have option of enabling hierarchical scheduling.
+If it is selected, fair queuing is done in hierarchical manner. If hierarchical
+scheduling is disabled, noop, deadline and AS should retain their existing
+CFQ is the only exception where one can not disable fair queuing as it is
+needed for provding fairness among various threads even in non-hierarchical
+Various user visible config options
+ - Enables hierchical fair queuing in noop. Not selecting this option
+ leads to old behavior of noop.
+ - Enables hierchical fair queuing in deadline. Not selecting this
+ option leads to old behavior of deadline.
+ - Enables hierchical fair queuing in AS. Not selecting this option
+ leads to old behavior of AS.
+ - Enables hierarchical fair queuing in CFQ. Not selecting this option
+ still does fair queuing among various queus but it is flat and not
+ hierarchical.
+ - This option enables blkio-cgroup controller for IO tracking
+ purposes. That means, by this controller one can attribute a write
+ to the original cgroup and not assume that it belongs to submitting
+ thread.
+ - Currently CFQ attributes the writes to the submitting thread and
+ caches the async queue pointer in the io context of the process.
+ If this option is set, it tells cfq and elevator fair queuing logic
+ that for async writes make use of IO tracking patches and attribute
+ writes to original cgroup and not to write submitting thread.
+ - Throws extra debug messages in blktrace output helpful in doing
+ doing debugging in hierarchical setup.
+ - Also allows for export of extra debug statistics like group queue
+ and dequeue statistics on device through cgroup interface.
+Config options selected automatically
+These config options are not user visible and are selected/deselected
+automatically based on IO scheduler configurations.
+ - Enables/Disables the fair queuing logic at elevator layer.
+ - Enables/Disables hierarchical queuing and associated cgroup bits.
+So far I have done very simple testing of running two dd threads in two
+different cgroups. Here is what you can do.
+- Enable hierarchical scheduling in io scheuduler of your choice (say cfq).
+- Enable IO tracking for async writes.
+ (This will automatically select CGROUP_BLKIO)
+- Compile and boot into kernel and mount IO controller and blkio io tracking
+ controller.
+ mount -t cgroup -o io,blkio none /cgroup
+- Create two cgroups
+ mkdir -p /cgroup/test1/ /cgroup/test2
+- Set weights of group test1 and test2
+ echo 1000 > /cgroup/test1/io.weight
+ echo 500 > /cgroup/test2/io.weight
+- Set "fairness" parameter to 1 at the disk you are testing.
+ echo 1 > /sys/block/<disk>/queue/iosched/fairness
+- Create two same size files (say 512MB each) on same disk (file1, file2) and
+ launch two dd threads in different cgroup to read those files. Make sure
+ right io scheduler is being used for the block device where files are
+ present (the one you compiled in hierarchical mode).
+ sync
+ echo 3 > /proc/sys/vm/drop_caches
+ dd if=/mnt/sdb/zerofile1 of=/dev/null &
+ echo $! > /cgroup/test1/tasks
+ cat /cgroup/test1/tasks
+ dd if=/mnt/sdb/zerofile2 of=/dev/null &
+ echo $! > /cgroup/test2/tasks
+ cat /cgroup/test2/tasks
+- At macro level, first dd should finish first. To get more precise data, keep
+ on looking at (with the help of script), at io.disk_time and io.disk_sectors
+ files of both test1 and test2 groups. This will tell how much disk time
+ (in milli seconds), each group got and how many secotors each group
+ dispatched to the disk. We provide fairness in terms of disk time, so
+ ideally io.disk_time of cgroups should be in proportion to the weight.
+ (It is hard to achieve though :-)).
+Details of cgroup files
+- io.ioprio_class
+ - Specifies class of the cgroup (RT, BE, IDLE). This is default io
+ class of the group on all the devices until and unless overridden by
+ per device rule. (See io.policy).
+ 1 = RT; 2 = BE, 3 = IDLE
+- io.weight
+ - Specifies per cgroup weight. This is default weight of the group
+ on all the devices until and unless overridden by per device rule.
+ (See io.policy).
+- io.disk_time
+ - disk time allocated to cgroup per device in milliseconds. First
+ two fields specify the major and minor number of the device and
+ third field specifies the disk time allocated to group in
+ milliseconds.
+- io.disk_sectors
+ - number of sectors transferred to/from disk by the group. First
+ two fields specify the major and minor number of the device and
+ third field specifies the number of sectors transferred by the
+ group to/from the device.
+- io.disk_queue
+ - Debugging aid only enabled if CONFIG_DEBUG_GROUP_IOSCHED=y. This
+ gives the statistics about how many a times a group was queued
+ on service tree of the device. First two fields specify the major
+ and minor number of the device and third field specifies the number
+ of times a group was queued on a particular device.
+- io.disk_queue
+ - Debugging aid only enabled if CONFIG_DEBUG_GROUP_IOSCHED=y. This
+ gives the statistics about how many a times a group was de-queued
+ or removed from the service tree of the device. This basically gives
+ and idea if we can generate enough IO to create continuously
+ backlogged groups. First two fields specify the major and minor
+ number of the device and third field specifies the number
+ of times a group was de-queued on a particular device.
+- io.policy
+ - One can specify per cgroup per device rules using this interface.
+ These rules override the default value of group weight and class as
+ specified by io.weight and io.ioprio_class.
+ Following is the format.
+ #echo DEV:weight:ioprio_class > /patch/to/cgroup/io.policy
+ weight=0 means removing a policy.
+ Examples:
+ Configure weight=300 ioprio_class=2 on /dev/hdb in this cgroup
+ # echo /dev/hdb:300:2 > io.policy
+ # cat io.policy
+ dev weight class
+ /dev/hdb 300 2
+ Configure weight=500 ioprio_class=1 on /dev/hda in this cgroup
+ # echo /dev/hda:500:1 > io.policy
+ # cat io.policy
+ dev weight class
+ /dev/hda 500 1
+ /dev/hdb 300 2
+ Remove the policy for /dev/hda in this cgroup
+ # echo /dev/hda:0:1 > io.policy
+ # cat io.policy
+ dev weight class
+ /dev/hdb 300 2
+About configuring request desriptors
+Traditionally there are 128 request desriptors allocated per request queue
+where io scheduler is operating (/sys/block/<disk>/queue/nr_requests). If these
+request descriptors are exhausted, processes will put to sleep and woken
+up once request descriptors are available.
+With io controller and cgroup stuff, one can not afford to allocate requests
+from single pool as one group might allocate lots of requests and then tasks
+from other groups might be put to sleep and this other group might be a
+higher weight group. Hence to make sure that a group always can get the
+request descriptors it is entitled to, one needs to make request descriptor
+limit per group on every queue.
+A new parameter /sys/block/<disk>/queue/nr_group_requests has been introduced
+and this parameter controlls the maximum number of requests per group.
+nr_requests still continues to control total number of request descriptors
+on the queue.
+Ideally one should set nr_requests to be following.
+nr_requests = number_of_cgroups * nr_group_requests
+This will make sure that at any point of time nr_group_requests number of
+request descriptors will be available for any of the cgroups.
+Currently default nr_requests=512 and nr_group_requests=128. This will make
+sure that apart from root group one can create 3 more group without running
+into any issues. If one decides to create more cgorus, nr_requests and
+nr_group_requests should be adjusted accordingly.
+Probably a better way to assign limit to group request descriptors is through
+sysfs interface. This is a future TODO item.

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