[PATCH 01/10] Documentation

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

o Documentation for io-controller.

Signed-off-by: Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
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+				IO Controller
+				=============
+
+Overview
+========
+
+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
+device.
+
+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.
+
+Rest of the IO schedulers (noop, deadline and AS) don't have any notion
+of fairness among various threads.
+
+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.
+
+Design
+======
+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 supposed to be 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.
+
+So it is yet to be seen if changing to time domain still retains BFQ gurantees
+or not.
+
+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
+behavior.
+
+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
+mode.
+
+Various user visible config options
+===================================
+CONFIG_IOSCHED_NOOP_HIER
+	- Enables hierchical fair queuing in noop. Not selecting this option
+	  leads to old behavior of noop.
+
+CONFIG_IOSCHED_DEADLINE_HIER
+	- Enables hierchical fair queuing in deadline. Not selecting this
+	  option leads to old behavior of deadline.
+
+CONFIG_IOSCHED_AS_HIER
+	- Enables hierchical fair queuing in AS. Not selecting this option
+	  leads to old behavior of AS.
+
+CONFIG_IOSCHED_CFQ_HIER
+	- Enables hierarchical fair queuing in CFQ. Not selecting this option
+	  still does fair queuing among various queus but it is flat and not
+	  hierarchical.
+
+Config options selected automatically
+=====================================
+These config options are not user visible and are selected/deselected
+automatically based on IO scheduler configurations.
+
+CONFIG_ELV_FAIR_QUEUING
+	- Enables/Disables the fair queuing logic at elevator layer.
+
+CONFIG_GROUP_IOSCHED
+	- Enables/Disables hierarchical queuing and associated cgroup bits.
+
+TODO
+====
+- Lots of cleanups, testing, bug fixing, optimizations, benchmarking etc...
+- Convert cgroup ioprio to notion of weight.
+- Anticipatory code will need more work. It is not working properly currently
+  and needs more thought.
+- Use of bio-cgroup patches.
+- Use of Nauman's per cgroup request descriptor patches.
+
+HOWTO
+=====
+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).
+	CONFIG_IOSCHED_CFQ_HIER=y
+
+- Compile and boot into kernel and mount IO controller.
+
+	mount -t cgroup -o io none /cgroup
+
+- Create two cgroups
+	mkdir -p /cgroup/test1/ /cgroup/test2
+
+- Set io priority of group test1 and test2
+	echo 0 > /cgroup/test1/io.ioprio
+	echo 4 > /cgroup/test2/io.ioprio
+
+- 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).
+
+	echo 1 > /proc/sys/vm/drop_caches
+
+	dd if=/mnt/lv0/zerofile1 of=/dev/null &
+	echo $! > /cgroup/test1/tasks
+	cat /cgroup/test1/tasks
+
+	dd if=/mnt/lv0/zerofile2 of=/dev/null &
+	echo $! > /cgroup/test2/tasks
+	cat /cgroup/test2/tasks
+
+- First dd should finish first.
+
+Some Test Results
+=================
+- Two dd in two cgroups with prio 0 and 4. Ran two "dd" in those cgroups.
+
+234179072 bytes (234 MB) copied, 10.1811 s, 23.0 MB/s
+234179072 bytes (234 MB) copied, 12.6187 s, 18.6 MB/s
+
+- Three dd in three cgroups with prio 0, 4, 4.
+
+234179072 bytes (234 MB) copied, 13.7654 s, 17.0 MB/s
+234179072 bytes (234 MB) copied, 19.476 s, 12.0 MB/s
+234179072 bytes (234 MB) copied, 20.1858 s, 11.6 MB/s
-- 
1.6.0.1