[dm-devel] [PATCH 01/28] io-controller: Documentation

[Vivek Goyal <vgoyal redhat com>]

o Documentation for io-controller.

Signed-off-by: Vivek Goyal <vgoyal redhat com>
Acked-by: Rik van Riel <riel redhat com>
---
 Documentation/block/00-INDEX          |    2 +
 Documentation/block/io-controller.txt |  464 +++++++++++++++++++++++++++++++++
 2 files changed, 466 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-iosched.txt
 	- Deadline IO scheduler tunables
+io-controller.txt
+	- IO controller for provding hierarchical IO scheduling
 ioprio.txt
 	- Block io priorities (in CFQ scheduler)
 request.txt
diff --git a/Documentation/block/io-controller.txt b/Documentation/block/io-controller.txt
new file mode 100644
index 0000000..f2bfce6
--- /dev/null
+++ b/Documentation/block/io-controller.txt
@@ -0,0 +1,464 @@
+				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 primarily needed 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 |
+			--------------------------------
+			 |	     |	     |       |
+			NOOP     DEADLINE    AS     CFQ
+
+Design
+======
+This patchset takes the inspiration from CFS cpu scheduler, CFQ and BFQ to
+come up with core of hierarchical scheduling. Like CFQ we give time slices to
+every queue based on their priority. Like CFS, this disktime given to a
+queue is converted to virtual disk time based on queue's weight (vdisktime)
+and based on this vdisktime we decide which is the queue next to be
+dispatched. And like BFQ we maintain a cache of recently served queues and
+derive new vdisktime of the queue from the cache if queue was recently served.
+
+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. 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. Noop, deadline and AS don't
+maintain separate queues per task, hence ther is only one io_queue per group.
+So once we can find right group, we also found right queue. CFQ maintains
+multiple io queues with-in group based on task context and maps the request
+to right queue in the group.
+
+sync requests are mapped to right group and queue based on the "current" task.
+Async requests can be mapped using either "current" task or based on owner of
+the page. (blkio cgroup subsystem provides this bio/page tracking mechanism).
+This option is controlled by config option "CONFIG_TRACK_ASYNC_CONTEXT"
+
+Going back to old behavior
+==========================
+In new scheme of things essentially we are creating hierarchical fair
+queuing logic in elevator layer and changing 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. So CFQ has to use fair queuing logic from common layer but it can choose
+to enable only flat support and not enable hierarchical (group scheduling)
+support.
+
+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.
+
+CGROUP_BLKIO
+	- 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.
+
+CONFIG_TRACK_ASYNC_CONTEXT
+	- 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.
+
+	  This should be primarily useful when lots of asynchronous writes
+	  are being submitted by pdflush threads and we need to assign the
+	  writes to right group.
+
+CONFIG_DEBUG_GROUP_IOSCHED
+	- 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_DEBUG_ELV_FAIR_QUEUING
+	- Enables some vdisktime related debugging messages.
+
+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.
+
+HOWTO
+=====
+You can do a 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
+
+- Enable IO tracking for async writes.
+	CONFIG_TRACK_ASYNC_CONTEXT=y
+
+  (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.
+
+What Works and What Does not
+============================
+Service differentiation at application level can be noticed only if completely
+parallel IO paths are created from application to IO scheduler and there
+are no serializations introduced by any intermediate layer. For example,
+in some cases file system and page cache layer introduce serialization and
+we don't see service difference between higher weight and lower weight
+process groups.
+
+For example, when I start an O_SYNC write out on an ext3 file system (file
+is being created newly), I see lots of activity from kjournald. I have not
+gone into details yet, but my understanding is that there are lot more
+journal commits and kjournald kind of introduces serialization between two
+processes. So even if you put these two processes in two different cgroups
+with different weights, higher weight process will not see more IO done.
+
+It does work very well when we bypass filesystem layer and IO is raw. For
+example in above virtual machine case, host sees raw synchronous writes
+coming from two guest machines and filesystem layer at host is not introducing
+any kind of serialization hence we can see the service difference.
+
+It also works very well for reads even on the same file system as for reads
+file system journalling activity does not kick in and we can create parallel
+IO paths from application to all the way down to IO scheduler and get more
+IO done on the IO path with higher weight.
+
+Details of new ioscheduler tunables
+===================================
+
+group_idle
+-----------
+
+"group_idle" specifies the duration one should wait for new request before
+group is expired. This is very similiar to "slice_idle" parameter of cfq. The
+difference is that slice_idle specifies queue idling period and group_idle
+specifies group idling period. Another difference is that cfq idling is
+dynamically updated based on traffic pattern. group idling is currently
+static.
+
+group idling takes place when a group is empty when it is being expired. If
+an empty group is expired and later it gets a request (say 1 ms), it looses
+its fair share as upon expiry it will be deleted from the service tree and
+a new queue will be selected to run and min_vdisktime will be udpated on
+service tree.
+
+There are both advantages and disadvantates of enabling group_idle. If
+enabled, it ensures that a group gets its fair share of disk time (as long
+as a group gets a new request with-in group_idle period). So even if a
+single sequential reader is running in a group, it will get the disk time
+depending on the group weight. IOW, enabling it provides very strong isolation
+between groups.
+
+The flip side is that it makes the group a heavier entity with slow switching
+between groups. There are many cases where CFQ disables the idling on the
+queue and hence queue gets expired as soon as requests are over in the queue
+and CFQ moves to new queue. This way it achieves faster switching and in many
+cases better throughput (most of the time seeky processes will not have idling
+enabled and will get very limited access to disk).
+
+If group idling is disabled, a group will get fairness only if it is
+continuously backlogged. So this weakens the fairness gurantees and isolation
+between the groups but can help achieve faster switching between queues/groups
+and better throughput.
+
+So one should set "group_idle" depending on one's use case and based on need.
+
+For the time being it is enabled by default.
+
+"fairness"
+----------
+IO controller has introduced a "fairness" tunable for every io scheduler.
+Currently this tunable can assume values 0, 1.
+
+If fairness is set to 1, then IO controller waits for requests to finish from
+previous queue before requests from new queue are dispatched. This helps in
+doing better accouting of disk time consumed by a queue. If this is not done
+then on a queuing hardware, there can be requests from multiple queues and
+we will not have any idea which queue consumed how much of disk time.
+
+So if "fairness" is set, it can help achive better time accounting. But the
+flip side is that it can slow down switching between queues and also lower the
+throughput.
+
+Again, this parameter should be set/reset based on the need. For the time
+being it is disabled by default.
+
+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).
+
+	  Currently allowed range of weights is from 100 to 1000.
+
+- 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_maj:dev_minor weight ioprio_class > /patch/to/cgroup/io.policy
+
+	weight=0 means removing a policy.
+
+	Examples:
+
+	Configure weight=300 ioprio_class=2 on /dev/hdb (8:16) in this cgroup
+	# echo 8:16 300 2 > io.policy
+	# cat io.policy
+	dev	weight	class
+	8:16	300	2
+
+	Configure weight=500 ioprio_class=1 on /dev/hda (8:0) in this cgroup
+	# echo 8:0 500 1 > io.policy
+	# cat io.policy
+	dev	weight	class
+	8:0	500	1
+	8:16	300	2
+
+	Remove the policy for /dev/hda in this cgroup
+	# echo 8:0 0 1 > io.policy
+	# cat io.policy
+	dev	weight	class
+	8:16	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.
+
+Some High Level Test setups
+===========================
+One of the use cases of IO controller is to provide some kind of IO isolation
+between multiple virtual machines on the same host. Following is one
+example setup which worked for me.
+
+
+			     KVM	     KVM
+			    Guest1	    Guest2
+			   ---------      ----------
+			  |  -----  |    |  ------  |
+			  | | vdb | |    | | vdb  | |
+			  |  -----  |    |   ------ |
+			   ---------      ----------
+
+			   ---------------------------
+			  | Host		      |
+			  |         -------------     |
+			  |        | sdb1 | sdb2 |    |
+			  |         -------------     |
+			   ---------------------------
+
+On host machine, I had a spare SATA disk. I created two partitions sdb1
+and sdb2 and gave this partitions as additional storage to kvm guests. sdb1
+to KVM guest1 and sdb2 KVM guest2. These storage appeared as /dev/vdb in
+both the guests. Formatted the /dev/vdb and created ext3 file system and
+started a 1G file writeout in both the guests. Before writeout I had created
+two cgroups of weight 1000 and 500 and put virtual machines in two different
+groups.
+
+Following is write I started in both the guests.
+
+dd if=/dev/zero of=/mnt/vdc/zerofile1 bs=4K count=262144 conv=fdatasync
+
+Following are the results on host with "deadline" scheduler.
+
+group1 time=8:16 17254 group1 sectors=8:16 2104288
+group2 time=8:16 8498  group2 sectors=8:16 1007040
+
+Virtual machine with cgroup weight 1000 got almost double the time of virtual
+machine with weight 500.
-- 
1.6.0.6