Re: [PATCH 01/10] Documentation

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Mar 12, 2009 at 03:45:55PM +0800, Yang Hongyang wrote:
> Don't forget to update the 00-INDEX file when you add a new doc.^!^
> 

Thanks. Will do it.

Vivek

> Vivek Goyal wrote:
> > o Documentation for io-controller.
> > 
> > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > ---
> >  Documentation/block/io-controller.txt |  221 +++++++++++++++++++++++++++++++++
> >  1 files changed, 221 insertions(+), 0 deletions(-)
> >  create mode 100644 Documentation/block/io-controller.txt
> > 
> > diff --git a/Documentation/block/io-controller.txt b/Documentation/block/io-controller.txt
> > new file mode 100644
> > index 0000000..8884c5a
> > --- /dev/null
> > +++ b/Documentation/block/io-controller.txt
> > @@ -0,0 +1,221 @@
> > +				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
> 
> 
> -- 
> Regards
> Yang Hongyang