Date:	Wed, 4 Nov 2009 12:52:44 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	nauman@...gle.com, dpshah@...gle.com, lizf@...fujitsu.com,
	ryov@...inux.co.jp, fernando@....ntt.co.jp, s-uchida@...jp.nec.com,
	taka@...inux.co.jp, guijianfeng@...fujitsu.com, jmoyer@...hat.com,
	righi.andrea@...il.com, m-ikeda@...jp.nec.com,
	akpm@...ux-foundation.org, riel@...hat.com,
	kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 01/20] blkio: Documentation

On Wed, Nov 04, 2009 at 10:51:00PM +0530, Balbir Singh wrote:
> * Vivek Goyal <vgoyal@...hat.com> [2009-11-03 18:43:38]:
> 
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > ---
> >  Documentation/cgroups/blkio-controller.txt |  106 ++++++++++++++++++++++++++++
> >  1 files changed, 106 insertions(+), 0 deletions(-)
> >  create mode 100644 Documentation/cgroups/blkio-controller.txt
> > 
> > diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
> > new file mode 100644
> > index 0000000..dc8fb1a
> > --- /dev/null
> > +++ b/Documentation/cgroups/blkio-controller.txt
> > @@ -0,0 +1,106 @@
> > +				Block IO Controller
> > +				===================
> > +Overview
> > +========
> > +cgroup subsys "blkio" implements the block io controller. There seems to be
> > +a need of various kind of IO control policies (like proportional BW, max BW)
> > +both at leaf nodes as well as at intermediate nodes in storage hierarchy. Plan
> > +is to use same cgroup based management interface for blkio controller and
> > +based on user options switch IO policies in the background.
> > +
> > +In the first phase, this patchset implements proportional weight time based
> > +division of disk policy. It is implemented in CFQ. Hence this policy takes
> > +effect only on leaf nodes when CFQ is being used.
> > +
> > +HOWTO
> > +=====
> > +You can do a very simple testing of running two dd threads in two different
> > +cgroups. Here is what you can do.
> > +
> > +- Enable group scheduling in CFQ
> > +	CONFIG_CFQ_GROUP_IOSCHED=y
> > +
> > +- Compile and boot into kernel and mount IO controller (blkio).
> > +
> > +	mount -t cgroup -o blkio none /cgroup
> > +
> > +- Create two cgroups
> > +	mkdir -p /cgroup/test1/ /cgroup/test2
> > +
> > +- Set weights of group test1 and test2
> > +	echo 1000 > /cgroup/test1/blkio.weight
> > +	echo 500 > /cgroup/test2/blkio.weight
> > +
> > +- 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.
> > +
> > +	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 blkio.disk_time and
> > +  blkio.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.
> > +
> > +Various user visible config options
> > +===================================
> > +CONFIG_CFQ_GROUP_IOSCHED
> > +	- Enables group scheduling in CFQ. Currently only 1 level of group
> > +	  creation is allowed.
> > +
> > +CONFIG_DEBUG_CFQ_IOSCHED
> > +	- Enables some debugging messages in blktrace. Also creates extra
> > +	  cgroup file blkio.dequeue.
> > +
> > +Config options selected automatically
> > +=====================================
> > +These config options are not user visible and are selected/deselected
> > +automatically based on IO scheduler configuration.
> > +
> > +CONFIG_BLK_CGROUP
> > +	- Block IO controller. Selected by CONFIG_CFQ_GROUP_IOSCHED.
> > +
> > +CONFIG_DEBUG_BLK_CGROUP
> > +	- Debug help. Selected by CONFIG_DEBUG_CFQ_IOSCHED.
> > +
> > +Details of cgroup files
> > +=======================
> > +- blkio.ioprio_class
> > +	- Specifies class of the cgroup (RT, BE, IDLE). This is default io
> > +	  class of the group on all the devices.
> > +
> > +	  1 = RT; 2 = BE, 3 = IDLE
> > +
> > +- blkio.weight
> > +	- Specifies per cgroup weight.
> > +
> > +	  Currently allowed range of weights is from 100 to 1000.
> > +
> > +- blkio.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.
> > +
> > +- blkio.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.
> > +
> > +- blkio.dequeue
> > +	- Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y. This
> > +	  gives the statistics about how many a times a group was dequeued
> > +	  from 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 dequeued from a particular device.
> 
> Hi, Vivek,
> 
> Are the parameters inter-related? What if you have conflicts w.r.t.
> time, sectors, etc?

What kind of conflicts?

time, sectors, and dequeue are read only files. They are mainly for 
monitoring purposes. CFQ provides fairness in terms of disk time, so one
can monitor whether time share received by a group is fair or not.

"sectors" just gives additional data of how many sectors were transferred.
It it not a necessary file. I just exported it to get some sense of both
time and amount IO done by cgroup.

So I am not sure what kind of conflicts you are referring to.

Thanks
Vivek
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