Date:	Tue, 18 Jan 2011 15:27:20 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Corrado Zoccolo <czoccolo@...il.com>,
	Chad Talbott <ctalbott@...gle.com>,
	Nauman Rafique <nauman@...gle.com>,
	Divyesh Shah <dpshah@...gle.com>, jmoyer@...hat.com,
	Shaohua Li <shaohua.li@...el.com>
Subject: Re: [PATCH 6/6 v3] blkio-cgroup: Document for blkio.use_hierarchy
 interface

On Mon, Dec 27, 2010 at 04:51:20PM +0800, Gui Jianfeng wrote:
> Document for blkio.use_hierarchy interface
> 
> Signed-off-by: Gui Jianfeng <guijianfeng@...fujitsu.com>
> ---
>  Documentation/cgroups/blkio-controller.txt |   70 ++++++++++++++++++++--------
>  1 files changed, 51 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
> index 4ed7b5c..bd01d6c 100644
> --- a/Documentation/cgroups/blkio-controller.txt
> +++ b/Documentation/cgroups/blkio-controller.txt
> @@ -91,30 +91,51 @@ Throttling/Upper Limit policy
>  
>  Hierarchical Cgroups
>  ====================
> -- Currently none of the IO control policy supports hierarhical groups. But
> -  cgroup interface does allow creation of hierarhical cgroups and internally
> -  IO policies treat them as flat hierarchy.
> +- Cgroup interface allows creation of hierarchical cgroups. Currently,
> +  internally IO policies are able to treat them as flat hierarchy or
> +  hierarchical hierarchy.

Gui, now we have 2 IO policies. Throttling policy still supports only flat
mode. Can you please make it clear here that CFQ will support both flat
and hierarhical mode while throttling supports only flat mode as of today.

> Both hierarchical bandwidth division and flat
> +  bandwidth division are supported. "blkio.use_hierarchy" can be used to
> +  switch between flat mode and hierarchical mode.
>  
> -  So this patch will allow creation of cgroup hierarhcy but at the backend
> -  everything will be treated as flat. So if somebody created a hierarchy like
> -  as follows.
> +  Consider the following CGroup hierarchy:
>  
> -			root
> -			/  \
> -		     test1 test2
> -			|
> -		     test3
> +			  Root
> +			/  |   \
> +		     Grp1  Grp2 tsk1
> +	            /  \
> +		 Grp3 tsk2
>  
> -  CFQ and throttling will practically treat all groups at same level.
> +  If blkio.use_hierarchy is disabled in all CGroups, CFQ will practically treat all groups
> +  at the same level.
>  
> -				pivot
> -			     /  |   \  \
> -			root  test1 test2  test3
> +			     Pivot tree
> +			    /  |   |   \
> +			Root Grp1 Grp2 Grp3
> +			 /     |
> +			tsk1  tsk2
>  
> -  Down the line we can implement hierarchical accounting/control support
> -  and also introduce a new cgroup file "use_hierarchy" which will control
> -  whether cgroup hierarchy is viewed as flat or hierarchical by the policy..
> -  This is how memory controller also has implemented the things.
> +  If blkio.use_hierarchy is enabled in Grp1 and Grp3, CFQ will treat groups and tasks as the

I would think that before this example, we can give one simpler example
where use_hierarhcy=1 in root cgroup and that would/should lead to all
children group under root having use_hierarchy=1 set automatically and
will look as follows.

		     	Pivot tree
			    |
		           root
			 /  |  | 
	             tsk1   G1 G2 
			   / \
			 grp3 tsk2

After this now you can give more complicated example as below where
use_hierarchy=1 is only set for sub branch of tree and not the whole
tree.

> +  same view in CGroup hierarchy, it looks as follows.
> +
> +
> +			     Pivot tree
> +			    /    |    \
> +			  Root  Grp1  Grp2
> +			  /     /  \
> +		       tsk1   Grp3 tsk2
> +
> +  Root, Grp1 and Grp2 are treated at the same level under Pivot tree. tsk1 stays under Root.
> +  Grp3 and tsk2 are treated at the same level under Grp1. Below is the mapping between
> +  task io priority and io weight:
> +
> +	    prio       0    1     2    3    4    5    6     7
> +	    weight  1000  868   740  612  484  356  228   100
> +
> +  Note1: Regardless of the use_hierarchy setting in Root group, Root group is always put onto
> +  Pivot tree.
> +
> +  Note2: Currently, "blkio.use_hierarchy" only effects proportional bandwidth division. For
> +  Throttling logic, it still continue to treat everything as flat.
>  

Ok, I see you mentioned throttling flat mode here. I guess it is better
to clarify it in the beginning itself.

>  Various user visible config options
>  ===================================
> @@ -169,6 +190,17 @@ Proportional weight policy files
>  	  dev     weight
>  	  8:16    300
>  
> +- blkio.use_hierarchy
> +	- Switch between hierarchical mode and flat mode as stated above.
> +	  blkio.use_hierarchy == 1 means hierarchical mode is enabled.
> +	  blkio.use_hierarchy == 0 means flat mode is enabled.
> +	  You can set this interface only if there isn't any child CGroup under
> +	  this CGroup. If one CGroup's blkio.use_hierarchy is set, the created
> +	  children will inherit it. it's not allowed to unset it in children.
> +	  The default mode in Root CGroup is flat.
> +	  blkio.use_hierarchy only works for proportional bandwidth division
> +	  as of today and doesn't have any effect on throttling logic.
> +

This is good. Makes use_hierarchy behavior for children and subtree very
clear.

>  - blkio.time
>  	- disk time allocated to cgroup per device in milliseconds. First
>  	  two fields specify the major and minor number of the device and
> -- 
> 1.6.5.2
> 
> 
> 
> 
> 
> 
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