Re: [PATCH 8/8] blkio-cgroup: Document for blkio.use_hierarchy.

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Vivek Goyal wrote:
> On Mon, Dec 13, 2010 at 09:45:22AM +0800, Gui Jianfeng wrote:
>> Document for blkio.use_hierarchy.
>>
>> Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
>> ---
>>  Documentation/cgroups/blkio-controller.txt |   58 +++++++++++++++++++---------
>>  1 files changed, 39 insertions(+), 19 deletions(-)
>>
>> diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
>> index 4ed7b5c..9c6dc9e 100644
>> --- a/Documentation/cgroups/blkio-controller.txt
>> +++ b/Documentation/cgroups/blkio-controller.txt
>> @@ -91,30 +91,44 @@ 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. 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 flat mode is enabled, CFQ and throttling 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 hierarchical mode is enabled, CFQ will treat groups and tasks as the same
>> +  view in CGroup hierarchy.
>> +
>> +			  Root
>> +		        /  |   \
>> +		    Grp1  Grp2 tsk1
>> +		    /  \
>> +		  Grp3 tsk2
>> +
>> +  Grp1, Grp2 and tsk1 are treated at the same level under Root group. 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
> 
> I am curious to know that why did you choose above mappings. Current prio
> to slice mapping seems to be.
> 
> 	prio	0	1	2	3	4	5	6	7
> 	slice	180	160	140	120	100	80	60	40
> 
> Now with above weights difference between prio 0 and prio 7 will be 10
> times as compared to old scheme of 4.5 times. Well then there is
> slice offset logic which tries to introduce more service differentation.
> anyway, I am not particular about it. Just curious. If it works well, then
> it is fine.

Currently, Since CFQ queue and CFQ group are treated at the same level, I'd
like to map ioprio to the whole range of io weight. So choose this mapping.

> 
>>  
>>  Various user visible config options
>>  ===================================
>> @@ -169,6 +183,12 @@ 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.
>> +	  The default mode is flat mode.
>> +
> 
> Can you please explicitly mentiond that blkio.use_hierarchy only effects
> CFQ and has impact on "throttling" logic as of today. Throttling will 
> still continue to treat everything as flat. 

Sure.

Gui

> 
> I am working on making throttling logic hierarchical. It has been going
> on kind of slow and expecting it to get ready for 2.6.39.
> 
> Vivek
>