[Devel] Re: [PATCH 01/10] Documentation

[Ryo Tsuruta]

Hi Vivek,

> dm-ioband
> ---------
> I have briefly looked at dm-ioband also and following were some of the
> concerns I had raised in the past.
> 
> - Need of a dm device for every device we want to control
> 
> 	- This requirement looks odd. It forces everybody to use dm-tools
> 	  and if there are lots of disks in the system, configuation is
> 	  pain.

I don't think it's a pain. Could it be easily done by writing a small
script?

> - It does not support hiearhical grouping.

I can implement hierarchical grouping to dm-ioband if it's really
necessary, but at this point, I don't think it's really necessary
and I want to keep the code simple.

> - Possibly can break the assumptions of underlying IO schedulers.
> 
> 	- There is no notion of task classes. So tasks of all the classes
> 	  are at same level from resource contention point of view.
> 	  The only thing which differentiates them is cgroup weight. Which
> 	  does not answer the question that an RT task or RT cgroup should
> 	  starve the peer cgroup if need be as RT cgroup should get priority
> 	  access.
> 
> 	- Because of FIFO release of buffered bios, it is possible that
> 	  task of lower priority gets more IO done than the task of higher
> 	  priority.
> 
> 	- Buffering at multiple levels and FIFO dispatch can have more
> 	  interesting hard to solve issues.
> 
> 		- Assume there is sequential reader and an aggressive
> 		  writer in the cgroup. It might happen that writer
> 		  pushed lot of write requests in the FIFO queue first
> 		  and then a read request from reader comes. Now it might
> 		  happen that cfq does not see this read request for a long
> 		  time (if cgroup weight is less) and this writer will 
> 		  starve the reader in this cgroup.
> 
> 		  Even cfq anticipation logic will not help here because
> 		  when that first read request actually gets to cfq, cfq might
> 		  choose to idle for more read requests to come, but the
> 		  agreesive writer might have again flooded the FIFO queue
> 		  in the group and cfq will not see subsequent read request
> 		  for a long time and will unnecessarily idle for read.

I think it's just a matter of which you prioritize, bandwidth or
io-class. What do you do when the RT task issues a lot of I/O?

> - Task grouping logic
> 	- We already have the notion of cgroup where tasks can be grouped
> 	  in hierarhical manner. dm-ioband does not make full use of that
> 	  and comes up with own mechansim of grouping tasks (apart from
> 	  cgroup).  And there are odd ways of specifying cgroup id while
> 	  configuring the dm-ioband device.
> 
> 	  IMHO, once somebody has created the cgroup hieararchy, any IO
> 	  controller logic should be able to internally read that hiearchy
> 	  and provide control. There should not be need of any other
> 	  configuration utity on top of cgroup.
> 
> 	  My RFC patches had tried to get rid of this external
> 	  configuration requirement.

The reason is that it makes bio-cgroup easy to use for dm-ioband.
But It's not a final design of the interface between dm-ioband and
cgroup.

> - Task and Groups can not be treated at same level.
> 
> 	- Because at any second level solution we are controlling bio
> 	  per cgroup and don't have any notion of which task queue bio
> 	  belongs to, one can not treat task and group  at same level.
> 	
> 	  What I meant is following.
> 
> 			root
> 			/ | \
> 		       1  2  A
> 			    / \
> 			   3   4
> 
> 	In dm-ioband approach, at top level tasks 1 and 2 will get 50%
> 	of BW together and group A will get 50%. Ideally along the lines
> 	of cpu controller, I would expect it to be 33% each for task 1
> 	task 2 and group A.
> 
> 	This can create interesting scenarios where assumg task1 is
> 	an RT class task. Now one would expect task 1 get all the BW
> 	possible starving task 2 and group A, but that will not be the
> 	case and task1 will get 50% of BW.
> 
>  	Not that it is critically important but it would probably be
> 	nice if we can maitain same semantics as cpu controller. In
> 	elevator layer solution we can do it at least for CFQ scheduler
> 	as it maintains separate io queue per io context. 	

I will consider following the CPU controller's manner when dm-ioband
supports hierarchical grouping.

> 	This is in general an issue for any 2nd level IO controller which
> 	only accounts for io groups and not for io queues per process.
> 
> - We will end copying a lot of code/logic from cfq
> 
> 	- To address many of the concerns like multi class scheduler
> 	  we will end up duplicating code of IO scheduler. Why can't
> 	  we have a one point hierarchical IO scheduling (This patchset).
> Thanks
> Vivek

Thanks,
Ryo Tsuruta
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