[Devel] Re: dm-ioband + bio-cgroup benchmarks

[Vivek Goyal]

On Thu, Sep 18, 2008 at 09:04:18PM +0900, Ryo Tsuruta wrote:
> Hi All,
> 
> I have got excellent results of dm-ioband, that controls the disk I/O
> bandwidth even when it accepts delayed write requests.
> 
> In this time, I ran some benchmarks with a high-end storage. The
> reason was to avoid a performance bottleneck due to mechanical factors
> such as seek time.
> 
> You can see the details of the benchmarks at:
> http://people.valinux.co.jp/~ryov/dm-ioband/hps/
> 

Hi Ryo,

I had a query about dm-ioband patches. IIUC, dm-ioband patches will break
the notion of process priority in CFQ because now dm-ioband device will
hold the bio and issue these to lower layers later based on which bio's
become ready. Hence actual bio submitting context might be different and
because cfq derives the io_context from current task, it will be broken.

To mitigate that problem, we probably need to implement Fernando's
suggestion of putting io_context pointer in bio. 

Have you already done something to solve this issue?

Secondly, why do we have to create an additional dm-ioband device for 
every device we want to control using rules. This looks little odd
atleast to me. Can't we keep it in line with rest of the controllers
where task grouping takes place using cgroup and rules are specified in
cgroup itself (The way Andrea Righi does for io-throttling patches)?

To avoid creation of stacking another device (dm-ioband) on top of every
device we want to subject to rules, I was thinking of maintaining an
rb-tree per request queue. Requests will first go into this rb-tree upon
__make_request() and then will filter down to elevator associated with the
queue (if there is one). This will provide us the control of releasing
bio's to elevaor based on policies (proportional weight, max bandwidth
etc) and no need of stacking additional block device.

I am working on some experimental proof of concept patches. It will take
some time though.

I was thinking of following.

- Adopt the Andrea Righi's style of specifying rules for devices and
  group the tasks using cgroups.

- To begin with, adopt dm-ioband's approach of proportional bandwidth
  controller. It makes sense to me limit the bandwidth usage only in
  case of contention. If there is really a need to limit max bandwidth,
  then probably we can do something to implement additional rules or
  implement some policy switcher where user can decide what kind of
  policies need to be implemented.

- Get rid of dm-ioband and instead buffer requests on an rb-tree on every
  request queue which is controlled by some kind of cgroup rules.

It would be good to discuss above approach now whether it makes sense or 
not. I think it is kind of fusion of io-throttling and dm-ioband patches
with additional idea of doing io-control just above elevator on the request
queue using an rb-tree.

Thanks
Vivek
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