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

[Andrea Righi]

Vivek Goyal wrote:
> On Thu, Sep 18, 2008 at 05:18:50PM +0200, Andrea Righi wrote:
>> Vivek Goyal wrote:
>>> On Thu, Sep 18, 2008 at 04:37:41PM +0200, Andrea Righi wrote:
>>>> Vivek Goyal wrote:
>>>>> 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. All sounds reasonable to me and I think this is be the right way
>>>> to proceed.
>>>>
>>>> I'll try to design and implement your rb-tree per request-queue idea into my
>>>> io-throttle controller, maybe we can reuse it also for a more generic solution.
>>>> Feel free to send me your experimental proof of concept if you want, even if
>>>> it's not yet complete, I can review it, test and contribute.
>>> Currently I have taken code from bio-cgroup to implement cgroups and to
>>> provide functionality to associate a bio to a cgroup. I need this to be
>>> able to queue the bio's at right node in the rb-tree and then also to be
>>> able to take a decision when is the right time to release few requests.
>>>
>>> Right now in crude implementation, I am working on making system boot.
>>> Once patches are at least in little bit working shape, I will send it to you
>>> to have a look.
>>>
>>> Thanks
>>> Vivek
>> I wonder... wouldn't be simpler to just use the memory controller
>> to retrieve this information starting from struct page?
>>
>> I mean, following this path (in short, obviously using the appropriate
>> interfaces for locking and referencing the different objects):
>>
>> cgrp = page->page_cgroup->mem_cgroup->css.cgroup
>>
> 
> Andrea,
> 
> Ok, you are first retrieving cgroup associated page owner and then 
> retrieving repsective iothrottle state using that
> cgroup, (cgroup_to_iothrottle). I have yet to dive deeper into cgroup

Correct.

> data structures but does it work if iothrottle and memory controller
> are mounted on separate hierarchies?

ehm... I've to check. I usually mount all the controllers into the same
hierarchy. :P

> bio-cgroup guys are also doing similar thing in the sense retrieving
> relevant pointer through page and page_cgroup and use that to reach
> bio_cgroup strucutre. The difference is that they don't retrieve first
> css object of mem_cgroup instead they directly store the pointer of
> bio_cgroup in page_cgroup (When page is being charged in memory controller).
> 
> While page is being charged, determine the bio_cgroup, associated with
> the task and store this info in page->page_cgroup->bio_cgroup.
> 
> static inline struct bio_cgroup *bio_cgroup_from_task(struct task_struct
> *p)
> {
>        return container_of(task_subsys_state(p, bio_cgroup_subsys_id),
>                                struct bio_cgroup, css);
> }
> 
> At any later point, one can look at bio and reach respective bio_cgroup
> by.
> 
> bio->page->page_cgroup->bio_cgroup.
> 
> Looks like now we are getting rid of page_cgroup pointer in "struct page"
> and we shall have to change the implementation accordingly.

Actually, only page_get_page_cgroup() implementation would change. And
we don't have to worry about the particular implementation (hash,
radix_tree, whatever..), in any case bio-cgroup has to simply use the
opportune interface: page_get_page_cgroup(struct *page).

-Andrea
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