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SubjectRe: Using block layer's I/O context api (block/blk-ioc.c) without CFQ.
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Hi,

2010/8/21 Corrado Zoccolo <czoccolo@gmail.com>:
> 2010/8/21 Pedro Eugênio Rocha <pedro.eugenio.rocha@gmail.com>:
>> Thanks for your reply,
>>
>> 2010/8/20 Corrado Zoccolo <czoccolo@gmail.com>:
>>> 2010/8/19 Pedro Eugênio Rocha <pedro.eugenio.rocha@gmail.com>:
>>>> Hi,
>>> Hi,
>>>>
>>>> Is there a way to use the generic block layer I/O context api
>>>> (block/blk-ioc.c), without
>>>> using cfq specific functions and structs? I'm writing an I/O scheduler
>>>> for my master
>>>> thesis, and I'd like to use my own functions for I/O context handling,
>>>> not cfq's. Sorry
>>>> if I'm missing something here.
>>> Sure. I think you can get an idea by grabbing an old enough kernel
>>> that still has anticipatory scheduler (if I recall correctly, 2.6.32
>>> should have it), and look at its functions.
>>
>> In mainstream kernel as in 2.6.32, it's used a generic get_io_context
>> function to get the current task io_context. In addition, you can create
>> a function like cfq_get_io_context to implement some scheduler
>> specific stuff. Oh right so far.
>>
>> But when the task is exiting there is an exit_io_context function, which
>> calls put_io_context. This function calls cfq_dtor, which seems to be
>> an specific cfq function, and maybe will not work with others I/O
>> schedulers. My newbie question is, those block/blk-ioc.c functions
>> are  supposed to be generic, or my scheduler should implement it's
>> own?
> Cfq and AS required extending the basic io_context to add per-process
> statistics. Therefore, cfq_io_context and as_io_context were added to
> the io_context, and proper deallocation is done when deallocating
> io_context (as_io_context is deallocated via its dtor function pointer
> just before calling cfq_dtor).
> If your I/O scheduler requires storing some stats per process, you
> have the choice of either reusing one of the existing as/cfq
> io_context extensions, or define your own (and in that case, you
> should add your destructor as well).

Right, I think I understood now! In my case, I was thinking in use just
the *ioc_data pointer in io_context. But in that case, I will not have a
callback to deallocate my structures, right? For now, I will clone those
cfq_io_context's.

Is there a reason to not bring those functions into elevator_type .ops?
I mean, create a interface like .elevator_create_io_context and
.elevator_destroy_io_context, and let the elevator.c do the callbacks.

Sorry, if I'm wrong.

>>
>>
>>> Or you can look at my post:
>>> http://lists.openwall.net/linux-kernel/2009/06/08/387, where I
>>> implemented a proof-of-concept I/O scheduler based on anticipatory
>>> scheduler. Most of the concepts in that scheduler were later ported to
>>> CFQ.
>>> Do you already have a scheduling algorithm in mind? And what will be
>>> your main purpose/target?
>>>
>>
>> Yes. Our main purpose is to share disk I/O bandwidth among
>> processes/tasks/cgroups/whatever providing strong QoS guarantees, such
>> as bandwidth, bursts and latency. It borrows many ideas from bfq and
>> pClock I/O schedulers. The implementation will be just a proof-of-concept,
>> not something you can use in a real environment. =)
>
> Sounds interesting. Do you plan to have simulations before the real
> implementation, and what are your reference workloads?
>

I think that what I'm doing now will be my real implementation, but I'm
just starting. We are thinking in scenarios with massive sequential reads
all over the disk, in addition to some latency bound applications, to
simulate a streaming and a web server, for instance.

Any suggestions would be welcome.

Thanks again,

> Cheers,
> Corrado
>
>> Thanks for your help!
>>
>>> Thanks
>>> Corrado
>>>>
>>>> Thanks,
>>>>
>>>> Pedro Eugênio Rocha
>>>> --
>>>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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>>>> Please read the FAQ at  http://www.tux.org/lkml/
>>>>
>>>
>>>
>>>
>>> --
>>> __________________________________________________________________________
>>>
>>> dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
>>> PhD - Department of Computer Science - University of Pisa, Italy
>>> --------------------------------------------------------------------------
>>> The self-confidence of a warrior is not the self-confidence of the average
>>> man. The average man seeks certainty in the eyes of the onlooker and calls
>>> that self-confidence. The warrior seeks impeccability in his own eyes and
>>> calls that humbleness.
>>>                                Tales of Power - C. Castaneda
>>>
>>
>>
>>
>> --
>> Pedro Eugênio Rocha
>>
>
>
>
> --
> __________________________________________________________________________
>
> dott. Corrado Zoccolo                          mailto:czoccolo@gmail.com
> PhD - Department of Computer Science - University of Pisa, Italy
> --------------------------------------------------------------------------
> The self-confidence of a warrior is not the self-confidence of the average
> man. The average man seeks certainty in the eyes of the onlooker and calls
> that self-confidence. The warrior seeks impeccability in his own eyes and
> calls that humbleness.
>                                Tales of Power - C. Castaneda
>



--
Pedro Eugênio Rocha
--
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