Re: [PATCH] Priorities in Anticipatory I/O scheduler

[Naveen Gupta]

2008/10/28 Dave Chinner <david@xxxxxxxxxxxxx>:
> On Tue, Oct 28, 2008 at 05:04:53PM -0700, Naveen Gupta wrote:
>> 2008/10/28 Dave Chinner <david@xxxxxxxxxxxxx>:
>> > On Tue, Oct 28, 2008 at 03:48:44PM -0700, Naveen Gupta wrote:
>> >> 2008/10/28 Dave Chinner <david@xxxxxxxxxxxxx>:
>> >> > On Tue, Oct 28, 2008 at 10:14:20AM -0700, Naveen Gupta wrote:
>> >> >> The anticipatory scheduler chooses it's next i/o to be of highest
>> >> >> available priority level.
>> >> >
>> >> > That sounds exactly like what the current RT class is supposed to
>> >> > be used for - defining the absolute priority of dispatch. How
>> >> > is this latency class different to the current RT class semantics
>> >> > that are defined for CFQ?
>> >> >
>> >>
>> >> I/O from RT class in CFQ can still see a bubble with this new latency
>> >> class. An easy way to check this would be to submit ios at multiple
>> >> levels both in CFQ and AS and check max latency of the highest levels.
>> >> I will let Jens or Satoshi comment on exact algorithm for RT class.
>> >
>> > You're missing my point entirely.
>> >
>> > You're defining a new class that has the exact same meaning as
>> > the current RT class definition, then mapping the BE class over
>> > the top of that, hence changing what that means for everyone.
>> >
>> > The fact that the *implementation* of AS and CFQ is different is
>> > irrelevant; if you use the RT class then on CFQ you get the current
>> > RT behaviour, if you use the RT class on AS you should get your new
>> > priority dispatch mechanism. We don't need a new API just because
>> > the implementations are different.
>> >
>>
>> There is nothing "real-time" about the current RT class anyways.
>
> That's an implementation problem, not an API definition problem.
>
>> It is
>> basically these small *implementation* differences that defines these
>> classes in current scheme of things, precise definitions of which
>> would be very hard to find if one started looking around.
>
> Please, disconnect what you think about implementation and ask
> yourself what makes sense from an API if you were trying to use this
> stuff.
>
> I want to be able to use this stuff to optimise filesystem I/O,
> but if the priority class I need to use is dependent on the elevator the
> *user selects* and can change dynamically, then I simply cannot
> make that optimisation.
>
>> Now the initial feedback was since this *implementation* is different
>> from anything we have in CFQ which is our current *standard* way of
>> thinking and comparing (that is the only thing that exists) why not
>> make them into a new class :).
>
> Because it make it impossible to optimise application code as the
> class that needs to be used is entirely dependent on the
> configuration of the machine that it is running on. Application
> writers are not going to probe the I/O scheduler the block device
> is using to determine if they should be using RT or LATENCY class
> prioritisation. From a user POV they do *exactly the same thing*,
> so they should use the same behavioural classes defined by the API.

I agree with you that we need an API which is valid across schedulers.
But one has to agree that this sort of thing has it's own limitations.
We are assuming that every scheduler which implements any kind of
priority has a valid implementation of RT, BE, Idle class, which in
this we we don't have. What happens tomorrow once we have a scheduler
which decides that it needs to divide b/w. Which class would one map
it to?

As I understand what you are asking for is: filesystem i/o can use BE
0 across all schedulers for journal updates. And you still have RT
levels to take care of any higher priority i/o which need not wait for
journal updates.

Here is what we can do:
1. Add 17 levels. top 8 RT, next 8 BE and last 1 idle. Though we know
they all are similar in implementation. It's just that RT > BE > idle
in importance. And if the LATENCY camp is still active, add another
class LATENCY which in context of AS is same as RT. So you get to keep
RT > BE and they get Latency.

2. Add 10 levels instead of current 8. top 1 level maps all 8 RT
levels. next 8 are BE and last 1 maps to idle. This also gives you
access to BE 0, while all RT levels are higher priority than BE. It
discourages people from using different RT levels unless we find a new
meaning for it in context of AS.


>
>> >> I see your problem, we could make the LATENCY class different from
>> >> and above BE class (instead of one-one mapping).
>> >
>> > Like the RT class is currently defined to be? ;)
>>
>> I agree with you and we could use RT (though you and I know that
>> basically it is best effort). LATENCY was invented due to a previous
>> suggestion.
>
> As someone who is actually trying to use this stuff, I'm saying that
> the LATENCY suggestion was a *bad idea* because of the complexity it
> introduces when trying to optimise performance by applying I/O
> priorities to different I/O types. I want *one* API that is
> implemented by all schedulers, not an API per scheduler.....
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@xxxxxxxxxxxxx
>
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