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

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Message-ID: <2846be6b0810290153rf4bf181ta07df64940f7aaca@mail.gmail.com>
Date:	Wed, 29 Oct 2008 01:53:20 -0700
From:	"Naveen Gupta" <ngupta@...gle.com>
To:	"Aaron Carroll" <aaronc@...ato.unsw.edu.au>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	akpm@...ux-foundation.org, s-uchida@...jp.nec.com,
	david@...morbit.com
Subject: Re: [PATCH] Priorities in Anticipatory I/O scheduler

2008/10/28 Aaron Carroll <aaronc@...ato.unsw.edu.au>:
> Naveen Gupta wrote:
>> 2008/10/28 Aaron Carroll <aaronc@...ato.unsw.edu.au>:
>>> Naveen Gupta wrote:
>>>> As I said earlier the organization of the AS levels is flat, so we
>>>> could use any class (RT, BE, LATENCY) and fold the remaining ones. The
>>>> other way which you would probably like is to increase number of
>>>> levels and map different classes so that they are not folded.
>>> As I said in my reply to the initial posting of this, I think there are
>>> only two sensible ways of handling this:
>>>
>>>  1) Maintain the full number of I/O priorities (1 IDLE, 8 BE, 8 RT);
>>
>> But then we are assuming that we are providing different quality of
>> service according to classes.
>
> Right.  The ideal solution is a scheduler-independent definition of
> RT (Jens?) which you can apply here.  However, it seems to me that you
> want to basically ignore RT and IDLE. If you're going to do that, at
> least implement sane alternate behaviour.
>
> This solution applies the the principle of least surprise; RT requests
> always have higher priority than BE requests, and within the class,
> higher level means higher priority.  In your implementation, BE 0 == RT x
> and IDLE == BE 7.  This is surprising behaviour.

Aaron, I took care of these in reply to Dave's email.

>
>>>  2) Collapse the levels and only deal with the classes;
>>
>> I am not sure if this is meaningful. When all we have is different
>> levels of BE, it wouldn't make sense to call them different classes.
>
> It's not meaningful as it stands.  This difference here is that you
> at least maintain the ordering of the classes with respect to priority.

I am not sure that giving one level to each class would be an
acceptable solution.

>
>
>           -- Aaron
>
>
>
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