From: Henrik Austad <henrik@austad.us>
To: Zhiqiang Zhang <zhangzhiqiang.zhang@huawei.com>
Cc: luca.abeni@unitn.it, juri.lelli@arm.com,
linux-kernel@vger.kernel.org, torvalds@linux-foundation.org
Subject: Re: [PATCH] Documentation/scheduler/sched-deadline.txt: correct definition of density as C_i/min{D_i,P_i}
Date: Fri, 3 Apr 2015 19:57:37 +0200 [thread overview]
Message-ID: <20150403175737.GA3847@icarus.home.austad.us> (raw)
In-Reply-To: <1428049113-62546-1-git-send-email-zhangzhiqiang.zhang@huawei.com>
On Fri, Apr 03, 2015 at 04:18:33PM +0800, Zhiqiang Zhang wrote:
> From the contex,the definition of the destiny of a task
I'm sure we would all like to have a perfect scheduler, however, knowing
the destiny of a task is a bit beyond what we can do now ;)
> C_i/min{D_i,T_i},where T_i is not referred before, should be
> substituted by C_i/min{D_i,P_i}.
Actually, I'd prefer we use T_i to describe the period and not P because:
- P is easy to confuse with priority - which has _nothing_ to do with
deadline scheduling
- I was going to state that "the litterature is consistent in its usage of
'T_i' for task i's period". But then I dived through some of the books
and of course it isn't. Buttazzo use T, Jane Liu use P and so on.
However, I state that *most* litterature use T_i do denote the period of
task i. Burns & Davis has a nice summary of RT-litterature [1].
So I'd rather prefer a s/P_i/T_i/ throughout the text.
I realise that I've reviewed quite a lot of this, and I have some vague
memories of this being discussed earlier, Juri? Luca?
> ----------------------------------------
>
> Signed-off-by: Zhiqiang Zhang <zhangzhiqiang.zhang@huawei.com>
> ---
> Documentation/scheduler/sched-deadline.txt | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
> index 21461a0..194664b 100644
> --- a/Documentation/scheduler/sched-deadline.txt
> +++ b/Documentation/scheduler/sched-deadline.txt
> @@ -169,8 +169,8 @@ CONTENTS
> of all the tasks executing on a CPU if and only if the total utilisation
> of the tasks running on such a CPU is smaller or equal than 1.
> If D_i != P_i for some task, then it is possible to define the density of
> - a task as C_i/min{D_i,T_i}, and EDF is able to respect all the deadlines
> - of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,T_i} of the
> + a task as C_i/min{D_i,P_i}, and EDF is able to respect all the deadlines
> + of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,P_i} of the
My argument for T_i vs. P_i aside, I do agree that we should not use T_i
here whilst using P_i in other places. We should strive to be internally
consistent above all else.
So *if* we are going to use P_i for period, then this is correct and should
be applied.
> densities of the tasks running on such a CPU is smaller or equal than 1
> (notice that this condition is only sufficient, and not necessary).
>
> --
> 1.9.0
>
Just my $0.02 etc etc :)
1) http://www-users.cs.york.ac.uk/~robdavis/papers/MPSurveyv5.0.pdf
--
Henrik Austad
next prev parent reply other threads:[~2015-04-03 17:57 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-04-03 8:18 [PATCH] Documentation/scheduler/sched-deadline.txt: correct definition of density as C_i/min{D_i,P_i} Zhiqiang Zhang
2015-04-03 10:52 ` Luca Abeni
2015-04-08 9:31 ` Juri Lelli
2015-04-08 10:34 ` Luca Abeni
2015-04-03 17:57 ` Henrik Austad [this message]
2015-04-03 19:47 ` Luca Abeni
2015-04-07 9:48 ` Luca Abeni
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