Re: [PATCH] Documentation/scheduler/sched-deadline.txt: correct definition of density as C_i/min{D_i,P_i}

From: Luca Abeni <luca.abeni@unitn.it>
To: Henrik Austad <henrik@austad.us>
Cc: Zhiqiang Zhang <zhangzhiqiang.zhang@huawei.com>,
	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 21:47:13 +0200	[thread overview]
Message-ID: <20150403214713.70e9614b@luca-1225C> (raw)
In-Reply-To: <20150403175737.GA3847@icarus.home.austad.us>

Hi Henrik,

On Fri, 3 Apr 2015 19:57:37 +0200
Henrik Austad <henrik@austad.us> wrote:
[...]
> > 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].
I think literature is more or less equally divided between "P" and
"T" (I suspect I personally used different letters in different
papers :)

> 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?
I remember there was a discussion (and I seem to remember that the
symbol used for the period was changed at least one time, but I might
be wrong), but I do not remember the details.

Next week I'll have some time for working on this; I'll search the old
emails and I'll try to reconstruct the discussion.

[...]
> > - 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.
I fully agree with this ;-)

			Luca