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Date:   Tue, 9 May 2023 22:54:17 +0200
From:   luca abeni <luca.abeni@santannapisa.it>
To:     Vineeth Remanan Pillai <vineeth@bitbyteword.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/deadline: accurate reclaim bandwidth for GRUB
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On Tue, 9 May 2023 22:48:29 +0200
luca abeni <luca.abeni@santannapisa.it> wrote:

> Hi,
> 
> On Tue, 9 May 2023 15:29:21 -0400
> Vineeth Remanan Pillai <vineeth@bitbyteword.org> wrote:
> [...]
> > > Is this understanding correct?    
> > Yes, the above two details are correct. In addition to that, I think
> > the existing equation had a small bug:
> > GRUB paper says, running time is depreciated as
> >    "dq = -U dt" where U is running_bw.
> > This is assuming that the whole cpu bandwidth could be reclaimed.
> > But in our case, we cap at Umax. So the equation should be
> >    "dq = -(U/Umax) dt"  
> 
> Yes, this is the approximation I was mentioning... Instead of using a
> division, I approximated it with a different equation using a sum.

Sorry, ignore this comment (and the following); I misinterpreted the
code (and my old notes).

I do not understand why the "max{}" doe not work well, I need to double
think about it.


			Luca