Re: Parallel programming book query

["Paul E. McKenney" <paulmck@linux.ibm.com>]

No, that would work.  Not something I would think of because there is
no globally accessible ceil() function in the Linux kernel.  ;-)

I have queued a patch with your Reported-by as shown at the end of this
email.  Please let me know if I messed anything up.

							Thanx, Paul

On Wed, Dec 19, 2018 at 05:13:01PM -0800, Mark Morrissey wrote:
> I agree that it is equivalent to:
> 
> p = s * ceil(m/s)
> 
> But why not just perform that calculation? If it is based on either
> efficiency or clock cycles, I don't see it. I admit to not having looked at
> the implementation of the ceil() library routine. If I should look at
> ceil(), then just slap me and I will understand.
> 
> --mark
> --
> Mark Morrissey, markem@pdx.edu
> Senior Instructor, Computer Science
> Portland State University
> 
> 
> On Wed, Dec 19, 2018 at 4:45 PM Paul E. McKenney <paulmck@linux.ibm.com>
> wrote:
> 
> > On Mon, Dec 17, 2018 at 01:04:37PM -0800, Mark Morrissey wrote:
> > > Hi Paul,
> > >
> > > I am looking at your parallel programming book, 12/5/2018 version. In
> > > formula E.6, you use the floor function rather than the ceiling function.
> > > Is this deliberate or perhaps in error?
> >
> > Good question!
> >
> > The intent is that the "+ s - 1" in the numerator does the upwards part
> > of the rounding, and the floor function does the rest.
> >
> > Or am I missing something here?  It should be possible to express this
> > in terms of the ceiling function, though that would be further removed
> > from C code implementing this equation.
> >
> > Adding Alexey and the list on CC.  This is on page 423 (PDF page 435) of
> >
> > https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook.2018.12.08a.pdf

------------------------------------------------------------------------

commit 9d317176364184ad6ef55fdab5535449fb8d110d
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date:   Wed Dec 19 20:30:28 2018 -0800

    SMPdesign: Use ceiling symbol, skip odd floor computation
    
    Equation E.6 in Quick Quiz 6.20 computes the floor function of
    "(m + 2 - 1) / s", which is just the ceiling function of "m / s".
    This commit therefore moves to this simpler formulation, on the
    strength of the ceil() function in the standard library.  The fact
    that ceil() is not globally available in the Linux kernel, which
    motivated the original, seems a bit irrelevant, hence the change.
    
    Reported-by: Mark Morrissey <markem@pdx.edu>
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/SMPdesign/SMPdesign.tex b/SMPdesign/SMPdesign.tex
index fe8f445851e3..0ba7ebd04fe6 100644
--- a/SMPdesign/SMPdesign.tex
+++ b/SMPdesign/SMPdesign.tex
@@ -1177,7 +1177,7 @@ this book.
 	$m$ rounded up to the next multiple of $s$, as follows:
 
 	\begin{equation}
-		p = s \left \lfloor \frac{m + s - 1}{s} \right \rfloor
+		p = s \left \lceil \frac{m}{s} \right \rceil
 	\label{sec:SMPdesign:p}
 	\end{equation}