Re: Parallel programming book query

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

Not a problem -- we change it all the time, after all.  That is the
only way that it can possibly keep up with a changing world, let
alone improve.  ;-)

						Thanx, Paul

On Thu, Dec 20, 2018 at 08:17:35AM -0800, Mark Morrissey wrote:
> Thanks, Paul.l  Didn't mean to cause a book change.
> 
> --mark
> --
> Mark Morrissey, markem@pdx.edu
> Senior Instructor, Computer Science
> Portland State University
> 
> 
> On Wed, Dec 19, 2018 at 8:35 PM Paul E. McKenney <paulmck@linux.ibm.com>
> wrote:
> 
> > 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}
> >
> >
> >