On Wed, Feb 08, 2023 at 07:26:58PM +0900, Akira Yokosawa wrote:
> On Wed, 8 Feb 2023 17:47:31 +0900, Akira Yokosawa wrote:
> > Hi,
> >
> > On Tue, 7 Feb 2023 19:41:02 -0800, Paul E. McKenney wrote:
> >> On Wed, Feb 08, 2023 at 12:07:20AM -0300, Leonardo Brás wrote:
> >>> Hello Paul,
> >>>
> >>> I have been reading the book, until I stumbled on Quick Quiz 3.7,
> >>> Table E.1: Performance of Synchronization Mechanisms
> >>> on 16-CPU 2.8 GHz Intel X5550 (Nehalem) System
> >>>
> >>> <Copying from source, since the PDF is a little tricky>
> >>>
> >>> The first part looks like:
> >>>
> >>> Clock period & 0.4 & 1.0 \\
> >>> Same-CPU CAS & 12.2 & 33.8 \\
> >>> Same-CPU lock & 25.6 & 71.2 \\
> >>> Blind CAS & 12.9 & 35.8 \\
> >>> CAS & 7.0 & 19.4 \\
> >>>
> >>> In this case, what would be the last lines "Blind CAS" and "CAS" referring to ?
> >>>
> >>> (For a second I thought it could be "In-Core Blind CAS" and "In-Core CAS" like
> >>> in Table 3.1, but that would not make sense: This "CAS" is faster than the
> >>> previous "Same-CPU CAS". )
> >>
> >> I was surprised myself, but those measurements are quite real. My best
> >> guess is that the two threads in the core are able to overlap their
> >> accesses, while the single CPU must do everything sequentially.
> >
> > Paul, do you remember how you obtained the data set?
> > There are several data sets under CodeSamples/cpu/data/, but I don't
> > see the one corresponds to the table.
> >
> > The code for collecting these data was added in CodeSamples/cpu/
> > by commit 81989d7483e2 ("cpu: Reproduce the old cache-to-cache
> > latency measurement code") in 2020. And the next commit 2fc05ca07edc
> > ("api-pthreads.h: Use clock_gettime() and check sched_setaffinity()")
> > improved the stability of reproduced code.
> >
> > This table was first added in commit 38fd945ff401 ("Fill out CPU
> > chapter, including adding Nehalem data.") in 2009.
> > The data have never been updated since.
> >
> > I'm kind of suspecting the "7.0 us" which surprised you at the time
> I mean, "7.0 ns"
>
> Thanks, Akira
>
> > might have been an outlier due to some disturbance discussed in
> > Appendix A.3 "What Time Is It?".
> >
> > I'm not sure, just guessing...
My surprise caused me to beat on it, and it was persistent.
But I cannot find the raw data, either, so maybe I should delete that
table. Though I really do like the fact that it is surprising, based
on a hope that it convinces readers to expect the unexpected.
Thanx, Paul
> > Thanks, Akira
> >
> >>
> >> Strange, but whatever the reason, true! ;-)
> >>
> >> Thanx, Paul
