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...
>
> Thanks, Akira
>
>>
>> Strange, but whatever the reason, true! ;-)
>>
>> Thanx, Paul
