Re: [PATCH] cpu: Fix numbers in Performance of Mechanisms table in qqz

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

On Sat, Jun 11, 2016 at 03:39:31PM +0900, Akira Yokosawa wrote:
> >From 1ff97081c713ff51d5bc2e15f8ba7649427fac4f Mon Sep 17 00:00:00 2001
> From: Akira Yokosawa <akiyks@gmail.com>
> Date: Sat, 11 Jun 2016 15:28:20 +0900
> Subject: [PATCH] cpu: Fix numbers in Performance of Mechanisms table in qqz
> 
> Numbers given in 'Comms Fabric' and 'Global Comms' rows in
> Table D.1 seem wrong.
> 
> Their costs are given in ns unit, so they should match those given
> in Table 3.1.
> 
> Also, their ratio should be calculated by cost(ns)/0.36(ns).
> 
> This commit fixes those numbers.

Great catch, but let's talk about the fix.

The 130ms is the time required for light to circumnavigate the earth
in a vacuum.  The 195ms is that for light to circumnavigate the earth
in glass, as in an optical fiber.  There is some variation due to
different refractive indexes of different types of fiber.

So making both of those entries be 195,000,000 seems like the right
approach.  An even better approach would be to use the ping time to
some system on the other side of the world, but I am coming up empty.
Pinging google.com gets me 216.58.217.46 for about 47ms.  If you are
far away from West Coast USA, please see what you get.  Not that I
would put it past Google to spread a single IP address worldwide to
defeat this, but worth a try...

On the Comms Fabric number, I suspect that I just googled this at
two different times and got two different numbers.  Not surprising,
as different products would be optimized differently at different
times.

So how about https://en.wikipedia.org/wiki/InfiniBand?  Choose the
value in effect when Nehalem was released for the QQ table, and choose
the value in effect when the AMD Opteron 844 was released for the
table in Chapter 3.

Seem reasonable?

Oh, and also add a Latex comment saying where the data came from!  ;-)

							Thanx, Paul

> Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
> ---
>  cpu/overheads.tex | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/cpu/overheads.tex b/cpu/overheads.tex
> index 311c43e..0cf83f3 100644
> --- a/cpu/overheads.tex
> +++ b/cpu/overheads.tex
> @@ -224,11 +224,11 @@ global agreement.
>  	\hline
>  	CAS cache miss		&          95.9	&         266.4 \\
>  	\hline
> -	Comms Fabric		&       4,500\textcolor{white}{.0}
> -						&	7,500\textcolor{white}{.0} \\
> +	Comms Fabric		&       3,000\textcolor{white}{.0}
> +						&	8,330\textcolor{white}{.0} \\
>  	\hline
> -	Global Comms		& 195,000,000\textcolor{white}{.0}
> -						& 324,000,000\textcolor{white}{.0} \\
> +	Global Comms		& 130,000,000\textcolor{white}{.0}
> +						& 361,000,000\textcolor{white}{.0} \\
>  \end{tabular}
>  \caption{Performance of Synchronization Mechanisms on 16-CPU 2.8GHz Intel X5550 (Nehalem) System}
>  \label{tab:cpu:Performance of Synchronization Mechanisms on 16-CPU 2.8GHz Intel X5550 (Nehalem) System}
> -- 
> 1.9.1
>