[Haskell-cafe] Unbelievable parallel speedup

[Simon Marlow]

On 03/06/2011 13:10, John D. Ramsdell wrote:
> I've enjoyed reading Simon Marlow's new tutorial on parallel and
> concurrent programming, and learned some surprisingly basic tricks.  I
> didn't know about the '-s' runtime option for printing statistics.  I
> decided to compute speedups for a program I wrote just as Simon did,
> after running the program on an unloaded machine with four processors.
>   When I did, I found the speedup on two processors was 2.4, on three
> it was 3.2, and on four it was 4.4!  Am I living in a dream world?
>
> I ran the test nine more times, and here is a table of the speedups.
>
> 2.35975	3.42595	4.39351
> 1.57458	2.18623	2.94045
> 1.83232	2.77858	3.41629
> 1.58011	2.37084	2.94913
> 2.36678	3.63694	4.42066
> 1.58199	2.29053	2.95165
> 1.57656	2.34844	2.94683
> 1.58143	2.3242	2.95098
> 2.36703	3.36802	4.41918
> 1.58341	2.30123	2.93933
>
> That last line looks pretty reasonable to me, and is what I expected.
> Let's look at a table of the elapse times.
>
> 415.67	176.15	121.33	94.61
> 277.52	176.25	126.94	94.38
> 321.37	175.39	115.66	94.07
> 277.72	175.76	117.14	94.17
> 415.63	175.61	114.28	94.02
> 277.75	175.57	121.26	94.10
> 277.68	176.13	118.24	94.23
> 277.51	175.48	119.40	94.04
> 415.58	175.57	123.39	94.04
> 277.62	175.33	120.64	94.45
>
> Notice that the elapse times for two and four processors is pretty
> consistent, and the one for three processors is a little inconsistent,
> but the times for the single processor case are all over the map.  Can
> anyone explain all this variance?

This looks like automatic CPU speed throttling to me.  The OS is 
decreasing the CPU clock speed automatically to save power.  Normally it 
happens in steps (0.75x, 0.5x max clock).  This would also explain why 
when using more cores the results are more stable: the OS has determined 
that there is lots of work to do, and has stopped throttling the CPU. 
If you can it off, do so.

Cheers,
	Simon