Re: need help making shell script use two CPUs/cores
Carl Johnson put forth on 1/24/2011 5:07 PM:
> Stan Hoeppner <stan@hardwarefreak.com> writes:
>
>> Now we have 4 CPUs on two memory channels. If not for caches, you'd see
>> no speedup past 2 Imagemagick processes. Which is pretty much the behavior
>> identified by another OP with an Athlon II x4 system--almost zeo speedup from 2
>> to 4 processes.
>
> I think you are referring to the data that I posted for my Athlon II x4
> system, but that is *NOT* what the data showed. I thought that the data
> clearly showed pretty good scaling up to 4 processors, so I don't know
> what you are seeing that everybody else is missing. I will copy some of
> the data below, but basically it showed that total time almost cut in
> half when it went from 1 to 2 processors, and again when it went from 2
> to 4 processors.
>
> Processors Time (seconds)
> P1 66
> P2 36
> P4 20
Perfect scaling here would be a run time of 16.5 seconds with 4 processes/cores
with this particular sample set of photos. 20 is 1/4th of 80, and closer to
1/3rd of 66. This isn't close to linear scaling, although it is a little better
than I expected from this particular CPU. One can clearly see the effects of
memory contention at only 2 processes, and that trend continues out to 4
processes, getting progressively worse, as expected. Past four processes,
likely at 5, and on from there, you'll see little, and then no scaling at all.
I must admit I am a bit surprised that a quad core AMD with only 512KB L2 per
core, and no L3, scales as well as it does to 4 processes. The images in this
sample test are relatively small though, so cache size probably isn't that much
in play. With larger image sizes I'm guessing we'd see less scaling than we do
here, as many more reads to main memory will be required to fetch the pixel
data, and thus memory contention among the 4 cores will be much higher.
--
Stan
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