> jhu schrieb:
>> Actually I think the question applies to all renderers. I was
>> wondering about
>> this when I noticed one of my scenes was using up almost 3 GiB of
>> memory. What's
>> the time ratio between the CPU doing calculations and the CPU waiting
>> for the
>> data fetch to complete?
>
> I just did an experiment which might give a hint:
>
> My Intel i7 QuadCore machine provides hyperthreading, that is each core
> can operate on two threads quasi-simultaneously, quickly switching
> between them in case one of them needs to wait for some not-on-chip
> component (such as main memory) to complete an operation.
>
> Running with the default of 8 threads (i.e. fully utilizing the
> hyperthreading mechanism), POV-Ray 3.7 beta took 54 seconds to complete
> a particular scene.
>
> Running with only 4 threads (i.e. utilizing all cores, but not
> hyperthreading), the same scene took 87 seconds to complete.
>
> I think this indicates that render threads do indeed spend quite some
> time waiting for memory accesses to complete, otherwise hyperthreading
> would not give much added benefit. OTOH I think it also shows that
> memory latency (i.e. the time it takes to complete a particular memory
> request) is more of an issue than memory bandwidth (i.e. the maximum
> speed at which memory content can be read), as hyperthreading cannot
> help with a memory bandwidth bottleneck either (it would only increase
> the number of threads standing by for data from the main memory).

We need a computer with 3 GHz RAM and a 3 MHz hex core CPU. RAW bus 
speed, doubled using DDR, then tripled using triple channel... Add a 
little cache and you get almost no memory wait. Then, you put some 
dedicated stack buffers on the die to further reduce main memory access.


Alain

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