iz (i@z.x) on 9/1/11 wrote:
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>David Kanter (dkanter@realworldtech.com) on 8/31/11 wrote:
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>>As always, we encourage feedback and discussions.
>
>The article mentions:
>
>A large frequency range is essential for the DVFS and good performance for consumer
>products. However it is undesirable for the frequency to be very coarse-grained
>without smaller intermediate steps. In many lightly loaded scenarios the CPU cores
>will often switch back to the base frequency to keep under the power and thermal
>limits. These frequency adjustments cost both performance and power, so it is better
>to have a variety of steady state frequencies available and avoid transitions. For
>example, 5-10% frequency steps would yield much better results.
>
>
>But I don't think that is actually very true. Looking at
>my laptop, which has fine grained control between 600MHz
>and 1400MHz, the cpu ends up at 1400MHz for 22% and at 600
>for 77%. CPU loads are very spiky in nature. The transition
>latencies are low enough that a 50% CPU load looks like an
>alternating sequence of 0% and 100% CPU loads.

Let's take your example. Suppose you are doing work where the maximum sustained frequency (due to TDP/power) for several cores is actually 1000MHz. You could achieve this by oscillating between 600MHz and 1400MHz and paying the cost of adjusting frequency, etc. or you could have a better designed system that runs closer to the sustained levels.

Also, keep in mind we are solely talking about turbo here, where you are beyond max frequency.

>Only thing that makes sense for fine grained frequency
>control is when the CPU does it automatically when waiting
>for memory to catch up. But OS/BIOS-level frequency control
>has no use for fine grained intermediate steps, because
>going straight to max speed and back to sleep saves more
>power and has better performance. Power saving between
>different frequency steps is too small and the latencies
>for changing between the steps too high to keep up with
>the machine load.

We are talking about turbo bins beyond the base frequency. What I'm saying is that for turbo you want to be able to run at {1.2f, 1.4f, 1.6f...) where f is the base frequency.

David