
Rockbox mail archiveSubject: Power Efficiency TradeoffsPower Efficiency Tradeoffs
From: Catalin Patulea <cat_at_vv.carleton.ca>
Date: Sat, 12 Jan 2008 19:09:35 0500 Hey everyone, I've been pondering a power efficiency tradeoff problem in dualcore embedded systems. (This obviously directly stems from the m:robe architecture, but the discussion should be fairly general.) Take a system with two identical cores.  One runs at some given clock f_1 and another that runs at f_2, different from f_1.  I believe (correct me if I'm wrong) that, in general, power consumption is proportional to the core frequency. Let the power consumptions of the cores be P_1 = k*f_1 and P_2 = k*f_2.  Take some task that takes a constant number of instructions to execute to completion. Assume that this task, given the requirements of the system, may be executed on either core. Then, since the number of instruction is constant, the amount of time required for the same task on each core is inversely proportional to the clock (with the simplifying assumption that these are 1 clocks per instruction machines): T_1 = N/f_1 and T_2 = N/f_2.  If you consider power to be constant and you integrate over time, you end up with the following equations for consumed *energy* for the same given task, on each core running at a different frequency: E_1 = P_1*T_1 = k*f_1 * N/f_1 = k*N E_2 = P_2*T_2 = k*f_2 * N/f_2 = k*N In other words, all other things being equal, a given task on a given machine takes a given constant amount of energy to complete, regardless of the clock frequency. Is this right? This basically means that underclocking brings no benefit in terms of energy consumption if the CPU is fully active. In fact, if we assume that the machine's idle mode consumes zero power, this also means there's no underclocking benefit for notfullyloaded CPUs. Clearly, though, since underclocking is so commonly accepted as a way of reducing power consumption, there must be an explanation. Can anyone shed some light? Ultimately, I would like to use the conclusion of this discussion to decide where to place certain code for the m:robe's audio system: in the ARM core, or in the (lowerclocked) DSP core. I know that these are far from identical architectures, but it's often helpful to examine an ideal situation before drawing real conclusions. Thanks, Catalin Received on 20080113 Page template was last modified "Tue Sep 7 00:00:02 2021" The Rockbox Crew  Privacy Policy 