Messages in this thread |  | | Date | Thu, 29 Aug 2002 10:51:34 +0100 | From | Padraig Brady <> | Subject | Re: [PATCH][2.5.32] CPU frequency and voltage scaling (0/4) |
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Linus Torvalds wrote: > On 28 Aug 2002, Alan Cox wrote: > >>Systems designers are designing on the basis of thermal slowdowns being >>the optimal way to build some systems. Its actually quite reasonable for >>many workloads. > > Absolutely. Thermal policy is often an overriding thing, where even > non-transmeta CPU's will simply do the decision "on their own", without > input from the OS. That's simply because some designs will literally not > work above certain temperatures and do not have the heat sink capacity to > get out of a tight spot by purely external cooling. > > But that's just one part of it. Even aside from thermal concerns, you want > to drop frequency aggressively when the machine is idle, because dropping > the frequency allows you to drop the voltage and effetively gets you a > cubed power reduction (which not only saves your battery, but also cools > the chip down so that when you _do_ start going full speed again you have > more thermal headroom). > > So in order to avoid the thermal shutdown, you need to be proactive about > the frequency. Which again means that a user-level "once a second" or > "once in a blue moon" approach is fundamentally flawed.
Just a data point from my application. First of all with VIA CPUs there is a 1ms delay per change when it resyncs things, so it's not practical to do it very often or for realtime apps etc. My application was purely a heat reduction exercise where the timescale from 0°C to 70°C was around 10 minutes at max voltage/frequency, so it could easily be controlled by a userspace application. In fact I didn't control it manually and only set the frequency (for which the appropriate voltage was chosen by the cpufreq code) at system startup.
> I don't disagree with _also_ being able to set the frequency statically. > However, I do disagree with an interface that seems to be _purely_ > designed for this, and nothing else. > > Linus
Pádraig.
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