On Mon, Jul 15, 2013 at 01:41:47PM -0700, Arjan van de Ven wrote: > On 7/15/2013 12:59 PM, Peter Zijlstra wrote: > >On Sat, Jul 13, 2013 at 07:40:08AM -0700, Arjan van de Ven wrote: > >>On 7/12/2013 11:49 PM, Peter Zijlstra wrote: > >>> > >>>Arjan; from reading your emails you're mostly busy explaining what cannot > >>>be > >>>done. Please explain what _can_ be done and what Intel wants. From what I > >>>can > >>>see you basically promote a max P state max concurrency race to idle FTW. > >> > >>> > >>>Since you can't say what the max P state is; and I think I understand the > >>>reasons for that, and the hardware might not even respect the P state you > >>>tell > >>>it to run at, does it even make sense to talk about Intel P states? When > >>>would > >>>you not program the max P state? > >> > >>this is where it gets complicated ;-( the race-to-idle depends on the type > >>of > >>code that is running, if things are memory bound it's outright not true, but > >>for compute bound it often is. > > > >So you didn't actually answer the question about when you'd program a less > >than > >max P state. > (oops missed this part in my previous reply) > > so race to halt is all great, but it has a core limitation, it is > fundamentally > assuming that if you go at a higher clock frequency, the code actually > finishes sooner. > This is generally true for the normal "compute" kind of instructions, but > if you have an instruction that goes to memory (and misses caches), that is > not the > case because memory itself does not go faster or slower with the CPU > frequency. > > so depending of the mix of compute and memory instructions, different > tradeoffs > might be needed. > > (for an example of this, AMD exposes a CPU counter for this as of recently > and added > patches to "ondemand" to use it)
OK, but isn't that part of why the micro controller might not make you go faster even if you do program a higher P state? But yes, I understand this issue in the 'traditional' cpufreq sense. There's no point in ramping the speed if all you do is stall more. But I was under the impression the 'hardware' was doing this. If not then we need the whole go-faster and go-slower thing and places to call them and means to determine to call them etc. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
