On Thursday, July 24, 2014 09:26:09 AM Peter Zijlstra wrote: > On Thu, Jul 24, 2014 at 02:53:20AM +0200, Rafael J. Wysocki wrote: > > I am used to slightly different terminology here. Namely, there are voltage > > domains (parts sharing a voltage rail or a voltage regulator, such that you > > can only apply/remove/change voltage to all of them at the same time) and > > clock > > domains (analogously, but for clocks). A power domain (which in your > > description > > above seems to correspond to a voltage domain) may be a voltage domain, a > > clock > > domain or a combination thereof. > > > > In addition to that, in a voltage domain it may be possible to apply many > > different levels of voltage, which case doesn't seem to be covered at all by > > the above (or I'm missing something). > > > > Also a P-state is not just a frequency level, but a combination of frequency > > and voltage that has to be applied for that frequency to be stable. You may > > regard them as Operation Performance Points of the CPU, but that very well > > may > > go beyond frequencies and voltages. Thus it actually is better not to talk > > about P-states as "frequencies". > > > > Now, P-states may or may not have to be coordinated between all CPUs in a > > package (cluster), by hardware or software, such that all CPUs in a cluster > > need to be kept in the same P-state. That you can regard as a "P-state > > domain", but it usually means a specific combination of voltage and > > frequency. > > I think Morton is aware of this, but for the sake of sanity dropped the > whole lot into something simpler (while hoping reality would not ruin > his life). > > > C-states in turn are states in which CPUs don't execute instructions. > > That need not mean the removal of voltage or even frequency from them. > > Of course, they do mean some sort of power draw reduction, but that may > > be achieved in many different ways. Some C-states require coordination > > too (for example, a single C-state may apply to a whole package or cluster > > at the same time) and you can think about "domains" here too, but there > > need not be a direct mapping to physical parameters such as the frequency > > or the voltage. > > One thing that wasn't clear to me is if you allow for C-domain and > P-domain to overlap or if they're always inclusive (where one is wholly > contained in the other).
On the CPUs I worked with so far they were always inclusive. Previously, the whole package was a P-state domain. Today some CPUs (Haswell server chips for example) have per-core P-states. > > Moreover, P-states and C-states may overlap. That is, a CPU may be in Px > > and Cy at the same time, which means that after leaving Cy it will execute > > instructions in Px. Things like leakage may depend on x in that case and > > the total power draw may depend on the combination of x and y. > > Right, and I suppose the domain thing makes it impossible to drop to the > lowest P state on going idle. Tricky that. That's the case for older chips. I'm not sure about the newest lot entirely to be honest, need to ask. > > The concern is that if a scaling governor is running in parallel with the > > above > > algorithm and it has its own utilization goal (it usually does), it may > > change > > the P-state under you to match that utilization goal and you'll end up with > > something different from what you expected. > > > > That may be addressed either by trying to predict what the scaling governor > > will > > do (and good luck with that) or by taking care of P-states by yourself. The > > latter would require changes to the algorithm I think, though. > > The idea was that we'll do P states ourselves based on these utilization > figures. If we find we cannot fit the 'new' task into the current set > without either raising P or waking an idle cpu (if at all available), we > compute the cost of either option and pick the cheapest. Yeah. One subtle thing is that ramping up P may affect the other guys (if the whole chip is a P-domain, for example), but I guess that can be taken into account. -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
