On Thu, 25 Aug 2005, Alex Williamson wrote: > I don't know that it's that uncommon. Simply having one non-arch > specific timer is enough to need to decided whether it's better than a > generic timer. I assume pretty much every arch has a cycle timer. For > smaller boxes, this might be the preferred timer given it's latency even > if something like an hpet exists (mmio access are expensive). How do > you hard code a value that can account for that? I agree, we could > easily go too far and produce some bloated algorithm, but maybe it's > simply a weighted product of a few variables.
I think a priority is something useful for the interpolators. Some of the decisions about which time sources to use also have criteria different from drift/latency/jitter/cpu. F.e. timers may not survive various power-saving configurations. Thus I would think that we need a priority plus some flags. Some of the criteria for choosing a time source may be: 1. If a system boots up with a single cpu then there is no question that the ITC/TSC should be used because of the fast access. 2. If the BIOS is capable of perfect ITC/TSC synchronization then use the ITC/TSC. However, this is typically restricted to SMP configs and there is an issue on IA64 that the PAL can indicate a nodrift configuration but Linux is not capable of perfects sync on bootup. 3. If a memory mapped global clock is available then make sure to first use a distributed timer over a centralized time source because distributed timer have fast local access even under contention. I.e. use Altix RTC over HPET/Cyclone. 4. Use any memory mapped global clock source 5. Abuse some other system component for time keeping (PIT, i/o devices etc) The criteria for drift/latency can only be established after we gone through the above. The low-power stuff adds additional complexity. We may need to dynamically change timer interpolators / time sources if the power situation changes. Nothing like that exists today for time interpolators since they are mostly used in server configurations. - 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/
