Re: Need better is_better_time_interpolator() algorithm

[Christoph Lameter]

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.

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