* Paul Jackson <[email protected]> wrote:
> > It has to be made sure that H1+H2+H3 != H4+H5+H6,
>
> Yeah - if you start trying to think about the general case here, the
> combinations tend to explode on one.
well, while i dont think we need that much complexity, the most generic
case is a representation of the actual hardware (cache/bus) layout,
where separate hardware component types have different IDs.
e.g. a simple hiearchy would be:
____H1____
_H2_ _H2_
H3 H3 H3 H3
[P1] [P2] [P3] [P4]
Then all that has to happen is to build a 'path' of ids (e.g. "H3,H2,H3"
is a path), which is a vector of IDs, and an array of already measured
vectors. These IDs never get added so they just have to be unique per
type of component.
there are two different vectors possible: H3,H2,H3 and H3,H2,H1,H2,H3,
so two measurements are needed, between P1 and P2 and P1 and P3. (the
first natural occurence of each path)
this is tree walking and vector building/matching. There is no
restriction on the layout of the hierarchy, other than it has to be a
tree. (no circularity) It's easy to specify such a tree, and there are
no 'mixup' dangers.
> I'm thinking we get off easy, because:
>
> 1) Specific arch's can apply specific short cuts.
>
> My intuition was that any specific architecture, when it
> got down to specifics, could find enough ways to cheat
> so that it could get results quickly, that easily fit
> in a single 'distance' word, which results were 'close
> enough.'
yes - but the fundamental problem is already that we do have per-arch
shortcuts: the cache_hot value. If an arch wanted to set it up, it could
do it. But it's not easy to set it up and the value is not intuitive. So
the key is to make it convenient and fool-proof to set up the data -
otherwise it just wont be used, or will be used incorrectly.
but i'd too go for the simpler 'pseudo-distance' function, because it's
so much easier to iterate through it. But it's not intuitive. Maybe it
should be called 'connection ID': a unique ID for each uniqe type of
path between CPUs. An architecture can take shortcuts if it has a simple
layout (most have). I.e.:
sched_cpu_connection_type(int cpu1, int cpu2)
would return a unique type ID for different. Note that 'distance' (or
'memory access latency', or 'NUMA factor') as a unit is not sufficient,
as it does not take cache-size nor CPU speed into account, which does
play a role in the migration characteristics.
Ingo
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