A pretty reasonable cost function could be
0.7 * time + 0.2 * memory + 0.1 * disk
that reflects the real-life costs of the hardware used to operate the machine. In fact, the "cost function" is better the more it mimics real life economical costs.
Why? well, the above states that 70% of the "cost of update" is in the CPU computational capabilities + RAM access time + disk access time, because it's normally harder to change those values. 20% is the cost of RAM (because it's more expensive) and 10% is the cost of disk memory (in case of huge drives, this cost can well be zero)
Taking the adaptive cache to new levels, we can also explore adaptive cost functions--nothing will have to change from the overall architecture for us to do that.
For example, if we have a requirement from our hosting service that we can only use a certain amount of memory, as we approach that maximum the efficiency of that parameter becomes much more important.
So the weighting constants would linearly adapt to the importance of the requirement.
I.e. assuming we have the basic cost function:
0.7 * time + 0.2 * memory + 0.1 * disk
The closer we get to administrator mandated maximum the more important the cache efficiency for that resource is. If the maximum is 65MB (theorhetically speaking) and the rest-state of Cocoon for our application is 30MB, then the weighting is adjusted proportionately from 0.2 to 1.0 for that particular resource and the other weightings are also proportionately adjusted.
When our application is running at 51MB we are 60% closer to our maximum. The new weightings should be:
0.28 * time + 0.68 * memory + 0.04 * disk
As you can see, an adaptive cost function working in conjunction with an adaptive cache can add a new level of adaptability to hard requirements without having to resort to too many explicit rules.
Again when we reach our maximum of 65MB we are 100% closer to our maximum. The new weightings would be:
0.0 * time + 1.0 * memory + 0.0 * disk
We can never truly guarantee that we will never cross the 65MB threshold, but we will have everything working to minimize the amount over we can go.
--
"They that give up essential liberty to obtain a little temporary safety
deserve neither liberty nor safety."
- Benjamin Franklin