3. Currentcy Accounting

On the device side, various schemes may be appropriate for debiting tasks for access to devices. This may reflect actual energy costs or there may be rate structures designed to accomplish some energy objective. The strategies fall into the following categories:

Debiting. The straightforward policy is pay-as-you-go using the actual energy cost of the devices until currentcy is spent. In another scenario, prices levied against a task may dynamically vary to accomplish a subgoal (e.g., an extra ``tax'' to discourage use or a ``sale price'' to encourage use).

Bidding. The task may offer a price it is willing to support for access to an energy consuming resource. The bid does not necessarily imply that the task will be debited that amount for an activity.

Pricing. The price of a resource, which may be dynamically changing over time, is a way to encode thresholds in terms of currentcy and may interact with bids (e.g., in a negotiation protocol). Pricing may be decoupled from debiting to enforce threshold levels without skewing accurate accounting for the resource. Pricing may also encode the power state of a device (e.g., the price of a disk access is discounted when the disk is already spinning and no spinup is required).

Examples of creative combinations of debiting, pricing, and bidding policies arise with the disk management policies in Section 8. We believe that expressing policies in terms of allocation and accounting operations on currentcy is a powerful way to unify resource management.