Compare feasible schedules with all declared movement costs
Include deadlines, migration, storage, factor uncertainty, and demand changes in conditional shifting claims.
On this page
The question for today
What you’ll understand
- Compute a boundary-consistent shifted-workload account.
- Check completion deadlines including serial movement and waiting.
- Distinguish modeled savings from realized effects and rebound scenarios.
A scheduler can move flexible work toward a lower predicted emissions factor, but moving the work can change more than the factor. It may require data transfer, additional storage time, a slower machine, a retry, or waiting that violates the result’s deadline. A defensible comparison keeps those changes inside an explicit scenario and distinguishes predicted accounting from verified consequences. Use flexible capacity without pretending that every workload is movable or that a lower regional scalar guarantees a net environmental benefit.
Constrain the schedule before optimizing it
Radovanović and colleagues describe carbon-aware capacity shaping for temporally flexible work using demand and carbon forecasts. The key engineering idea is a constrained schedule: work must still complete within its allowed horizon and capacity. Interactive requests with immediate response requirements have different flexibility from a batch analysis due tomorrow. State release times, dependencies, duration estimates, and deadline slack before deciding that a future interval is actually available.
Spatial movement adds data placement, allowed regions, network capacity, and destination availability. Temporal movement can accumulate a queue that later competes with other delayed work. Forecasts of both electricity factors and execution time can be wrong. The original workload-shifting limitations research examines why flexibility assumptions matter; its results do not establish a universal achievable reduction. Evaluate fallback schedules and deadline risk rather than optimizing a forecasted scalar while ignoring feasibility.
Keep movement energy inside the comparison
Compute a shifted account from disjoint execution, transfer, and storage energy components, each with the factor matching its location and interval. A slower destination can use more execution energy even if its factor is lower. A preexisting copy may avoid some transfer, whereas repeated checkpoints or retention can add costs. The boundary must say whether each energy term is IT-only or facility-inclusive; do not apply a second facility multiplier to an already inclusive component.
Separate an average-factor inventory scenario from a marginal-impact scenario. Forecast average factors can rank a declared accounting objective, while a consequential claim needs an appropriate response model and horizon. Compare predicted and realized inputs afterward using the same workload and factor conventions. If movement costs or uncertainty reverse the sign, report that possibility. A numerical optimization can be internally correct while its omitted components make the headline claim unsupported.
Distinguish efficiency from total demand
A scheduling improvement for fixed work does not determine the effect of changing demand. Lower cost or improved capacity can allow more jobs to run. For example, multiplying per-job energy by a larger job count can increase the total even when the per-job figure falls. This is a conditional rebound scenario, not a claim that every efficiency improvement inevitably increases consumption. Measure actual workload volume and report absolute totals alongside normalized ratios.
Keep a reversible scheduling policy and an auditable record of why each shift was chosen: forecast version, candidate region or interval, work estimate, transfer assumptions, and deadline margin. Monitor completed work, failures, and additional storage rather than evaluating only jobs that successfully shifted. A policy that drops difficult work can look efficient by selection. The decision should remain inspectable when an updated factor, duration estimate, or demand forecast changes the preferred schedule.
Work it through
A lower execution factor does not erase transfer and storage
A fictional immediate job uses 2 kWh at 600 g CO2e/kWh. A shifted alternative uses 2.2 kWh for execution at 200 g/kWh, 0.3 kWh for transfer at 400 g/kWh, and 0.1 kWh for storage at 300 g/kWh. These are disjoint facility-inclusive accounts. Waiting takes 2 hours, serial transfer 0.25 hour, and execution 1 hour; deadline is 4 hours after release.
Immediate accounting is 2×600=1200 g CO2e. Shifted execution alone accounts for 2.2×200=440 g, but that is not the complete shifted boundary.
Transfer adds 0.3×400=120 g and storage adds 0.1×300=30 g. Shifted total is 440+120+30=590 g, a modeled accounting reduction of 610 g, or approximately 50.83% of 1200 g.
Completion occurs after 2+0.25+1=3.25 hours, within the 4-hour deadline. A 3-hour deadline would reject this serial schedule regardless of its lower accounting total. The energy accounts already include their stipulated facility allocation.
The shifted schedule is feasible under the fictional 4-hour deadline and lowers the included average-factor account from 1200 to 590 g. Omitted lifecycle effects and marginal grid response remain outside this claim.
The living notebook
Explore the model
Interactive lab · queue
Deferred work still needs enough capacity
Reset, set Arrival rate λ to 2 and Service rate μ to 3; predict the initial fluid backlog of 4 reaches zero at t=4 s before Play. Reset, set λ to 4 and μ to 3; predict backlog grows at 1 unit/s and equals 4.5 after one Step. This tiny constant-rate queue has no deadline scheduler, carbon forecast, transfer energy, or regional optimization.
- Fluid backlog
- 4
- Net rate · λ − μ
- −1 /s
- M/M/1 mean residence
- 1 s
Fluid backlog starts at 4 and follows max(0, 4 + (λ − μ)t). The separate M/M/1 residence-time formula assumes Poisson arrivals, exponential service and λ < μ. Parameter changes pause playback and recompute the model at the current elapsed time. Reset restores both parameters and returns time to zero. Playback starts paused. Adjust either parameter, then choose Play or Step.
Look forA favorable future factor is useful only if the available capacity can finish the work.
Pause & reason
Check your understanding
Your turn
Try it yourself
In a fictional demand scenario, a change reduces energy per identical job to 0.8 times its old value, while the number of jobs becomes 1.5 times the old count. With all other boundaries unchanged, what happens to total energy? Does this prove rebound in every deployment?
Take a moment to make a prediction or work it out on paper. Open a hint when you need one.
Give me a hint
Multiply the per-job ratio by the workload-volume ratio.
Compare with the solution
Total energy becomes 0.8×1.5=1.2 times the baseline, a 20% increase despite a 20% per-job reduction. This is a stipulated demand scenario, not a universal forecast. If job volume remained fixed, the same per-job change would instead reduce the total by 20%. Demand measurement and scenario assumptions determine the aggregate interpretation.
Keep the idea
Take it with you
- Deadlines, dependency paths, and capacity constrain workload flexibility.
- Include disjoint movement and storage costs under compatible factor boundaries.
- A fixed-work improvement does not determine a changing-demand total.
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