Computation Offload Break-Even Calculation Audit

Computation Offload Break-Even Calculation Audit

Ada keeps bytes, radio charge, setup, and tail in one ledger before deciding where the workload belongs

ADA · CALCULATION AUDIT

Computation Offload Break-Even Calculation Audit

The chapter gives a node a 6 mA radio on a 50 kbps link and a local inference that costs 30 mA-s, then asks when it is cheaper to send data than to compute it. With no setup overhead the crossover lands near 31,250 bytes, about 30 KB; add an 8 mA-s connection setup and it drops to about 22 KB. This audit works the offload break-even to the byte, so a placement decision survives a battery review.

Companion to the chapter Compute Offloading and Placement — every number here comes from that chapter.

Offloading is not a cloud magic trick; it is an energy equation. Keep local compute, payload bytes, setup charge, and radio tail in the same ledger before deciding where the workload belongs.

See the relationship before changing it

The figure reads from left to right. The blue card is payload size. The middle card applies this page's rule. The green card is payload radio charge. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only payload size, so the numeric fixture does not switch without explanation.

Walk the arrows. Larger payloads spend more radio charge until local computation becomes the cheaper placement.

Derive the baseline in four named moves

1. 1

   Name the input. The chapter baseline is 22917 bytes.

2. 2

   Name the relationship. radio charge = bytes x 0.00096 mA-s per byte

3. 3

   Substitute with units. 22,917 bytes x 0.00096 = 22.00 mA-s

4. 4

   Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change payload size

Try Predict the direction of radio charge = bytes x 0.00096 mA-s per byte. Test another payload size, then compare payload radio charge.

Payload size

22917 bytes

Chapter baseline

Payload radio charge

Observe Larger payloads spend more radio charge until local computation becomes the cheaper placement. Reset payload size to 22917 and compare payload radio charge.

Explain Larger payloads spend more radio charge until local computation becomes the cheaper placement.

Check yourself

What should you do before trusting a moved-control result?

Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.

What does this small model leave out?

Answer: Only payload size moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only payload size. It is not a field prediction.

Try

The chapter gives a node a 6 mA radio on a 50 kbps link and a local inference that costs 30 mA-s , then asks when it is cheaper to send data than to compute it. Calculate this case.

Observe

This audit works the offload break-even to the byte, so a placement decision survives a battery review. Check shows this.

Explain

Radio charge grows with payload bytes divided by the 50 kbps throughput, while local inference remains a fixed 30 mA-s; their equality creates the payload break-even, and connection setup shifts that crossing downward. Check confirms it.

Technical boundaries

The “Computation Offload Break-Even Calculation Audit” compares only the displayed local and radio workloads. Wireless retries, headers, ramp and tail time, memory traffic, queueing, thermal throttling, and result-download cost are excluded.

1. The BLE example derives its per-byte radio charge from current and throughput

The chapter gives a 6 mA active radio and an effective 50 kbps link, so one byte costs:

e_byte = 6 mA x (8 bit / 50000 bit/s) = 0.00096 mA-s per byte

2. The no-setup break-even is about 31,250 bytes

The local inference costs 15 mA for 2 seconds, or 30 mA-s. Dividing by the per-byte radio charge gives:

D* = 30 mA-s / 0.00096 mA-s per byte = 31250 bytes, about 30 KB

3. Setup overhead moves the decision boundary downward

If the link spends 8 mA-s before payload transfer, only 22 mA-s remains for bytes at the break-even point:

D* = (30 - 8) / 0.00096 = 22916.7 bytes, about 22 KB

Review item	Arithmetic shown	Decision signal
2 KB feature vector	2048 x 0.00096 = 1.97 mA-s	Offload can beat the 30 mA-s local run before fixed overheads dominate.
80 KB raw input	81920 x 0.00096 = 78.64 mA-s	Compute locally because radio charge alone is already more than 2.6x the local compute charge.
Cellular tail example	25 + (120 x 0.4) + (40 x 8) = 393 mA-s	Setup and tail flip the payload-only estimate against offloading when local compute is 90 mA-s.

What the audit buys you: a placement decision that survives a battery review. Payload size, throughput, and radio state timing are separate measured terms; round only after the comparison is complete.

Every number above is taken from this chapter's own worked example and re-derived step by step.