calc_monthly_egress_cost multiplied monthly_bytes (in bytes) by the raw cost_per_gb number without any unit conversion
This produces a result ~1,000,000,000x too large (e.g., $1.87 trillion instead of $233 for 1 MB/s at $0.09/GB)
Fix: use _ensure_unit to attach dollar/gigabyte to cost_per_gb, then convert to dollar/byte before multiplying
Root cause
# before -- wrong: monthly_bytes is in bytes, cost_per_gb is raw $/GB numbermonthly_bytes=b_s*(30*ureg.day)# e.g. 2,592,000,000,000 bytecost=monthly_bytes*cost_per_gb# byte * 0.09 (raw) -- no unit conversion!# after -- correct: cost_per_gb gets units, then converted to $/bytecost_rate=_ensure_unit(cost_per_gb,ureg.dollar/ureg.gigabyte,"cost_per_gb")cost=monthly_bytes*cost_rate.to(ureg.dollar/ureg.byte)
Tests added
TestMonthlyEgressCost -- 4 tests: known-answer (raw + Quantity), zero bandwidth, linear scaling
TestFleetTCO -- 3 tests: known-answer, zero quantity, linear scaling
TestMTBFNode -- 3 tests: single component, two GPUs halves MTBF, mixed components
All three functions previously had zero test coverage in test_formulas.py.
Test plan
pytest mlsysim/tests/test_formulas.py -v passes (54 tests, was 43)
🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.
## 📋 Pull Request Information
**Original PR:** https://github.com/harvard-edge/cs249r_book/pull/1597
**Author:** [@Shashank-Tripathi-07](https://github.com/Shashank-Tripathi-07)
**Created:** 4/28/2026
**Status:** ✅ Merged
**Merged:** 4/29/2026
**Merged by:** [@profvjreddi](https://github.com/profvjreddi)
**Base:** `dev` ← **Head:** `fix/egress-cost-unit-bug`
---
### 📝 Commits (1)
- [`4e1301f`](https://github.com/harvard-edge/cs249r_book/commit/4e1301f9d6634f5a5a73ff13e41a8efa09c0ef08) fix(mlsysim): correct unit conversion in calc_monthly_egress_cost
### 📊 Changes
**2 files changed** (+86 additions, -1 deletions)
<details>
<summary>View changed files</summary>
📝 `mlsysim/mlsysim/core/formulas.py` (+2 -1)
📝 `mlsysim/tests/test_formulas.py` (+84 -0)
</details>
### 📄 Description
## Summary
- `calc_monthly_egress_cost` multiplied `monthly_bytes` (in bytes) by the raw `cost_per_gb` number without any unit conversion
- This produces a result ~1,000,000,000x too large (e.g., \$1.87 **trillion** instead of \$233 for 1 MB/s at \$0.09/GB)
- Fix: use `_ensure_unit` to attach `dollar/gigabyte` to `cost_per_gb`, then convert to `dollar/byte` before multiplying
## Root cause
```python
# before -- wrong: monthly_bytes is in bytes, cost_per_gb is raw $/GB number
monthly_bytes = b_s * (30 * ureg.day) # e.g. 2,592,000,000,000 byte
cost = monthly_bytes * cost_per_gb # byte * 0.09 (raw) -- no unit conversion!
# after -- correct: cost_per_gb gets units, then converted to $/byte
cost_rate = _ensure_unit(cost_per_gb, ureg.dollar / ureg.gigabyte, "cost_per_gb")
cost = monthly_bytes * cost_rate.to(ureg.dollar / ureg.byte)
```
## Tests added
- `TestMonthlyEgressCost` -- 4 tests: known-answer (raw + Quantity), zero bandwidth, linear scaling
- `TestFleetTCO` -- 3 tests: known-answer, zero quantity, linear scaling
- `TestMTBFNode` -- 3 tests: single component, two GPUs halves MTBF, mixed components
All three functions previously had zero test coverage in `test_formulas.py`.
## Test plan
- [ ] `pytest mlsysim/tests/test_formulas.py -v` passes (54 tests, was 43)
---
<sub>🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.</sub>
Blocking a user prevents them from interacting with repositories, such as opening or commenting on pull requests or issues. Learn more about blocking a user.
📋 Pull Request Information
Original PR: https://github.com/harvard-edge/cs249r_book/pull/1597
Author: @Shashank-Tripathi-07
Created: 4/28/2026
Status: ✅ Merged
Merged: 4/29/2026
Merged by: @profvjreddi
Base:
dev← Head:fix/egress-cost-unit-bug📝 Commits (1)
4e1301ffix(mlsysim): correct unit conversion in calc_monthly_egress_cost📊 Changes
2 files changed (+86 additions, -1 deletions)
View changed files
📝
mlsysim/mlsysim/core/formulas.py(+2 -1)📝
mlsysim/tests/test_formulas.py(+84 -0)📄 Description
Summary
calc_monthly_egress_costmultipliedmonthly_bytes(in bytes) by the rawcost_per_gbnumber without any unit conversion_ensure_unitto attachdollar/gigabytetocost_per_gb, then convert todollar/bytebefore multiplyingRoot cause
Tests added
TestMonthlyEgressCost-- 4 tests: known-answer (raw + Quantity), zero bandwidth, linear scalingTestFleetTCO-- 3 tests: known-answer, zero quantity, linear scalingTestMTBFNode-- 3 tests: single component, two GPUs halves MTBF, mixed componentsAll three functions previously had zero test coverage in
test_formulas.py.Test plan
pytest mlsysim/tests/test_formulas.py -vpasses (54 tests, was 43)🔄 This issue represents a GitHub Pull Request. It cannot be merged through Gitea due to API limitations.