I changed nothing and my LLM server got 27% more expensive
I ran the same cost check four times in a row against a server I didn't touch. Same model, same machine, same prompts, same settings. This is what came back, in dollars per million output tokens: Run $/M output tokens 95% CI Change vs previous run 1 $8.77 $8.19 to $9.35 2 $8.86 $7.80 to $9.93 +1.0% 3 $11.26 $10.31 to $12.21 +27.1% 4 $10.02 $7.41 to $12.62 −11.0% Run 3 says the server got 27% more…
The same server experienced a 27% increase in cost, followed by an 11% decrease, despite no changes being made. This discrepancy occurred because timing on a real machine can be influenced by various factors, such as other processes, background indexing, thermal state, memory pressure, and OS scheduling. On a shared cloud node, there may also be noisy neighbors and other tenants on the same host or network.
The author found that measuring run-to-run noise directly helps determine the confidence interval and identify if a change is due to measurement noise or a true configuration change. The recommended approach is to run the unchanged configuration multiple times and analyze the standard deviation to estimate noise. With enough repeats and a quieter environment, one can better understand if a change is significant or just within the noise floor.
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