The rapid development of large language models (LLMs) has led to significant advancements in code completion tasks. While larger models have higher accuracy, they also cost much more to run. Meanwhile, model cascading has been proven effective to conserve computational resources while enhancing accuracy in LLMs on natural language generation tasks. It generates output with the smallest model in a set, and only queries the larger models when it fails to meet predefined quality criteria. However, this strategy has not been used in code completion tasks, primarily because assessing the quality of code completions differs substantially from assessing natural language, where the former relies heavily on the functional correctness. To address this, we propose letting each model generate and execute a set of test cases for their solutions, and use the test results as the cascading threshold. We show that our model cascading strategy reduces computational costs while increases accuracy compared to generating the output with a single model. We also introduce a heuristics to determine the optimal combination of the number of solutions, test cases, and test lines each model should generate, based on the budget. Compared to speculative decoding, our method works on black-box models, having the same level of cost-accuracy trade-off, yet providing much more choices based on the server's budget. Ours is the first work to optimize cost-accuracy trade-off for LLM code generation with model cascading.

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