Same Task, More Tokens: the Impact of Input Length on the Reasoning Performance of Large Language Models
Summary
LLMs advertise ever-larger context windows, but it is unclear whether a model that solves a task on a short prompt can still solve it when the same task sits inside a longer one. Earlier long-context benchmarks changed the task and the length together, so the two effects could not be separated. Levy, Jacoby and Goldberg isolate length with FLenQA, a True/False reasoning dataset. Each item needs two key facts to be combined. The authors keep those facts fixed and surround them with irrelevant padding of different sizes, sources and positions. On average across the tested models, accuracy falls from 0.92 to 0.68 at only about 3000 tokens, far below the models' maximum context sizes. The decline appears in every padding variant, though its strength varies. Chain-of-Thought prompting raises accuracy overall but generally does not close the length-related gap. Next-word prediction quality also fails to predict long-input reasoning. The work argues that a long-context model must be evaluated across all the lengths it claims to support.
Key points
- FLenQA has three tasks, each with 100 base instances and balanced True/False labels:
- Monotone Relations (MonoRel), a new task;
- People In Rooms (PIR), a new task inspired by bAbI;
- a simplified Ruletaker.
- Key sentences were expanded into coherent paragraphs with GPT-4 and then checked by hand. Person names come from the Faker library. Code to regenerate the data from scratch is released to limit contamination.
- The evaluated models are GPT3.5 (gpt-3.5-turbo-1106), GPT4 (gpt-4-1106-preview), Gemini Pro, Mistral Medium and Mixtral 8x7B.
- Chain-of-Thought gives a roughly constant gain at every length. GPT4 is the only model whose advantage from CoT widens as inputs grow.
- Several failure modes grow with input length:
- a bias toward answering "False";
- responses that give no answer at all;
- stating the answer before the reasoning steps (odds-ratio 3.643 for incorrect answers);
- weaker coverage of the key facts in CoT output (odds-ratio 3.138).
Cited by
- HippoRAG: Neurobiologically Inspired Long-Term Memory for Large Language Models
- RULER: What's the Real Context Size of Your Long-Context Language Models?
- Long-context LLMs Struggle with Long In-context Learning
Cite
@inproceedings{levy2024same,
title = {{Same Task, More Tokens: the Impact of Input Length on the Reasoning Performance of Large Language Models}},
author = {Levy, Mosh and Jacoby, Alon and Goldberg, Yoav},
booktitle = {Proceedings of the 62nd Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)},
year = {2024},
doi = {10.18653/v1/2024.acl-long.818},
url = {https://doi.org/10.18653/v1/2024.acl-long.818}
}References
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