  Prompt tuning (PT), a parameter-efficient technique that only tunes the
additional prompt embeddings while keeping the backbone pre-trained language
model (PLM) frozen, has shown promising results in language understanding
tasks, especially in low-resource scenarios. However, effective prompt design
methods suitable for generation tasks such as summarization are still lacking.
At the same time, summarization guided through instructions (discrete prompts)
can achieve a desirable double objective of high quality and controllability in
summary generation. Towards a goal of strong summarization performance under
the triple conditions of parameter-efficiency, data-efficiency, and
controllability, we introduce PromptSum, a method combining PT with a
multi-task objective and discrete entity prompts for abstractive summarization.
Our model achieves competitive ROUGE results on popular abstractive
summarization benchmarks coupled with a strong level of controllability through
entities, all while only tuning several orders of magnitude less parameters.
