Brief Review — UL2R, U-PaLM: Transcending Scaling Laws with 0.1% Extra Compute

UL2R, Fine-Tune PaLM Using UL2 Objective, Formed U-PaLM

Sik-Ho Tsang
3 min readAug 31, 2023
U-PaLM, Fine-Tune PaLM Using UL2 Objective, More Efficient Than PaLM: Compute (training flops) versus Quality (average of 20+ NLP zero and few-shot tasks)

Transcending Scaling Laws with 0.1% Extra Compute
UL2R, U-PaLM, by Google
2022 arXiv v2 (

@ Medium)

LM Tuning / Prompting
2020 [Human Feedback Model] 2021 [T5+LM, Prompt Tuning] 2022 [GPT-3.5, InstructGPT] [LoRA] [Chain-of-Thought Prompting] [T0] [FLAN] [Flan-PaLM] 2023 [LIMA]
==== My Other Paper Readings Are Also Over Here ====

  • UL2Restore (UL2R) is proposed, which is a method to continue training a SOTA LLM (e.g., PaLM) on a few more steps with UL2’s mixture-of-denoiser objective. UL2R substantially improves existing language models and their scaling curves with almost negligible extra computational costs and no new sources of data.
  • U-PaLM model family of 8B, 62B, and 540B scales, is established by training PaLM with UL2R. An approximately 2× computational savings rate is achieved.
  • Later, U-PaLM is further instruction-finetuned as Flan-U-PaLM.


  1. UL2Restore (UL2R)
  2. U-PaLM
  3. Results

1. UL2Restore (UL2R)

The key idea is UL2R or UL2Restore to continue training an existing causal language model with a mixture of new objectives — specifically, the UL2 training objective mixture.

  • This restoration is expected to only cost roughly 0.1% to 1% of the original training FLOPs and requires no new data sources, making it highly efficient and convenient.

The UL2 objective combines prefix language modeling and long-short span corruption (e.g., infilling) tasks, that can teach LLM to leverage bidirectional attention (i.e., PrefixLM) or leverage infilling-style pretraining.

2. U-PaLM

2.1. Improved Scaling Properties on Few-shot Learning

Computation cost (training flops) versus Quality (average of 20+ NLP zero and few-shot tasks).

U-PaLM substantially outperforms the original PaLM models both at 8B scale and 540B scale. Note that the dotted lines represent a pathway before and after UL2R training.

  • UL2R training improves the scaling curve of PaLM substantially, i.e., UL2R provides a more compute-efficient performance improvement compared to training the original PaLM models for longer with the standard causal language modeling objective.

2.2. BigBench


U-PaLM outperforms PaLM on 19 out of the 21 tasks at 540B scale.

2.3. Chain-of-Thought (CoT) Reasoning

CoT Reasoning

U-PaLM 540B outperforms both PaLM 540B and Minverva 540B.

2.4. Infilling Ability

Infilling Ability

With UL2R training which uses UL2 objective, which has the infilling objective, the second and third examples demonstrate U-PaLM’s ability to infill multiple slots.



Sik-Ho Tsang

PhD, Researcher. I share what I learn. :) Linktree: for Twitter, LinkedIn, etc.