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FLOPS: Forward Learning with OPtimal Sampling
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Proposes FLOPS, a plug-and-play Gaussian query allocator that adaptively distributes a fixed query budget across data points in a mini-batch for perturbation-based forward learning (LR and SPSA), with theoretical optimality results and experiments on ViT fine-tuning, black-box prompt tuning, and multimodal alignment.
Problem
Forward (perturbation-based) learning needs many queries per data point for low-variance gradient estimates, which hurts scalability; existing methods allocate queries equally across data points regardless of estimation difficulty.
Approach
Treat per-example query counts as a random allocation vector drawn from a Gaussian allocator whose parameters are optimized via the likelihood-ratio method to minimize summed gradient-estimate trace variance; reparameterize mean via loss (tanh-linear) and covariance via an RBF kernel on CLS-token cosine similarity, leaving only four allocator parameters.
Key results
- On ViT-base, OPS-LR reaches 65.8% on ImageNet versus 57.3% for equally allocated LR and 80.5% for BP.
- Black-box prompt tuning with OPS-LR reaches 65.1% on ImageNet versus 62.5% for LR and 57.9% for manual prompt.
- Multimodal alignment with OPS-LR reportedly reduces training time by almost half relative to BP, with allocator overhead negligible in wall time.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · ImageNet | accuracy (%): 57.3 -> 65.8 | Helped |
| Use optimal query allocation with SPSA instead of equal allocation with DeepZero | ViT-base · ImageNet | accuracy (%): 55.6 -> 60.8 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · Caltech101 | accuracy (%): 85.5 -> 88.9 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · Food101 | accuracy (%): 87.6 -> 88.4 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · Flowers102 | accuracy (%): 86.5 -> 91.6 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · CIFAR10 | accuracy (%): 87.7 -> 93.2 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · CIFAR100 | accuracy (%): 80.8 -> 88.7 | Helped |
| Use optimal query allocation with LR instead of equal allocation with LR | ViT-base · EuroSAT | accuracy (%): 82.9 -> 89.3 | Helped |
| Use optimal query allocation rather than equal allocation (ablation) | Forward learning; ViT-base on CIFAR100 · CIFAR100 | gradient cosine similarity | Helped |
| Use the Gaussian allocator instead of the Bernoulli allocator (ablation) | Forward learning; ViT-base on CIFAR100 · CIFAR100 | gradient cosine similarity | Helped |
| Use a correlated Gaussian allocation instead of a diagonal covariance allocation (ablation) | Gaussian allocator · CIFAR100 | gradient cosine similarity | Helped |
| Increase the query budget per data point (ablation) | Forward-learning gradient estimators · CIFAR100 | gradient cosine similarity | Helped |
Surprising findings
- OPS-LR on ViT-base CIFAR100 (88.7%) is nearly as accurate as BP but substantially more robust under natural and adversarial perturbations than BP in the robustness table.
- Mezo with only 2 queries per data point performs very poorly (e.g., 9.3% on ImageNet), indicating insufficient queries destabilize perturbation-based training.
Limitations
- Closing the accuracy gap with backpropagation remains open; OPS-LR still trails BP on clean ImageNet (65.8 vs 80.5).
- Memory cost of many queries is high for attention modules (O(n^2)); efficient attention integration left for future work.
- Gradient estimation quality for multi-head attention in deep layers is low, which may limit performance on large datasets.
Experimental setup
Datasets: ImageNet Caltech101 Food101 Flowers102 CIFAR10 CIFAR100 EuroSAT LCS-558K
Benchmarks: ImageNet Caltech101 Food101 Flowers102 CIFAR10/100 EuroSAT
Metrics: classification accuracy cosine similarity between estimated and true gradients wall-clock time
Baselines: BP MeZO DeepZero LR SPSA Manual Prompt
Builds on: SPSA MeZO DeepZero likelihood ratio gradient estimation Gaussian process / RBF kernel
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