fix: halve VAE decode tiles on OOM retry instead of jumping to 256px

[losewayy]

Summary

Fixes #2108.

prepare_vae_decode_retry_tiling clamps the first spatial-tiling retry to min(size/2, 256 / scale_factor), so the fallback jumps straight to 256px image tiles even when much larger tiles would fit. Removing the clamp makes every retry halve the tile size (1024→512→256…, 1536→768→384…), matching the behavior already used once tiling is enabled and what #2108 asks for.

Each failed attempt only costs a graph build plus a failed workspace allocation, so trying intermediate sizes is cheap compared to running many more small tiles.

Test evidence (RTX 5070 Ti Laptop 12 GB, CUDA, Qwen Image 2.1 Q6_K)

At 1536x1536, decode previously fell back to 121 tiles at 256px in 32.6 s. With progressive halving it lands on 9 tiles at 768px in 10.3 s. For a 1024px decode the first retry is now 512px; #2108's own measurements show 512px tiles decode ~2-4x faster than 256px when they fit.

Summary by CodeRabbit

• Bug Fixes
  • Adjusted how latent tile sizes are reduced when retrying VAE decoding, allowing retries to use the appropriate smaller tile size.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Repository UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: 018d6d5d-0a3c-4da2-9dcc-e2dbfc308b80

---

📥 Commits

Reviewing files that changed from the base of the PR and between f89d9b1 and f3d7f70.

---

📒 Files selected for processing (1)

• src/core/backend_fit.cpp

---

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review.

---

---

📝 Walkthrough

Walkthrough

The VAE decode retry now halves the current tile size, with a minimum of 4 and no larger than the current size. It no longer applies a 256-pixel limit when tiling was previously disabled.

Changes

VAE decode retry tiling

Layer / File(s)	Summary
Retry tile calculation src/core/backend_fit.cpp	The retry calculation uses half the current tile size, with a minimum of 4 and a cap at the current size. It removes the prior 256-pixel limit.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: leejet

---

Merge Risk: ⚪ Minimal · up to f3d7f

VAE decode retries now reduce tile sizes progressively, with existing retry behavior preserved. No actionable merge risk remains.

Architecture Summary

Architecture risk: 🔵 Low · up to f3d7f

The change affects 1 system.

Changed systems: src

Architecture concerns
No architecture-level concerns identified.

Review details

Systems and components

• observed — src (service) was modified; 1 changed file maps to changed impact.

Before / after behavior

• observed — Modified behavior in src/core/backend_fit.cpp: The smaller-tile calculation no longer limits a first spatial tile to 256 / scale_factor; it now uses half the current size, with a minimum of 4, capped at the current size.

---

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: VAE decode retries now halve tile sizes instead of jumping to 256px.
Description check		The description explains the change, references issue #2108, gives technical rationale, and includes concrete test evidence. It does not use the template headings for Related Issue / Discussion or Add…
Linked Issues check		Issue #2108 requires VAE decode auto-fit retries to use intermediate tile sizes instead of jumping to the 256-pixel minimum. The change removes the first-retry clamp in `prepare_vae_decode_retry_tilin…
Out of Scope Changes check		The whole pull-request diff contains one focused change in src/core/backend_fit.cpp. It only changes VAE decode retry tile-size selection. No unrelated source, API, configuration, or documentation c…

---

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

---

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

---

_{Comment @coderabbitai help to get the list of available commands.}