Reviewer leaks ~70 MB/min of graphics memory while an animated GIF is on screen (macOS) — repaint-rate threshold identified

abdo · August 9, 2026, 4:38am

Memory leaks are unfortunately a common report on macOS:

github.com/ankitects/anki

Fix MacOS memory leak (#4569)

main ← tequa:davidmoran/fix-metal-macos-memory-leak

opened 12:06AM - 22 Feb 26 UTC

tequa

+12 -0

## Fix macOS memory growth caused by unbounded Metal shader cache accumulation i…n QtWebEngine compositor ### Problem <img width="709" height="109" alt="image" src="https://github.com/user-attachments/assets/09c8ad96-767a-4766-b7a1-34643eacbeba" /> Forum discussion from over a year ago: https://forums.ankiweb.net/t/memory-leak-over-time/50931/32 On macOS, Anki's memory footprint grows continuously while the application is idle — typically several MB per minute — and never recovers - memory could reach 20 to 30 gigs after a week or so. This behavior was observed via macOS Activity Monitor and confirmed with Instruments. Initial investigation ruled out the obvious candidates: - Python `tracemalloc` found no growing Python allocations - `leaks` found no unreachable objects — the memory is reachable, not leaked in the traditional sense - Reducing timer frequency had no effect - The `QtWebEngineProcess` helper process was stable; growth was entirely in the main Python process ### Root Cause (confirmed via Instruments → Allocations + Call Tree) <img width="755" height="113" alt="image" src="https://github.com/user-attachments/assets/e8760043-66a3-4409-a748-2e2aa3843fe1" /> <img width="581" height="212" alt="image" src="https://github.com/user-attachments/assets/4901e6b0-af3a-405b-9158-2d09374fc35d" /> Every second, Qt's scene graph fires a render-sync timer, which calls into `RenderWidgetHostViewQtDelegateItem` — QtWebEngine's internal QML item that composites web content using OpenGL. On each sync it allocates a new `QOpenGLFramebufferObject` rather than reusing the existing one. Each new FBO has a different geometry identity, so Apple's OpenGL→Metal translation layer (`GLDContextRec::buildPipelineState`) compiles a fresh Metal shader pipeline state for it via `AGXG15XFamilyDevice::newRenderPipelineStateWithDescriptor`. The compiled pipeline state is inserted into `MTLCompilerFSCache::getElement` — a cache that is reachable and therefore not flagged as a leak, but that grows without bound for the lifetime of the process. **Full call chain:** ``` QTimerInfoList::activateTimers → QQuickWidget::event → QQuickRenderControl::sync → RenderWidgetHostViewQtDelegateItem::updatePaintNode → QOpenGLFramebufferObject::QOpenGLFramebufferObject ← new FBO every 500ms → glDrawArrays → GLDContextRec::buildPipelineState → AGXG15XFamilyDevice::newRenderPipelineStateWithDescriptor → MTLCompilerFSCache::getElement ← accumulates forever ``` This is a Qt/Chromium interaction bug specific to the macOS OpenGL→Metal compatibility layer; it does not affect Linux or Windows. ### Fix Set `--disable-gpu-compositing` in `QTWEBENGINE_CHROMIUM_FLAGS` on macOS (in `setupGL()`, `qt/aqt/__init__.py`). This switches Chromium's page compositor from GPU mode (which creates FBOs on every render sync) to CPU/software mode (a plain bitmap uploaded as a texture). No `QOpenGLFramebufferObject` is created, no Metal pipeline states are compiled, and `MTLCompilerFSCache` never grows. The flag is appended to any existing value of `QTWEBENGINE_CHROMIUM_FLAGS` rather than overwriting it, so user-set or developer flags (e.g. `--remote-allow-origins`) are preserved. ### Confirmation of fix Anki running for 8 hours: <img width="663" height="42" alt="image" src="https://github.com/user-attachments/assets/6b14cab5-ccfe-4cd2-9db8-472b486cff08" /> ### Trade-off CPU compositing means Chromium composites page layers on the CPU (Skia) rather than the GPU. Web content renders identically; CSS animations and video still work. For Anki's workload — card text, LaTeX images, simple SVG — this is indistinguishable in practice. If visible degradation is ever reported, the fallback `--use-angle=swiftshader` provides a software GPU path instead.

I’m not sure if there’s anything Anki can do if it’s a Qt issue.

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Reviewer leaks ~70 MB/min of graphics memory while an animated GIF is on screen (macOS) — repaint-rate threshold identified

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