A deep dive into QML memory management internals
summary
QML objects are allocated on the process heap and deleted deterministically via the QObject parent tree, while JavaScript values live on a separate V4 32 buckets heap (sizes 16–512B; >512B go to malloc). The V4 heap grows by doubling chunk sizes (now capped at 2MB since Qt 5.3) which used to produce 32–64MB jumps; values shrink from heavy QVariants to 8‑byte QV4 values. Garbage collection runs on the main thread as a stop-the-world mark-and-sweep, can free empty chunks back to the OS since 5.5, and is observable via qv4_mm_stats, Qt Creator profiler, and Valgrind/massif.
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chapters (27)
- 0:00Intro
- 3:03Scope
- 4:37Memory management basics
- 5:55Heaps
- 8:05QML and JavaScript
- 8:52QML object types
- 10:16Creating QML objects
- 12:03QML defined properties
- 14:50QML garbage collectors
- 16:44QML objects
- 17:58JavaScript
- 18:50JavaScript type
- 23:54QML growth strategy
- 25:49Garbage collector
- 27:17Mock phase
- 28:34Sweep phase
- 30:24Garbage collector objectives
- 32:11Garbage collector after unloading
- 33:55Wrapup
- 36:05Questions
- 36:37Tools
- 37:36QML stats
- 39:41JavaScript timeline taps
- 40:48More to this
- 42:22Massive
- 44:47Wrap up
- 46:13Conclusion
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