Finding and Fixing Ghostty's Largest Memory Leak

4 months ago

Ghostty was consuming absurd amounts of memory, with reports of up to 37 GB after 10 days of uptime.
The memory leak was present since Ghostty 1.0 but became noticeable with the rise of CLI applications like Claude Code.
Ghostty uses a PageList data structure for terminal memory management, with standard and non-standard pages allocated via mmap.
A scrollback pruning optimization reused the oldest page as the newest without properly resizing non-standard pages, leading to memory leaks.
The fix involves never reusing non-standard pages during scrollback pruning, ensuring proper memory deallocation.
Virtual memory tags on macOS were added to help identify and debug memory allocations related to the PageList.
Ghostty employs various leak detection methods, including debug builds, unit tests, CI runs with valgrind, and macOS Instruments.
The community's detailed diagnostics and reproduction efforts were crucial in identifying and fixing the leak.

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