Bug #21848
open
GC compaction freezes during code reload with 100% CPU in rb_vm_ci_free -> st_general_delete
Description
Ruby 4.0.1 freezes with 100% CPU usage when garbage collection runs during Rails code reloading (hot reload in development). The process becomes unresponsive and requires kill -9 to terminate.
Environment:
- Ruby: 4.0.1 (Homebrew, ARM64)
- Platform: macOS 15.6.1 (Darwin 24.6.0, Apple Silicon)
- Rails: 8.x with Puma 7.2.0
- Mode: Development with code reloading enabled
Steps to reproduce:
- Run a Rails application with Puma in development mode (bundle exec rails server -e development -b 0.0.0.0)
- Make a several code change to trigger hot reload, reload the site in the browser in between
- Process freezes with ~100% CPU
Expected behavior:
Code reloads successfully, GC completes normally.
Actual behavior:
Process freezes indefinitely at 100% CPU. Stack trace from sample shows GC stuck in hash table lookup while freeing VM call info:
gc_sweep (in libruby.4.0.dylib)
gc_compact_plane
gc_sweep_page
gc_sweep_plane
rb_gc_obj_free_vm_weak_references
rb_vm_ci_free
st_general_delete
find_table_bin_ind
vm_ci_hash_cmp <-- stuck here, spinning
Full relevant stack (Puma worker thread):
Thread: puma srv tp 005
rb_ary_each -> rb_yield -> invoke_block_from_c_bh -> rb_vm_exec
-> int_upto -> rb_yield (nested loops)
-> vm_search_method_slowpath0 -> rb_vm_search_method_slowpath
-> newobj_of -> newobj_cache_miss -> gc_continue
-> gc_sweep -> gc_compact_plane -> gc_sweep_page
-> gc_sweep_plane -> rb_gc_obj_free_vm_weak_references
-> rb_vm_ci_free -> st_general_delete
-> find_table_bin_ind -> vm_ci_hash_cmp
Workaround:
Disabling auto-compaction prevents the freeze:
GC.auto_compact = false
Analysis:
The freeze occurs during GC sweep/compaction when freeing cached VM call info entries (rb_vm_ci_free). This is triggered by code reloading which invalidates method caches. The GC appears to get stuck in an infinite loop or
extremely slow operation in find_table_bin_ind while looking up entries in the call info hash table.
Files
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