100% CPU usage - V8 StringTable performance collapse

process_info.txt
sample1.txt
sample2.txt
sample3.txt
sample4.txt

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Claude Code 100% CPU Bug

Summary

Claude Code node process (PID 29085) consuming 98-100% CPU indefinitely due to V8 StringTable performance issue.

Environment

• OS: macOS 26.0.1 (25A362)
• Node: /opt/homebrew/Cellar/node/24.10.0/bin/node
• Claude Code: 2.0.27
• Parent Process: npm exec @anthropic-ai/claude-code@2.0.27
• Working Directory: /Users/max/hobby/mac/bgc
• Memory: 3.2GB → 3.5GB physical footprint (growing)
• Uptime: 17+ minutes at 100% CPU

Root Cause Analysis

Sample 1 (Initial)

• Heavy ICU (International Components for Unicode) garbage collection
• icu_77::RuleBasedBreakIterator destructors
• Memory deallocation in ICU library

Sample 2 (Follow-up) - Primary Issue

V8 String Table performance collapse:

Array.map →
  SetKeyedProperty →
    StringTable::LookupString →
      String::SlowEquals (996+ samples)
      CompareCharsEqual (328+ samples per call)

KeyedStoreIC_Megamorphic →
  StringTable::TryStringToIndexOrLookupExisting

ObjectPrototypeHasOwnProperty →
  StringTable lookups

JSON.stringify →
  JsonStringifier::SerializeString

Sample 3 (10-second, Confirmation)

Memory growth confirmed: 3.4GB → 3.5GB peak

V8 heap expansion detected:

• PagedSpaceBase::TryExpand
• MemoryAllocator::AllocatePage
• mmap system calls for new pages

Same StringTable bottleneck (worse):

• StringTable::LookupString - 2541 samples
• String::SlowEquals - 1000+ samples
• Hash table rehashing under load

Issue is stable and worsening over time.

Performance Anti-Pattern Detected

1. Megamorphic property access - V8 IC (Inline Cache) deoptimization
2. String table overload - thousands of unique string keys
3. Linear string comparison - String::SlowEquals with byte-by-byte comparison
4. Hash table rehashing - NameDictionary::Rehash during object property additions

Call Stack Pattern

RunMicrotasks →
  Promise chains →
    Array.map (on large dataset) →
      Object property assignments with dynamic keys →
        V8 string internalization →
          Linear search in string table →
            Byte-by-byte string comparison

Likely Trigger

Code is processing large arrays with:

• Dynamic object key creation
• Heavy use of hasOwnProperty
• JSON stringification
• ICU text segmentation (word/sentence breaking)

Impact

• 100% CPU usage on single core
• Process unresponsive
• Memory gradually increasing to 3.2GB
• System thermal throttling

Reproduction

Unknown - occurred during normal Claude Code operation in /Users/max/hobby/mac/bgc directory.

Files Attached

• sample1.txt - Initial 5s sample showing ICU GC (1.1MB)
• sample2.txt - Follow-up 5s sample showing V8 StringTable issue (138KB)
• sample3.txt - Confirmation 10s sample showing memory growth (295KB)
• process_info.txt - Process details
• Screenshot showing Activity Monitor CPU usage (pending)