RSS Ratio Detector - antimetal/system-agent GitHub Wiki

RSS Component Ratio Detector

⚠️ DRAFT/WIP: Documentation for in-development feature on mem_monitor branch

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Overview

The RSS Component Ratio Detector (ebpf/src/memgrowth_rss_ratio.bpf.c) analyzes the composition of process memory to identify heap-based memory leaks. By tracking the ratio of anonymous memory to file-backed memory, it distinguishes genuine leaks from normal cache growth.

Key Insight

Memory leaks manifest differently than cache growth:

Memory Type Normal App Leaking App Cache-Heavy App
Anonymous (heap/stack) 40-75% 85-95% 20-40%
File-backed (code/libs) 25-60% 5-15% 60-80%

This detector leverages these patterns to identify leaks with high accuracy.

RSS Component Tracking

The detector monitors four RSS components from the kmem:rss_stat tracepoint:

struct process_memory_state {
    // RSS components
    __u64 rss_anon;      // Anonymous pages (heap, stack)
    __u64 rss_file;      // File-backed pages (code, libs, mmap)
    __u64 rss_swap;      // Swap entries
    __u64 rss_shmem;     // Shared memory pages
    
    // Growth rates by type
    __s64 anon_growth_rate;  // bytes/sec (can be negative)
    __s64 file_growth_rate;  // bytes/sec (can be negative)
    
    // Memory ratios (% * 10 for precision)
    __u16 anon_ratio;    // Anonymous % * 10 (0-1000)
    __u16 swap_ratio;    // Swap % * 10 (0-1000)
};

Detection Algorithm

Primary Detection Criteria

#define ANON_RATIO_THRESHOLD 800  // 80% anonymous memory

// Criterion 1: High anonymous memory ratio
if (state->anon_ratio > ANON_RATIO_THRESHOLD) {
    confidence += 40;
}

// Criterion 2: Anonymous growth exceeds file growth
if (state->anon_growth_rate > state->file_growth_rate * 2) {
    confidence += 30;
}

// Criterion 3: Swap pressure increasing
if (state->swap_ratio > 100 && growing) {
    confidence += 20;
}

// Criterion 4: File memory stable while anon grows
if (state->file_growth_rate <= 0 && 
    state->anon_growth_rate > 100KB/s) {
    confidence += 10;
}

Ratio Calculation

// Calculate with precision (% * 10)
__u64 total = rss_anon + rss_file + rss_shmem;
anon_ratio = (rss_anon * 1000) / total;

// Example: 850 = 85.0% anonymous memory

Real-World Detection Patterns

Pattern 1: Classic Heap Leak

Time:     T0      T1      T2      T3      T4
Anon:     100MB   150MB   200MB   250MB   300MB  ↗️
File:     50MB    52MB    51MB    53MB    52MB   →
Ratio:    66%     74%     79%     82%     85%    
Action:   Monitor Monitor Warn    Alert   Critical

Pattern 2: Cache Growth (Not a Leak)

Time:     T0      T1      T2      T3      T4
Anon:     100MB   102MB   105MB   103MB   104MB  →
File:     50MB    100MB   150MB   200MB   250MB  ↗️
Ratio:    66%     50%     41%     34%     29%
Action:   Normal  Normal  Normal  Normal  Normal

Pattern 3: Memory Pressure with Swapping

Time:     T0      T1      T2      T3      T4
Anon:     100MB   200MB   300MB   250MB   200MB
File:     50MB    50MB    50MB    30MB    20MB
Swap:     0MB     0MB     50MB    100MB   150MB  ↗️
Ratio:    66%     80%     85%     89%     91%
Action:   Normal  Warn    Alert   Critical OOM-Risk

Confidence Scoring

Component Weight Scoring Criteria
Anonymous Ratio 0-40 >90%: 40, >85%: 35, >80%: 30, >75%: 20
Growth Differential 0-30 Anon-File >10MB/s: 30, >1MB/s: 25, >100KB/s: 20
Swap Pressure 0-20 >20%: 20, >10%: 15, >5%: 10
Duration 0-10 Sustained pattern: 10

Advantages Over Simple RSS Monitoring

1. Distinguishes Leak Types

Scenario Total RSS Anon Ratio Detection
Heap leak Growing ↗️ High (>80%) ✅ Detected
Cache growth Growing ↗️ Low (<40%) ✅ Correctly ignored
Mixed growth Growing ↗️ Stable (~60%) ✅ Low confidence

2. Early Detection

  • Detects pattern changes before significant growth
  • Identifies leaks in small processes (<100MB)
  • Scales automatically with process size

3. Swap-Aware

  • Detects memory pressure before OOM
  • Identifies thrashing patterns
  • Correlates swap growth with leak probability

Performance Characteristics

Metric Value Impact
Ratio Calculation ~50 instructions Per RSS update
Growth Rate Calc ~100 instructions Per RSS update
Confidence Scoring ~200 instructions On ratio change
Total CPU <350 instructions <0.03% overhead
Memory per Process 52 bytes additional 520KB for 10K procs

Configuration

struct rss_ratio_config {
    __u16 anon_ratio_threshold;     // Default: 800 (80%)
    __u16 swap_ratio_threshold;     // Default: 100 (10%)
    __u64 min_growth_differential;  // Default: 100KB/s
    __u32 sample_window_ms;         // Default: 5000ms
    __u8 enable_swap_detection;     // Default: 1
    __u8 confidence_threshold;      // Default: 60
};

Testing

Test with test/memory-leak-simulators/anon_ratio.c:

// Allocate primarily anonymous memory
for (int i = 0; i < 100; i++) {
    void* heap = malloc(10 * 1024 * 1024);  // 10MB heap
    memset(heap, i, 10 * 1024 * 1024);      // Touch pages
    
    // Minimal file mapping for comparison
    if (i % 10 == 0) {
        mmap(NULL, 1024*1024, PROT_READ, MAP_PRIVATE, fd, 0);
    }
    sleep(1);
}

Expected: Anonymous ratio >90%, Detection <30 seconds, Confidence 85-95

False Positive Prevention

The detector includes safeguards:

✅ Balanced growth recognition - Won't trigger if file and anon grow proportionally
✅ Cache detection - Low confidence when file memory dominates
✅ Startup grace period - Ignores initial allocation bursts
✅ Minimum threshold - Only tracks processes >10MB RSS

Limitations

❌ Language-specific patterns - JVM has unique memory patterns
❌ Shared memory - May misclassify heavy shmem usage
❌ Memory-mapped databases - Could trigger false positives
❌ Container overhead - cgroup accounting differences

Integration with Other Detectors

RSS Ratio provides unique insights:

  • WHAT is growing (heap vs cache) - This detector
  • HOW it's growing (trend) - Linear Regression
  • WHETHER it matches patterns - Threshold Detector

Combined, they provide comprehensive leak detection.

See Also

Last updated: 2025-01-19 | Branch: mem_monitor | Status: DRAFT

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