Silk Debug

Silk Debug Tool

A CLI tool for analyzing Django Silk profiling data to debug slow requests, detect N+1 queries, and optimize database performance.

Tool Location

.venv/bin/python scripts/silk_debug.py

Quick Reference

Analyze a Specific Request

When given a Silk request ID (UUID), use --full for comprehensive analysis:

.venv/bin/python scripts/silk_debug.py <request_id> --full

This shows:

• Request info (path, method, status, time, query count)
• Duplicate/similar queries (N+1 detection)
• Slow queries (>5ms by default)
• Queries grouped by table
• Query execution timeline
• Python cProfile data (if enabled)

List and Filter Requests

# List recent requests
.venv/bin/python scripts/silk_debug.py --list

# Sort by different criteria
.venv/bin/python scripts/silk_debug.py --list --sort queries     # Most queries
.venv/bin/python scripts/silk_debug.py --list --sort duration    # Slowest total time
.venv/bin/python scripts/silk_debug.py --list --sort db_time     # Most DB time

# Filter requests
.venv/bin/python scripts/silk_debug.py --list --path /api/events --min-queries 20
.venv/bin/python scripts/silk_debug.py --list --method POST --min-time 100

Aggregate Analysis

# Overall statistics
.venv/bin/python scripts/silk_debug.py --stats

# Endpoint summary (grouped by path pattern, shows P95)
.venv/bin/python scripts/silk_debug.py --endpoints

# Find slow endpoints
.venv/bin/python scripts/silk_debug.py --slow-endpoints --slow-endpoint-threshold 100

Interpreting Results

N+1 Query Detection

When you see duplicate queries like:

🔴 15x similar queries:
SELECT "events_ticket"."id" FROM "events_ticket" WHERE "events_ticket"."event_id" = '<UUID>'

This indicates an N+1 problem. Fix with:

• select_related() for ForeignKey fields
• prefetch_related() for reverse relations or M2M fields

Slow Queries

Common causes of slow queries:

1. COUNT on complex DISTINCT: Pagination wrapping complex visibility subqueries
   • Fix: Materialize IDs in Python first, then filter with simple IN clause
2. Missing indexes: Full table scans
   • Fix: Add database indexes on filtered/joined columns
3. Complex JOINs: Multiple related tables
   • Fix: Optimize query structure or denormalize if appropriate

Timeline Analysis

The timeline shows query execution order with visual bars:

  1. +    0.0ms [  2.5ms] █ "accounts_reveluser"
  7. +   60.2ms [  2.8ms] █ "__count"

Look for:

• Large gaps between queries (indicates Python processing time)
• Queries that could run in parallel but are sequential
• Expensive queries that block subsequent operations

Common Optimization Patterns

Expensive COUNT with DISTINCT

When you see:

SELECT COUNT(*) FROM (SELECT DISTINCT ... complex subquery ...)

Fix by materializing IDs:

# Before (slow COUNT)
qs = Event.objects.for_user(user).filter(...).distinct()

# After (fast COUNT)
event_ids = list(Event.objects.for_user(user).values_list("id", flat=True).distinct())
qs = Event.objects.full().filter(id__in=event_ids)

Redundant Visibility Checks

When the same for_user() query appears multiple times:

• Create a method that accepts already-checked objects
• Cache visibility results within the request

Batch Operations

When creating multiple objects:

• Use bulk_create() instead of individual .save() calls
• Fetch shared data (settings, related objects) once before the loop
• Send notifications in batches, not per-item

CLI Options Reference

Single Request Analysis

• --full, -f: Run all analyses
• --duplicates, -d: Show N+1 candidates
• --slow, -s: Show slow queries
• --slow-threshold N: Slow query threshold in ms (default: 5)
• --tables, -t: Group queries by table
• --timeline: Show execution timeline
• --traceback, -tb: Show code locations for duplicates
• --profile, -prof: Show Python cProfile data

Listing and Filtering

• --list, -l: List requests
• --limit N: Number of results (default: 20)
• --sort {recent,queries,duration,db_time}: Sort order
• --path, -p: Filter by path (contains)
• --method, -m: Filter by HTTP method
• --status: Filter by status code
• --min-queries N: Minimum query count
• --min-time N: Minimum response time (ms)
• --min-db-time N: Minimum DB time (ms)

Aggregate Views

• --stats: Show aggregate statistics
• --endpoints: Show endpoint summary with P95
• --slow-endpoints: Group slow requests by endpoint
• --slow-endpoint-threshold N: Threshold in ms (default: 200)
• --min-count N: Minimum requests for endpoint summary