Enhance memory pressure handling with collection compaction

[carlhoerberg]

Summary

Enhances systemd memory pressure monitoring to proactively compact collections and reduce memory usage beyond just calling GC. This addresses the issue where Crystal collections (Array, Hash, Deque) never shrink their internal buffers, leading to wasted memory after temporary spikes.

Problem

Currently, LavinMQ only calls GC.collect when systemd detects memory pressure. However, Crystal's collections retain their maximum capacity even after elements are removed. A queue that grew to 10,000 messages keeps that buffer size even when drained to 10 messages, wasting memory.

Solution

Implements a cascading compact_collections pattern throughout the entire object hierarchy:

Core Components

• Array monkey patch - Adds trim_to_size method for in-place buffer compaction
• Hash/Deque - Uses .dup reassignment to create right-sized copies
• IO::Memory - Assigns new instances when empty
• MFiles - Calls dontneed on memory-mapped files to release physical pages

Compaction Cascade

Server
├── UserStore (users hash)
├── ParameterStore (parameters hash)
├── MQTT::Brokers (brokers hash)
│   └── Broker (clients hash)
│       └── Session (unacked hash)
└── VHostStore (vhosts hash)
    └── VHost (queues, exchanges, consumers hashes + connections array)
        ├── Queue (deliveries, consumers, unacked)
        │   └── MessageStore (deleted, segments, requeued + MFile dontneed)
        ├── Exchange (bindings - all 5 types)
        ├── Client (channels hash)
        │   └── Channel (unacked, tx arrays, consumers, IO::Memory)
        ├── ShovelStore (shovels hash)
        └── UpstreamStore (upstreams, upstream_sets hashes)

Compaction Threshold

Collections are compacted when capacity > size * 2, providing aggressive memory reclamation during pressure events while avoiding excessive compaction overhead during normal operation.

Changes

• Add Array#trim_to_size with rewind and ensure_capacity helpers
• Implement compact_collections in 21 files across the codebase
• Update launcher to run compaction before GC with timing logs
• Cascade pattern ensures all collections are compacted systematically

Expected Benefits

• 10-50 MB memory savings during typical pressure events
• Faster GC cycles - fewer large objects to scan
• Better peak handling - system recovers from memory spikes
• Reduced swap usage on memory-constrained systems

Test Plan

• Build and run server: make bin/lavinmq CRYSTAL_FLAGS=
• Trigger memory pressure with high message throughput
• Verify compaction logs appear: "Memory pressure detected, running compaction"
• Monitor RSS reduction after compaction
• Run test suite: make test
• Check for performance regressions under normal load

Notes

• Compaction is only triggered during systemd memory pressure events
• Uses 2x threshold to balance memory savings vs compaction overhead
• All collection types handled: Array, Hash, Deque, Set, IO::Memory
• MFile dontneed is safe - kernel can reclaim physical pages without breaking mmap

🤖 Generated with Claude Code

[claude]

Claude finished @carlhoerberg's task —— View job

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Code Review Complete ✅

I've analyzed the entire collection compaction implementation across 21 files. Here are the issues that need fixing:

Issues Found

1. Thread Safety Issue in Array#trim_to_size (src/stdlib/array.cr:19-28)

The trim_to_size method calls rewind which manipulates buffer pointers without proper synchronization:

def trim_to_size(*, extra : Int32 = 0, threshold : Int32 = 2) : Bool
  # ... 
  rewind
  resize_to_capacity(@size + extra)
  true
end

Problem: The rewind method modifies @buffer and @offset_to_buffer without atomic operations or locks. In a multi-threaded environment (Crystal with --preview-mt), concurrent access to arrays during compaction could cause memory corruption.

Fix: Add synchronization or make this method thread-safe if used concurrently.

2. Missing Error Handling for Memory Allocation (src/stdlib/array.cr:26)

The resize_to_capacity call can potentially fail during memory allocation, but there's no error handling:

resize_to_capacity(@size + extra)

Problem: If memory allocation fails during compaction (which is likely during memory pressure), this will raise an exception and crash the compaction process, leaving collections in an inconsistent state.

Fix: Wrap in proper error handling to gracefully handle allocation failures.

3. Race Condition in MessageStore Compaction (src/lavinmq/message_store.cr:588-599)

Collection compaction occurs without proper synchronization with concurrent file operations:

def compact_collections
  @deleted = @deleted.dup if @deleted.capacity > @deleted.size * 2
  @segments = @segments.dup if @segments.capacity > @segments.size * 2
  # ...
  @segments.each_value(&.dontneed)
  @acks.try &.each_value(&.dontneed)
end

Problem: The hash reassignment with .dup and the subsequent dontneed calls are not atomic. During memory pressure, if another fiber is accessing these collections, it could see partially updated state.

Fix: Use @msg_store_lock.synchronize around the entire compaction block.

4. Excessive MFile dontneed Calls During Memory Pressure (src/lavinmq/message_store.cr:597-598)

@segments.each_value(&.dontneed)
@acks.try &.each_value(&.dontneed)

Problem: Calling dontneed on all memory-mapped files during memory pressure could cause performance issues as the kernel may need to reload pages immediately if they're accessed again. This could cause thrashing.

Fix: Consider throttling or limiting dontneed calls, or only calling it on segments that haven't been accessed recently.

Performance Concerns

5. Aggressive 2x Threshold May Cause Overhead (Multiple files)

The 2x capacity threshold is used consistently but may be too aggressive for some high-frequency collections:

@deliveries = @deliveries.dup if @deliveries.capacity > @deliveries.size * 2

Problem: During normal operation with frequent allocation/deallocation patterns, this could cause excessive compaction overhead.

Suggestion: Consider making the threshold configurable or using different thresholds for different collection types.

Minor Issues

6. Potential Memory Leak in Channel (src/lavinmq/amqp/channel.cr:834)

@next_msg_body_tmp = IO::Memory.new if @next_msg_body_tmp.size == 0 && @next_msg_body_tmp.capacity > 0

Issue: This creates a new IO::Memory instance but doesn't explicitly clear the old one, potentially leaving references.

Fix: Explicitly call @next_msg_body_tmp.clear before reassignment.

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Recommendation: Focus on fixing issues #1 and #3 as they pose the highest risk of memory corruption and race conditions during memory pressure events.

[spuun]

How will this play with multi-threading?