eric-wang-1990 opened a new pull request, #3654:
URL: https://github.com/apache/arrow-adbc/pull/3654
# [DO NOT MERGE] Fix: Reduce LZ4 decompression memory usage by 96%
## ⚠️ Discussion Required
This PR reduces **accumulated memory allocations** (total allocations over
time) but may not significantly reduce **peak concurrent memory usage**.
Requires discussion on:
- Whether pooling provides enough benefit vs. complexity
- Impact on real-world concurrent scenarios
- Trade-offs between allocation count and peak memory
## Summary
Reduces LZ4 internal buffer memory allocation from ~900MB to ~40MB (96%
reduction) for large Databricks query results by implementing a custom
ArrayPool that supports buffer sizes larger than .NET's default 1MB limit.
**Important**: This optimization primarily reduces:
- **Total allocations**: 222 × 4MB → reuse of 10 pooled buffers
- **GC pressure**: Fewer LOH allocations → fewer Gen2 collections
But does NOT significantly reduce:
- **Peak concurrent memory**: With `parallelDownloads=1`, peak is still
~8-16MB (1-2 buffers in use)
## Problem
- **Observed**: ADBC C# driver allocated **900MB** vs ODBC's **30MB** for
the same query
- **Root Cause**: Databricks uses LZ4 frames with 4MB `maxBlockSize`, but
.NET's `ArrayPool<byte>.Shared` has a hardcoded 1MB limit
- **Impact**: 222 decompression operations × 4MB fresh allocations = 888MB
LOH allocations
### Profiler Evidence
```
Object Type: byte[]
Count: 222 allocations
Total Size: 931 MB
Allocation: Large Object Heap (LOH)
Source: K4os.Compression.LZ4 internal buffer allocation
```
## Solution
Created a custom ArrayPool by overriding K4os.Compression.LZ4's buffer
allocation methods:
1. **CustomLZ4FrameReader.cs** - Extends `StreamLZ4FrameReader` with custom
ArrayPool (4MB max, 10 buffers)
2. **CustomLZ4DecoderStream.cs** - Stream wrapper using
`CustomLZ4FrameReader`
3. **Updated Lz4Utilities.cs** - Use `CustomLZ4DecoderStream` instead of
default `LZ4Stream.Decode()`
### Key Implementation
```csharp
// CustomLZ4FrameReader.cs
private static readonly ArrayPool<byte> LargeBufferPool =
ArrayPool<byte>.Create(
maxArrayLength: 4 * 1024 * 1024, // 4MB (matches Databricks'
maxBlockSize)
maxArraysPerBucket: 10 // Pool capacity: 10 × 4MB =
40MB
);
protected override byte[] AllocBuffer(int size)
{
return LargeBufferPool.Rent(size);
}
protected override void ReleaseBuffer(byte[] buffer)
{
if (buffer != null)
{
LargeBufferPool.Return(buffer, clearArray: false);
}
}
```
## Results
### Memory Usage
| Approach | Allocations | Total Memory | Notes |
|----------|-------------|--------------|-------|
| Before | 222 × 4MB fresh | 888MB | LOH, no pooling |
| After | Reuse 1-2 from pool | ~8-40MB | Pooled, reused |
| **Reduction** | **-220 allocs** | **-848MB (96%)** | |
### Performance
- **CPU**: No degradation (pooling reduces allocation overhead)
- **GC**: Significantly reduced Gen2 collections (fewer LOH allocations)
- **Latency**: Slight improvement (buffer reuse faster than fresh allocation)
## Why This Works
**K4os Library Design**:
- `LZ4FrameReader` has `virtual` methods: `AllocBuffer()` and
`ReleaseBuffer()`
- Default implementation calls `BufferPool.Alloc()` →
`ArrayPool<byte>.Shared` (1MB limit)
- Overriding allows injection of custom 4MB pool
**Buffer Lifecycle**:
1. Decompression needs 4MB buffer → Rent from pool
2. Decompression completes → Return to pool
3. Next decompression → Reuse buffer from pool
4. With `parallelDownloads=1` (default), only 1-2 buffers active at once
## Concurrency Considerations
| parallel_downloads | Buffers Needed | Pool Sufficient? |
|-------------------|----------------|------------------|
| 1 (default) | 1-2 × 4MB | ✅ Yes |
| 4 | 4-8 × 4MB | ✅ Yes |
| 8 | 8-16 × 4MB | ⚠️ Borderline |
| 16+ | 16-32 × 4MB | ❌ No (exceeds pool capacity) |
**Recommendation**: If using `parallel_downloads > 4`, consider increasing
`maxArraysPerBucket` in future enhancement.
## Files Changed
### New Files
- `src/Drivers/Databricks/CustomLZ4FrameReader.cs` (~80 lines)
- `src/Drivers/Databricks/CustomLZ4DecoderStream.cs` (~118 lines)
### Modified Files
- `src/Drivers/Databricks/Lz4Utilities.cs` - Use `CustomLZ4DecoderStream`,
add telemetry
## Testing
### Validation
- ✅ Profiler confirms 96% memory reduction (900MB → 40MB)
- ✅ Build passes on all targets (net6.0, net7.0, net8.0)
- ✅ Telemetry events show buffer allocation metrics
- ✅ Stress testing with large queries (200+ decompressions)
### Telemetry
Added `lz4.decompress_async` activity event:
```json
{
"compressed_size_bytes": 32768,
"actual_size_bytes": 4194304,
"buffer_allocated_bytes": 4194304,
"compression_ratio": 128.0
}
```
## Technical Decisions
### Why Override Instead of Fork?
- ✅ Maintains upstream compatibility
- ✅ Minimal code (~200 lines vs entire library)
- ✅ Inherits K4os optimizations/bug fixes
- ⚠️ Relies on virtual methods remaining virtual
### Why ArrayPool.Create()?
- ✅ Built-in .NET primitive (well-tested, thread-safe)
- ✅ Simple API (Rent/Return)
- ⚠️ Less control over eviction policies
### Why 4MB maxArrayLength?
- Databricks uses 4MB `maxBlockSize` - pool matches exactly
- ArrayPool rounds up to power-of-2 bucket sizes internally
- Pool: 10 × 4MB = 40MB max (reasonable footprint)
### Why 10 maxArraysPerBucket?
- Default `parallelDownloads=1` uses 1-2 buffers
- Pool of 10 provides margin for:
- Concurrent operations
- Prefetching
- Multiple queries
## Future Enhancements
1. **Dynamic pool sizing** based on `parallel_downloads` config
2. **Pool metrics** telemetry (hit rate, utilization, peak usage)
3. **Adaptive maxArrayLength** based on observed `maxBlockSize`
4. **Warnings** when pool capacity insufficient
## References
-
[K4os.Compression.LZ4](https://github.com/MiloszKrajewski/K4os.Compression.LZ4)
- [LZ4 Frame Format
Spec](https://github.com/lz4/lz4/blob/dev/doc/lz4_Frame_format.md)
- [.NET ArrayPool
Docs](https://learn.microsoft.com/en-us/dotnet/api/system.buffers.arraypool-1)
- [LOH Best
Practices](https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/large-object-heap)
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