SochDB Performance Optimizations

Summary

This document details the performance improvements made to SochDB's storage layer, achieving ~15× improvement from ~55k ops/sec to ~800k-1.3M ops/sec for sequential inserts, making it competitive with SQLite (~1.16M ops/sec).

Benchmark Results

Benchmark	Before	After	Improvement
SochDB Embedded (WAL)	~55k ops/sec	~761k ops/sec	13.8×
SochDB put_raw	N/A	~1.30M ops/sec	Direct storage
SochDB insert_row_slice	N/A	~1.29M ops/sec	Zero-alloc API
SQLite File (WAL)	~1.16M ops/sec	—	Baseline

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Root Cause Analysis

Discovery: 10× Overhead in Database Layer

Initial profiling revealed that raw DurableStorage::put() achieved 1.5M ops/sec, but the Database layer only achieved ~150k ops/sec. This pointed to overhead in the transaction/database abstraction layer rather than the storage engine itself.

Key Bottleneck: Group Commit

The group commit mechanism, designed to batch multiple transactions for better fsync amortization, was counterproductive for sequential single-threaded inserts:

Group Commit Flow (BEFORE):
┌─────────────────────────────────────────────────┐
│  1. Acquire mutex                               │
│  2. Add to pending batch                        │
│  3. Wait on condvar for batch fill OR timeout   │  ← BLOCKING!
│  4. Leader does fsync                           │
│  5. Wake all waiters                            │
└─────────────────────────────────────────────────┘

For sequential inserts, each operation was waiting for either:

• Other transactions to join the batch (none coming)
• A timeout (adding latency)

Solution: Disabled group commit for the embedded connection benchmark, allowing direct WAL writes without the coordination overhead.

Architecture Context

SochDB has a properly layered architecture:

┌─────────────────────────────────────────────────────────────┐
│  Client Layer                                               │
│  ├── Connection (alias for DurableConnection) ← DEFAULT     │
│  ├── InMemoryConnection (SochConnection) ← Testing only     │
│  └── EmbeddedConnection (optional, wraps Database)          │
├─────────────────────────────────────────────────────────────┤
│  Storage Layer                                              │
│  ├── Database (tables, schemas, MVCC coordination)          │
│  ├── DurableStorage (WAL, transactions, sync modes)         │
│  └── TxnWal (transaction-aware WAL with CRC32)              │
└─────────────────────────────────────────────────────────────┘

The DurableConnection properly routes through the WAL/MVCC kernel, making all optimizations in the storage layer visible to the client API.

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Optimizations Applied

1. Single-Pass CRC32 Calculation

File: sochdb-storage/src/txn_wal.rs

Before: CRC was calculated in a separate pass over the data.

After: CRC32 is calculated incrementally as data is written to the buffer.

pub fn write_no_flush_refs(&self, txn_id: TxnId, key: &[u8], value: &[u8]) -> Result<()> {
    let mut writer = self.writer.lock();
    
    // Single-pass: write and compute CRC simultaneously
    let mut crc = 0u32;
    
    // Write and accumulate CRC for each field
    writer.write_all(&txn_id.to_le_bytes())?;
    crc = crc32c::crc32c_append(crc, &txn_id.to_le_bytes());
    
    // ... continues for all fields
}

Impact: Eliminated redundant memory traversal, ~5-10% improvement.

---

2. Zero-Allocation WAL Writes

File: sochdb-storage/src/txn_wal.rs

Before: Each WAL write allocated a Vec<u8> buffer.

After: New write_no_flush_refs() method writes directly to BufWriter using references.

// OLD: Allocates Vec for each write
fn write(&self, key: Vec<u8>, value: Vec<u8>)

// NEW: Zero allocation, uses references
fn write_no_flush_refs(&self, txn_id: TxnId, key: &[u8], value: &[u8])

Impact: Eliminated per-write heap allocations.

---

3. DashMap for Lock-Free Concurrent Access

Files:

• sochdb-storage/src/durable_storage.rs
• sochdb-storage/src/database.rs

Before: RwLock<HashMap> for MVCC tracking and table metadata.

After: DashMap for lock-free concurrent reads.

// OLD: Lock contention on every access
tables: RwLock<HashMap<String, TableMetadata>>
active_txns: RwLock<HashMap<TxnId, TxnState>>

// NEW: Lock-free concurrent access
tables: DashMap<String, TableMetadata>
active_txns: DashMap<TxnId, TxnState>

Impact: Reduced lock contention, especially beneficial for concurrent workloads.

---

4. Zero-Allocation Row Insertion API

Files:

• sochdb-storage/src/packed_row.rs
• sochdb-storage/src/database.rs

Before: insert_row() required a HashMap<String, SochValue> allocation per row.

After: New insert_row_slice() accepts &[(&str, SochValue)] directly.

// OLD: Allocates HashMap per row
pub fn insert_row(&self, txn: TxnHandle, table: &str, 
                  row: HashMap<String, SochValue>) -> Result<u64>

// NEW: Zero allocation, uses slice
pub fn insert_row_slice(&self, txn: TxnHandle, table: &str,
                        row: &[(&str, SochValue)]) -> Result<u64>

PackedRow Enhancement:

// NEW: Pack from slice of references
pub fn pack_slice(values: &[Option<&SochValue>]) -> Vec<u8>

// NEW: Unpack to pre-sized Vec
pub fn unpack_to_vec(&self) -> Vec<Option<SochValue>>

Impact: Achieved 1.29M ops/sec, matching raw storage performance.

---

5. Cached Schema Lookup

File: sochdb-storage/src/database.rs

Before: Schema was fetched and parsed on every insert.

After: Schema is cached in DashMap<String, Vec<String>> after first access.

packed_schemas: DashMap<String, Vec<String>>  // table_name -> column_names

Impact: Eliminated repeated schema parsing overhead.

---

6. Lazy Query Iterator

File: sochdb-storage/src/database.rs

Before: execute() collected all results into a Vec before returning.

After: execute_iter() returns a lazy iterator that fetches rows on demand.

pub fn execute_iter(self) -> QueryRowIterator<'a> {
    QueryRowIterator {
        inner: self.storage.scan_prefix(&self.path_prefix).into_iter(),
        // ...
    }
}

Impact: Reduced memory usage for large result sets, enables streaming.

---

7. Group Commit Bypass for Sequential Workloads

File: sochdb-storage/src/durable_storage.rs

For benchmarks and sequential workloads, group commit can be disabled:

let config = DurableStorageConfig {
    group_commit: false,  // Disable for sequential inserts
    sync_mode: 1,         // NORMAL sync every 100 commits
    ..Default::default()
};

Impact: Eliminated condvar wait overhead, ~5× improvement for sequential inserts.

---

Architecture Insight

Performance Stack (ops/sec):
┌─────────────────────────────────────────┐
│  insert_row (HashMap)     ~150k         │  ← HashMap allocation overhead
├─────────────────────────────────────────┤
│  insert_row_slice         ~1.29M        │  ← Zero-allocation path
├─────────────────────────────────────────┤
│  put_raw                  ~1.30M        │  ← Direct storage bypass
├─────────────────────────────────────────┤
│  DurableStorage::put      ~1.50M        │  ← Raw storage layer
└─────────────────────────────────────────┘

Bottleneck removed: Group commit coordination

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Key Takeaways

1. Group commit is workload-dependent: Great for concurrent multi-tenant, counterproductive for sequential single-threaded.

2. Allocation matters at scale: HashMap per row adds up to significant overhead at 1M+ ops/sec.

3. Lock-free structures help: DashMap eliminates lock contention for concurrent access patterns.

4. Layer overhead accumulates: Raw storage was fast; overhead was in the abstraction layers.

5. Measure before optimizing: Profiling revealed the true bottleneck was group commit, not I/O.

---

Future Optimizations

See Architecture for remaining optimization areas:

• Memtable size limits: Flush to disk when threshold exceeded
• WAL compaction: Checkpoint/compaction to reclaim disk space
• Adaptive group commit: Switch modes based on workload pattern
• Connection pooling: For multi-tenant scenarios

---

Remaining Architectural Gaps

Based on the comprehensive architecture analysis in task.md, the following optimizations are planned:

High Priority (P0-P1)

Gap	Current State	Target	Impact
Epoch-Based GC	O(n) scan all keys	O(expired_versions) per cycle	10-100× GC reduction
Client GroupCommit	Duplicate implementation in client/storage	Single source in storage layer	Eliminates confusion
Adaptive Group Commit	Fixed thresholds (sync every 100 commits)	Little's Law: W* = √(τ/λ)	Better latency tuning

Medium Priority (P2)

Gap	Current State	Target	Impact
SSI Validation	read_set/write_set tracked but not validated	Full dangerous structure detection	Serializability
Vector Index WAL	HNSW uses periodic snapshots	WAL integration for crash recovery	ACID for embeddings
Cardinality Estimation	Static column_cardinalities	HyperLogLog++ streaming updates	5× plan quality

Lower Priority (P3)

Gap	Current State	Target	Impact
Clock-Pro Buffer	TinyLFU via moka	Clock-Pro + FIFO ghost cache	2× hit rate for scans
io_uring Integration	Sync fallback on non-Linux	SQ polling + batched submission	3× IOPS on Linux
Tiered Compaction	Implicit leveled strategy	Hybrid tiered/leveled	70% less write amp

Formulas Reference

Optimal Group Commit Wait Time (Little's Law):

W* = √(τ/λ)

Where:
- τ = fsync latency (~5ms on NVMe)
- λ = arrival rate (ops/sec)

For λ = 10,000 TPS, τ = 5ms:
  W* = √(0.005/10000) = 0.707ms
  Expected batch size = λ × W* = 7 transactions

MVCC GC with Epoch-Based Reclamation:

Memory bound = 3 × epoch_duration × write_rate

For epoch_duration = 100ms, write_rate = 100K/s:
  Max retained = 30,000 versions = ~1.9MB

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Reproducing Benchmarks

cd sochdb
cargo run -p benchmarks --release

This runs comparisons against SQLite and tests various SochDB APIs.