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Change max batch to one

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Andrew Noyes
2025-08-26 16:54:29 -04:00
parent 431befe9bd
commit c915fed4cc
4 changed files with 30 additions and 8 deletions
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threading_performance_report.md
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@@ -2,7 +2,7 @@
## Summary
WeaselDB achieved 1.3M requests/second throughput using a two-stage ThreadPipeline with futex wake optimization, delivering 488ns serial CPU time per request while maintaining 0% CPU usage when idle. Higher serial CPU time means more CPU budget available for serial processing.
WeaselDB achieved 1.3M requests/second throughput using a two-stage ThreadPipeline with futex wake optimization, delivering 550ns serial CPU time per request while maintaining 0% CPU usage when idle. Higher serial CPU time means more CPU budget available for serial processing.
## Performance Metrics
@@ -23,23 +23,36 @@ WeaselDB achieved 1.3M requests/second throughput using a two-stage ThreadPipeli
**Optimized Pipeline Mode**:
- **Throughput**: 1.3M requests/second
- **Serial CPU time per request**: 488ns (validated with nanobench)
- **Serial CPU time per request**: 550ns (validated with nanobench)
- **Theoretical maximum serial CPU time**: 769ns (1,000,000,000ns ÷ 1,300,000 req/s)
- **Serial efficiency**: 63.4% (488ns ÷ 769ns)
- **Serial efficiency**: 71.5% (550ns ÷ 769ns)
- **CPU usage when idle**: 0%
### Key Optimization: Futex Wake Reduction
### Key Optimizations
**Futex Wake Reduction**:
- **Previous approach**: Futex wake at every pipeline stage (10% CPU overhead)
- **Optimized approach**: Futex wake only at final stage to wake producers. Stages now do their futex wait on the beginning of the pipeline instead of the previous stage.
- **Result**: 23% increase in serial CPU budget (396ns → 488ns)
- **Benefits**: Higher throughput per CPU cycle + idle efficiency
**CPU-Friendly Spin Loop**:
- **Added**: `_mm_pause()` intrinsics in polling loop to reduce power consumption and improve hyperthreading efficiency
- **Maintained**: 100,000 spin iterations necessary to prevent thread descheduling
- **Result**: Same throughput with more efficient spinning
**Stage-0 Batch Size Optimization**:
- **Changed**: Stage-0 max batch size from unlimited to 1
- **Result**: Additional 12.7% increase in serial CPU budget (488ns → 550ns)
- **Overall improvement**: 38.9% increase from baseline (396ns → 550ns)
### Request Flow
```
I/O Threads (8) → HttpHandler::on_batch_complete() → ThreadPipeline
↑ ↓
| Stage 0: Noop thread
| (488ns serial CPU per request)
| (550ns serial CPU per request)
| (batch size: 1)
| ↓
| Stage 1: Connection return
| (optimized futex wake)