build-perf-diagnostics

## Performance Analysis Methodology 1. **Generate a binlog**: `dotnet build /bl:{} -m` 2. **Replay to diagnostic log with performance summary**: ```bash dotnet msbuild build.binlog -noconlog -fl -flp:v=diag;logfile=full.log;performancesummary ``` 3. **Read the performance summary** (at the end of `full.log`): ```bash grep "Target Performance Summary\|Task Performance Summary" -A 50 full.log ``` 4. **Find expensive targets and tasks**: The PerformanceSummary section lists all targets/tasks sorted by cumulative time 5. **Check for node utilization**: grep for scheduling and node messages ```bash grep -i "node.*assigned\|building with\|scheduler" full.log | head -30 ``` 6. **Check analyzers**: grep for analyzer timing ```bash grep -i "analyzer.*elapsed\|Total analyzer execution time\|CompilerAnalyzerDriver" full.log ``` ## Key Metrics and Thresholds - **Build duration**: what's "normal" — small project <10s, medium <60s, large <5min - **Node utilization**: ideal is >80% active time across nodes. Low utilization = serialization bottleneck - **Single target domination**: if one target is >50% of build time, investigate - **Analyzer time vs compile time**: analyzers should be <30% of Csc task time. If higher, consider removing expensive analyzers - **RAR time**: ResolveAssemblyReference >5s is concerning. >15s is pathological ## Common Bottlenecks ### 1. ResolveAssemblyReference (RAR) Slowness - **Symptoms**: RAR taking >5s per project - **...