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…rmance regression Key optimizations: 1. Add JobManager.WriteJobsYAML(*strings.Builder) to write the jobs section directly to an existing builder instead of allocating a 96KB intermediate buffer + string copy in RenderToYAML. 2. Add JobManager.renderJobTo(*strings.Builder, *Job) that writes each job directly to the provided builder, eliminating one 8KB buffer per job. 3. Reduce generateYAML body/yaml builder initial capacity from 96KB to 64KB (sufficient for ~60KB compiled output). 4. Replace yaml.WriteString(line + "\n") with two separate WriteString/WriteByte calls in generateEngineExecutionSteps and other hot paths to eliminate temporary string allocations. 5. Replace string concatenation in generateWorkflowBody (onSection+"\n\n", etc.) with two WriteString calls. Results: -27% in B/op (992KB → 722KB), -2% in allocs/op, ~21% faster wall time vs the regressed version (1.67ms → 1.32ms). Agent-Logs-Url: https://github.com/github/gh-aw/sessions/e57b41f5-70f8-457a-aa52-7d03cee44cd6 Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Agent-Logs-Url: https://github.com/github/gh-aw/sessions/e57b41f5-70f8-457a-aa52-7d03cee44cd6 Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
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[WIP] Fix performance regression in CompileSimpleWorkflow
perf: fix 14% CompileSimpleWorkflow regression by eliminating intermediate YAML buffer allocations
May 2, 2026
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Pull request overview
This PR optimizes workflow YAML generation to reduce allocations in CompileSimpleWorkflow, primarily by streaming YAML directly into existing builders instead of building intermediate strings.
Changes:
- Added direct-write helpers in
JobManagerso jobs can be rendered into an existingstrings.Builder. - Updated workflow body generation to use the direct-write job path and reduced initial YAML builder capacity from 96 KiB to 64 KiB.
- Replaced several
WriteString(line + "\n")hot-loop patterns withWriteString(line)plusWriteByte('\n').
Show a summary per file
| File | Description |
|---|---|
pkg/workflow/jobs.go |
Adds direct-to-builder job rendering helpers and keeps string-returning wrappers for compatibility. |
pkg/workflow/compiler_yaml.go |
Switches workflow body generation to the new direct-write jobs API and retunes builder sizing/newline writes. |
pkg/workflow/compiler_yaml_ai_execution.go |
Removes temporary string concatenation in AI execution step emission loops. |
pkg/workflow/compiler_yaml_main_job.go |
Applies the same newline-write optimization across several main-job step generation loops. |
Copilot's findings
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- Files reviewed: 4/4 changed files
- Comments generated: 3
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| // 64 KB covers the common simple-workflow case without a reallocation; | ||
| // larger workflows will trigger a single grow (to ~128 KB) which is | ||
| // acceptable given the reduction in baseline allocation compared to the | ||
| // previous 96 KB constant. | ||
| const initialBuilderCapacity = 64 * 1024 |
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| // RenderToYAML generates the jobs section of a GitHub Actions workflow. | ||
| // Prefer WriteJobsYAML when an existing *strings.Builder is available to avoid | ||
| // an extra allocation. | ||
| func (jm *JobManager) RenderToYAML() string { | ||
| var yaml strings.Builder |
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| // Generate jobs section using JobManager — write directly to avoid an | ||
| // intermediate string allocation. | ||
| c.jobManager.WriteJobsYAML(yaml) |
Collaborator
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@copilot review all comments |
…AML, add WriteJobsYAML test - Restore initialBuilderCapacity to 96KB in generateYAML to avoid regressions on Copilot/Claude workflows (70-90KB). The perf win is from eliminating copies, not from reducing capacity. - Pre-size the RenderToYAML compatibility builder with Grow(64KB) so existing callers don't regress vs the old pre-sized buffer. - Add TestJobManager_WriteJobsYAML with three table cases that verify WriteJobsYAML appends correctly to an already-populated builder and that output matches RenderToYAML(). Agent-Logs-Url: https://github.com/github/gh-aw/sessions/6befe686-4a16-4b17-877f-0b92cceca11e Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
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Addressed all three review points in commit
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CompileSimpleWorkflowregressed 14% (1.47ms → 1.67ms) due to excessive intermediatestrings.Builderallocations in the YAML generation path, driving GC to consume ~30% of CPU time.Root cause
Three compounding allocation sources per compile:
RenderToYAML()internal builderrenderJob()per-job buffer × ~5 jobsgenerateYAMLouteryaml+bodybuildersWriteString(line + "\n")in hot loopsThe
RenderToYAMLandrenderJobpattern built strings into their own builders then copied the result into the caller's builder — paying both the allocation and the copy.Changes
jobs.go— AddWriteJobsYAML(*strings.Builder)andrenderJobTo(*strings.Builder, *Job)that write directly to the caller's builder.RenderToYAML()/renderJob()become thin wrappers for existing callers;RenderToYAMLis pre-sized withGrow(64 KB)so its compatibility builder avoids regrowth on typical outputs.compiler_yaml.go— UseWriteJobsYAMLingenerateWorkflowBody; retain 96 KB initial capacity foryaml/bodybuilders (covers 70–90 KB Copilot/Claude workflows without reallocation); replaces + "\n\n"concatenations with twoWriteStringcalls.compiler_yaml_ai_execution.go,compiler_yaml_main_job.go— ReplaceWriteString(line + "\n")hot-loop pattern withWriteString(line)+WriteByte('\n').jobs_test.go— AddTestJobManager_WriteJobsYAMLwith three table-driven cases that verifyWriteJobsYAMLappends correctly to an already-populated builder and that its output matchesRenderToYAML().Results
Lands ~13% below the historical baseline (1.47 ms).