Measured GitHub-hosted PR preview sizing
基于测量的 GitHub 托管 PR 预览规格
PR previews build the full workspace and browser-worker VFS image. A lower per-minute runner price does not guarantee lower job cost because GitHub rounds each job upward to whole minutes. Moving previews to persistent self-hosted machines also changes isolation and is outside this decision.
English
Problem
PR previews build the full workspace and browser-worker VFS image. A lower per-minute runner price does not guarantee lower job cost because GitHub rounds each job upward to whole minutes. Moving previews to persistent self-hosted machines also changes isolation and is outside this decision.
Decision
The preview workflow uses standard GitHub-hosted ubuntu-24.04. Build, cache, deployment, protected-image verification, and comment semantics remain unchanged. The sizing reference owns comparison requirements. The separate CI failover runbook retains its independent runner-switch decision; previews do not use those switches.
Measurements
Experiment 34012729982 succeeds for all eight size/cache combinations plus one cache seed. Every measured job checks out the same experiment revision, uses Node 24.19.0 and pnpm 11.7.0, and executes immutable install, full workspace build, preview/VFS packing, and local upload shaping with gzip integrity verification. Warm jobs restore one exact run-private pnpm cache; cold jobs skip restoration but contain pnpm bootstrap files. No compiled outputs are restored.
| Runner | Cold / warm job seconds | Rounded minutes each | USD each | Workspace seconds cold / warm | Preview seconds cold / warm |
|---|---|---|---|---|---|
| standard, 2 vCPU | 202 / 203 | 4 | 0.024 | 138.92 / 147.21 | 12.65 / 12.94 |
| larger, 4 vCPU | 177 / 162 | 3 | 0.036 | 124.21 / 114.86 | 10.77 / 9.88 |
| larger, 8 vCPU | 154 / 154 | 3 | 0.066 | 110.51 / 110.77 | 9.20 / 9.21 |
| larger, 16 vCPU | 124 / 125 | 3 | 0.126 | 90.57 / 84.99 | 7.62 / 7.33 |
Using published rates, measured jobs total $0.504; the 60-second standard seed adds $0.006. The $0.510 gross compute estimate includes setup, restoration, measurement upload, and cleanup, but excludes storage and account discounts. Standard costs 80.95% less than 16-core and 33.33% less than 4-core in each sampled cache state. It adds 78 seconds against the corresponding 16-core job.
Standard jobs expose two vCPUs and 7.75 GiB RAM. Workspace maximum process RSS is 2.86 / 2.76 GiB; preview maximum process RSS is 0.76 / 0.74 GiB. Both complete without an OOM or timeout. GNU time RSS is not simultaneous process-tree memory. These samples establish successful execution, not a permanent memory guarantee.
The comparison fixes source, lockfile, commands, and runtime versions, not physical CPUs or image release: standard and 4-core use image 20260831.293.1; 8-core and 16-core use 20260823.283.1. CPUs vary among AMD EPYC 9V74/7763 and Intel Xeon 8370C/8573C. One sample per cache state measures the offered labels, not isolated CPU scaling or statistical repeatability.
The experiment does not deploy or access Cloudflare credentials. Measurement upload takes zero to one second; warm-cache restore takes six to ten seconds. For context, production job 101428009994 (run 34011495156) spends 14 seconds uploading, one second verifying, and two seconds commenting on a different SHA. Adding that overhead to this experiment is a projection, not a measured standard-runner publication result. The actual PR preview workflow owns deployment confirmation.
Alternatives considered
Keep 16-core. It provides the shortest measured job, but costs $0.102 more per sample for a 78-second improvement. Preview builds do not justify that premium for this cost-focused decision.
Select 4-core or 8-core. Both succeed and shorten builds, but their rounded sample costs exceed standard Ubuntu. Four-core retains more RAM and disk headroom if future workloads exhaust standard capacity; such a change requires new measurements.
Move to self-hosted. Rejected by scope: previews remain on GitHub CI. The existing Linux and Windows registrations can share persistent hosts; their dependency, store-volume, and cleanup assumptions do not apply to fresh hosted VMs. No failover or trust condition changes.
Consequences
Previews trade approximately 78 seconds of sampled build-job latency for lower compute cost. Production Cloudflare latency, image rollout variance, future build growth, and broader success rates remain observable limitations. No hourly or monthly savings are extrapolated from this single experiment. The temporary benchmark workflow and its safety test are absent from the final tree; the experiment commits and linked run preserve the method and evidence.
The executed focused regression pins hosted routing, PR triggers and permissions, immutable full builds, restore-only caching, publication shaping, protected-image checks, and idempotent comments. A physical self-hosted routing mutation fails its routing assertion; restoration passes all three tests. No model-visible runtime behavior changes, so no Session snapshot changes are required.
中文
问题
PR（Pull Request）预览构建完整工作区及浏览器 worker VFS 镜像。较低的每分钟运行器价格不能保证较低的作业成本，因为 GitHub 将每个作业向上取整至整分钟。将预览移至持久化自托管机器还会改变隔离方式，不属于本决策范围。
决策
预览工作流 使用标准 GitHub 托管 ubuntu-24.04。构建、缓存、部署、受保护镜像验证及评论语义保持不变。规格参考 负责比较要求。独立的 CI 故障切换手册 保留其运行器切换决策；预览不使用这些开关。
测量
实验 34012729982 的八种规格/缓存组合及一个缓存预热作业均成功。每个测量作业检出同一个实验修订，使用 Node 24.19.0 与 pnpm 11.7.0，并执行不可变安装、完整工作区构建、预览/VFS 打包，以及含 gzip 完整性验证的本地上传内容整理。热作业恢复同一个运行私有精确 pnpm 缓存；冷作业跳过恢复，但包含 pnpm 引导安装文件。不恢复编译产物。
| 运行器 | 冷 / 热作业秒数 | 各自取整分钟数 | 各自美元费用 | 冷 / 热工作区秒数 | 冷 / 热预览秒数 |
|---|---|---|---|---|---|
| 标准，2 vCPU | 202 / 203 | 4 | 0.024 | 138.92 / 147.21 | 12.65 / 12.94 |
| 大型，4 vCPU | 177 / 162 | 3 | 0.036 | 124.21 / 114.86 | 10.77 / 9.88 |
| 大型，8 vCPU | 154 / 154 | 3 | 0.066 | 110.51 / 110.77 | 9.20 / 9.21 |
| 大型，16 vCPU | 124 / 125 | 3 | 0.126 | 90.57 / 84.99 | 7.62 / 7.33 |
按公开费率，测量作业合计 $0.504；60 秒标准预热作业增加 $0.006。$0.510 总计算费用估算包含设置、恢复、测量上传及清理，但不含存储和账户折扣。在每种采样缓存状态下，标准运行器比 16 核低 80.95%，比 4 核低 33.33%。相比对应的 16 核作业增加 78 秒。
标准作业提供两个 vCPU 与 7.75 GiB 内存。工作区最大进程 RSS 为 2.86 / 2.76 GiB；预览最大进程 RSS 为 0.76 / 0.74 GiB。两者均未发生 OOM 或超时并完成。GNU time RSS 不是进程树同时占用的内存总量。这些样本证明成功执行，而非永久内存保证。
比较固定源代码、锁文件、命令和运行时版本，但不固定物理 CPU 或镜像版本：标准与 4 核使用镜像 20260831.293.1；8 核与 16 核使用 20260823.283.1。CPU 包括 AMD EPYC 9V74/7763 与 Intel Xeon 8370C/8573C。每种缓存状态的单个样本测量所提供的标签，而非独立 CPU 扩展性或统计可重复性。
实验不部署，也不访问 Cloudflare 凭据。测量上传耗时零至一秒；热缓存恢复耗时六至十秒。作为背景，生产作业 101428009994 (run 34011495156) 在不同 SHA 上上传耗时 14 秒、验证一秒、评论两秒。将该开销加至本实验属于推算，而非已测量的标准运行器发布结果。实际 PR 预览工作流负责部署确认。
考虑过的替代方案
保留 16 核。 它提供最短的测量作业，但为 78 秒改善使每个样本增加 $0.102。对于本次以成本为重点的决策，预览构建不值得这项溢价。
选择 4 核或 8 核。 两者均成功并缩短构建，但取整后的样本费用高于标准 Ubuntu。若未来工作负载耗尽标准容量，4 核可保留更多内存与磁盘余量；这样的变更需要新测量。
移至自托管。 因范围限制而拒绝：预览保留在 GitHub CI。现有 Linux 与 Windows 注册实例可能共享持久化主机；其依赖、store 卷及清理假设不适用于全新的托管 VM。不改变故障切换或信任条件。
影响
预览以约 78 秒采样构建作业延迟换取更低的计算费用。生产 Cloudflare 延迟、镜像发布差异、未来构建增长及更广泛的成功率仍是可观测限制。不从本次单一实验外推每小时或每月节省。最终文件树不包含临时基准工作流及其安全测试；实验提交与链接的运行保留方法和证据。
已执行的针对性回归 固定托管路由、PR 触发器与权限、不可变完整构建、只恢复缓存、发布内容整理、受保护镜像检查及幂等评论。实际修改为自托管路由会使路由断言失败；恢复后全部三个测试通过。不改变模型可见运行时行为，因此不需要修改 Session 快照。