GHA free windows runners are 2vCPU instances, and are our longest-running jobs most of the time now.
Test using Warp's 4x windows runners to get a feel for runtime improvements and whether it might be worth migrating.
First run should save caches (even on a PR) so a re-run can test with caches also coming via warp.
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Apparently Windows 2025 with VS2026 is coming in an image next week, so will look out for that and re-run when it's released.
We are using about 70-90k minutes per month on GH, if these runners were 25% faster we'd be looking at a ~$350 cost, and if they were 50% faster could be as low as ~$250/month.
8x runners cost double the 4x runners, so unless they're > 50% faster than the 4x, they'd cost us more.
Route the Windows native vcpkg caches through the existing internal cache wrappers so bitcoin/bitcoin can use WarpBuilds/cache on Warp Windows runners while forks keep the GitHub Actions cache path.
Also fix the runner expression quoting so the Windows Warp runner label is parsed as a string.
Allow bitcoin/bitcoin pull requests from branches in the main repository to save Windows vcpkg Warp caches so a trial PR can compare cold and warm rerun timings.
Keep fork pull requests restore-only to avoid letting untrusted PR code write to the shared cache namespace.
-- Somewhat predictably -- even the smallest 4x Warp runner gives speedups of 1.8x and 1.5x on a cache-hit run.
At ~850 runs per month, switching over to Warp runners which are billed at:
warp-windows-2025-vs2026-x64-4x Windows Server 2025 (VS 2026) 4 vCPU 16GB 256GB SSD $0.016/minute
Would see an extra >=$800 added to the CI bill.
If we had infinite or much larger budget it would be a no-brainer, but with [msan/valgrind]-fuzz often also taking an hour, not sure there' much benefit tbh, as the whole run time will not change too much...