uffd: record warmup faults and prefetch them on later forks#218
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sjmiller609 wants to merge 1 commit intohypeship/uffd-page-serverfrom
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uffd: record warmup faults and prefetch them on later forks#218sjmiller609 wants to merge 1 commit intohypeship/uffd-page-serverfrom
sjmiller609 wants to merge 1 commit intohypeship/uffd-page-serverfrom
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Adds a hot-page recorder + prefetch primitive on top of the userfaultfd page server. During a template's first warmup fork the server can record every served page (Config.RecordHotPages); the resulting HotPageList is stable-sorted, deduplicated, and saved to disk in a small binary format alongside the template. Later forks call Server.Prefetch(forkID, list) to issue UFFDIO_COPY for every recorded page against their userfaultfd before the guest unpauses, eliminating the fault round-trips on those addresses. The prefetcher is installed by the platform-specific listener once the fork's uffd has been received and registered, so callers can race Prefetch and the fault loop safely. EEXIST/EAGAIN are tolerated the same way the fault handler does to absorb first-touch races with vCPUs. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Stacked on: #216 (uffd page server) — review #213 → #214 → #216 first.
Summary
HotPage/HotPageListtypes with sort+dedup snapshot, atomicSave, andLoadHotPageList(binary varint format with aHPL1magic).Config.RecordHotPagesflag turns on per-fault recording in the page-fault loop.Server.Prefetch(forkID, list)issuesUFFDIO_COPYfor every entry in a hot-page list against the fork's userfaultfd before the guest unpauses.EEXIST/EAGAINare tolerated to absorb first-touch races with vCPUs.Why
Even with the shared mem-file + UFFD page server, a fresh fork still pays a fault round-trip on every page the guest needs to boot — that's tens of thousands of page-fault round-trips on the critical path. Recording the hot set during a template's first warmup fork and prefetching it on every later fork eliminates those round-trips entirely.
Template.HotPagesPath(reserved in PR 2) finally has a producer/consumer.Test plan
go test ./lib/uffd/...(covers HotPageList sort/dedup/save/load + bad-magic + truncation)RecordHotPages: true, save the list, fork without prefetch and time boot; fork with prefetch and time boot; confirm the second is faster🤖 Generated with Claude Code