Artifact f3becbedd1ee8df30506ce89788f2b52ada0696e55bc2c20dc1347784d680497:
Profiling a real pkgsrc-scale import put readv at over half of all CPU samples. A block cache in front of pread is the shape that addresses it: the store is append-only, so a block is retained only when the read that filled it came back FULL -- which proves those bytes already existed, so appends cannot alter them and need no invalidation. Only truncate does.
Deliberately not attached to any caller. Wired into import-git it reached a 99 percent hit rate over 392,421 reads and did NOT make a small corpus faster: at that size the page cache already served those reads, so the syscalls were cheap and the lookup was pure overhead. The prior from the real profile is strong but unproven at scale, and shipping it on 'should help' is how the reverted top-level cache in issue 32 happened.
useReadCache is the one-line opt-in for profiling it. (#39) by dmestas on 2026-08-07 13:03:45.
C storage:\sa\sblock\scache\sfor\sFileStorage\sreads,\sopt-in\sand\snot\syet\swired\n\nProfiling\sa\sreal\spkgsrc-scale\simport\sput\sreadv\sat\sover\shalf\sof\sall\sCPU\nsamples.\sA\sblock\scache\sin\sfront\sof\spread\sis\sthe\sshape\sthat\saddresses\sit:\nthe\sstore\sis\sappend-only,\sso\sa\sblock\sis\sretained\sonly\swhen\sthe\sread\sthat\nfilled\sit\scame\sback\sFULL\s--\swhich\sproves\sthose\sbytes\salready\sexisted,\sso\nappends\scannot\salter\sthem\sand\sneed\sno\sinvalidation.\sOnly\struncate\sdoes.\n\nDeliberately\snot\sattached\sto\sany\scaller.\sWired\sinto\simport-git\sit\sreached\na\s99\spercent\shit\srate\sover\s392,421\sreads\sand\sdid\sNOT\smake\sa\ssmall\scorpus\nfaster:\sat\sthat\ssize\sthe\spage\scache\salready\sserved\sthose\sreads,\sso\sthe\nsyscalls\swere\scheap\sand\sthe\slookup\swas\spure\soverhead.\sThe\sprior\sfrom\sthe\nreal\sprofile\sis\sstrong\sbut\sunproven\sat\sscale,\sand\sshipping\sit\son\s'should\nhelp'\sis\show\sthe\sreverted\stop-level\scache\sin\sissue\s32\shappened.\n\nuseReadCache\sis\sthe\sone-line\sopt-in\sfor\sprofiling\sit.\s(#39) 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