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windows: PIPE default ON + compat_fadvise WILLNEED cache-warmer (pread path)

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Cut Windows decode disk I/O from 2.06s/tok to 1.70s/tok (budget=4), meeting the
<=2s/tok target. Two changes on the pread expert-load path:

1. compat.h: replace the posix_fadvise no-op with a real WILLNEED cache-warmer
   (overlapped ReadFile into a scratch buffer -> populates the standby page cache
   so the later synchronous pread faults from RAM). Re-arms the existing
   expert_prefetch/PILOT/next-block prefetch chain on Windows. DONTNEED stays a
   no-op (matches macOS; Windows standby-list trimming self-regulates).
   Measured: hit rate 16.4% -> 27.6%.

2. glm.c: flip PIPE (async expert-load thread pool) from default OFF to default ON
   on Windows. Dispatches expert pread onto worker threads so loads overlap the
   matmul, instead of blocking serial load-then-compute. PIPE=0 opts out.
   Measured: expert-disk 65.9s -> 54.3s (-18%).

Also adds compat_fadvise assertions to tests/test_compat_direct.c (data integrity
after cache-warmer, safe no-op on bad fd / non-WILLNEED).

mmap (CreateFileMapping/MapViewOfFile) was implemented and tested at length but
reverted: it regressed on Windows (RSS bloat from touched mapped pages collapsed
the expert cache via ullAvailPhys — a fundamental Windows-vs-Linux difference).
Full findings + the dead-end analysis recorded in issue_diskio.md.
This commit is contained in:
woolcoxm
2026-07-15 06:14:15 -04:00
parent 9c24b1eceb
commit 25219de45b
5 changed files with 178 additions and 5 deletions
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c/tests/test_compat_direct.c
+17
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@@ -51,6 +51,23 @@ int main(void){
if(compat_open_direct("no_such_file.tmp")>=0) return fail("open missing file must fail");
if(compat_fsize(-1)>=0) return fail("compat_fsize on bad fd must be negative");
/* compat_fadvise: WILLNEED warms the page cache (background read into throwaway
* buffer), DONTNEED is a documented no-op. After a WILLNEED the buffered fd's
* subsequent pread must still return the exact bytes — the cache-warmer must not
* corrupt data. Bad fd / non-WILLNEED advice must be safe no-ops (return 0). */
int wfd = open(TMPF, COMPAT_O_RDONLY);
if(wfd<0) return fail("open buffered for fadvise");
if(posix_fadvise(wfd, 0, (off_t)FSZ, POSIX_FADV_WILLNEED)!=0) return fail("WILLNEED returned nonzero");
if(posix_fadvise(wfd, 0, (off_t)FSZ, POSIX_FADV_DONTNEED)!=0) return fail("DONTNEED should be a safe no-op (return 0)");
if(posix_fadvise(-1, 0, (off_t)FSZ, POSIX_FADV_WILLNEED)!=0) return fail("WILLNEED on bad fd should no-op (return 0)");
if(posix_fadvise(wfd, 0, 0, POSIX_FADV_WILLNEED)!=0) return fail("WILLNEED with len<=0 should no-op");
/* verify data integrity through the buffered fd after the cache-warmer ran */
uint8_t *verify=malloc(FSZ);
if(pread(wfd, verify, FSZ, 0)!=(ssize_t)FSZ) return fail("fadvise: pread size");
if(memcmp(verify, pat, FSZ)!=0) return fail("fadvise: data corrupted by cache-warmer");
free(verify);
close(wfd);
close(dfd);
compat_aligned_free(buf); free(pat); remove(TMPF);
puts("compat direct tests: ok");