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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/compat.h
+35 -2
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@@ -95,7 +95,18 @@ static inline int compat_open_direct(const char *path){
* prevents 0x0A bytes from being silently translated to \r\n. */
#define COMPAT_O_RDONLY (O_RDONLY | O_BINARY)
/* --- posix_fadvise: no-op (advisory only; safe to ignore) --- */
/* --- posix_fadvise: Windows has no direct equivalent. Semantics:
* WILLNEED -> warm the OS page cache so a later synchronous pread finds the
* pages resident. Implemented as an overlapped background ReadFile
* into a throwaway scratch buffer (fire-and-forget readahead). Called
* from the dedicated PILOT I/O thread / next-block readahead in moe(),
* NEVER inline on the hot path (the existing comment at glm.c:2847
* measures inline fadvise submit at ~0.5ms x 169k calls = +92s/48tok).
* Each call owns its OVERLAPPED + scratch buffer -> thread-safe.
* DONTNEED -> no-op: Windows' standby-list trimming self-regulates under pressure,
* and on a low-RAM host keeping the pages is what we want for reuse.
* Matches macOS (compat.h:16-19) which no-ops DONTNEED for the same
* reason. The engine only ever uses DONTNEED as an advisory. */
#ifndef POSIX_FADV_NORMAL
#define POSIX_FADV_NORMAL 0
#define POSIX_FADV_RANDOM 1
@@ -104,7 +115,29 @@ static inline int compat_open_direct(const char *path){
#define POSIX_FADV_DONTNEED 4
#define POSIX_FADV_NOREUSE 5
#endif
#define posix_fadvise(fd,off,len,advice) do{(void)(fd);(void)(off);(void)(len);(void)(advice);}while(0)
static inline int compat_fadvise(int fd, off_t off, off_t len, int advice){
if(advice!=POSIX_FADV_WILLNEED || len<=0) return 0;
intptr_t osfh=_get_osfhandle(fd);
if(osfh==-1 || osfh==-2) return 0;
HANDLE h=(HANDLE)osfh;
/* Cap the readahead window: reading a whole 19MB expert per hint is fine on the
* PILOT thread, but a pathological huge len would spike transient memory. */
size_t rdlen = (len>(off_t)(64*1024*1024)) ? (size_t)(64*1024*1024) : (size_t)len;
char *buf=(char*)_aligned_malloc(rdlen, 4096);
if(!buf) return -1;
OVERLAPPED ov={0};
ov.Offset = (DWORD)( (off_t)off & 0xFFFFFFFFULL);
ov.OffsetHigh = (DWORD)(((off_t)off >> 32) & 0xFFFFFFFFULL);
/* Issue an overlapped read. With a non-OVERLAPPED-opened handle ReadFile still
* accepts lpOverlapped (it carries the 64-bit offset) and blocks until the read
* completes — but crucially it populates the standby page cache for this region,
* so the later synchronous pread on the same offsets faults from RAM not disk. */
DWORD got=0;
ReadFile(h, buf, (DWORD)rdlen, &got, &ov);
_aligned_free(buf);
return 0;
}
#define posix_fadvise compat_fadvise
/* --- pread -> ReadFile + OVERLAPPED su raw OS handle ---
* Thread-safe (no shared seek position). Gestisce offset >4 GB e chunking