Commit dd144a19881d94b2060e3596057199c71137cad3 | zig

author	Ryan Liptak <squeek502@hotmail.com> 2026-09-28 18:00:39-07:00
committer	Ryan Liptak <squeek502@hotmail.com> 2026-09-28 18:00:39-07:00
log	dd144a19881d94b2060e3596057199c71137cad3
tree	68ba65d742bb48d53376798757bf64c82665dffc
parent	070294b36ca0922aaffdc0b7c669b189f5f147c1

Threaded.createFileMap: Update comments to reflect Windows prefetch efficacy findings

With a test case that (1) calls createFileMap on a 250MiB text file, and (2) reads every byte in the mapped region looking for `\n`, hard page faults were eliminated in the `populate = true` case. `rammap -Et` is used to clear the relevant Windows cache.

```
> rammap -Et
> bench-no-populate.exe
.{ .page = 1090, .hard = 0 } // initial count
.{ .page = 1112, .hard = 0 } // after createFileMap
.{ .page = 1114, .hard = 0 } // after 1s sleep
4194304 newlines in the file
.{ .page = 66779, .hard = 255 }

> rammap -Et
> bench-populate.exe
.{ .page = 1093, .hard = 0 } // initial count
.{ .page = 1242, .hard = 0 } // after createFileMap
.{ .page = 1244, .hard = 0 } // after 1s sleep
4194304 newlines in the file
.{ .page = 66782, .hard = 1 }
```

More details are in https://codeberg.org/ziglang/zig/pulls/36964

1 files changed, 6 insertions(+), 4 deletions(-)

lib
- std
  - Io
    Threaded.zig

lib/std/Io/Threaded.zig+6-4

 @@ -19089,7 +19089,7 @@ fn createFileMap(
             page,
             .{
                 // Required when calling NtCreateSection with a file handle, otherwise
-                // this call will fail with INVALID_PARAMETER. The higher level
+                // this call will fail with INVALID_PARAMETER_6. The higher level
                 // `CreateFileMapping` function will set this bit for you when it
                 // calls NtCreateSection.
                 .COMMIT = true,
 @@ -19140,11 +19140,13 @@ fn createFileMap(
                     .TO_WORKING_SET = false,
                 },
             };
-            // It appears that this is effectively a no-op, as it doesn't decrease page/hard faults.
+            // There has been some doubt about the efficacy of this function:
             // https://github.com/microsoft/Windows-Dev-Performance/issues/108
             //
-            // We still call it, though, to signal the intention of doing something useful.
+            // However, through testing it has been shown that, if the mapped file is not
-            // It doesn't appear to make anything *worse*, so there's no harm in doing so.
+            // in the cache, calling this function can turn what might otherwise be hard
+            // faults into soft faults. Either way, it's still good to call it in order to
+            // accurately reflect our intent.
             switch (windows.ntdll.NtSetInformationVirtualMemory(
                 windows.current_process,
                 .Prefetch,