In sequential_heuristic():

- spell 16384 as 16384 and not as BKVASIZE.  16384 is (not quite) just a
  magic size that works well in practice.  BKVASIZE should be MAXBSIZE
  (65536), but is 16384 because i386's don't have enough kva for it to
  be MAXBSIZE; 16384 works (not so well) for it for much the same reasons
  that it works well in the heuristic.
- expand and/or add comments about this and other details.
- don't explicitly inline this function.
- fix some other style bugs.

1 files changed, 20 insertions, 13 deletions

diff --git a/sys/kern/vfs_vnops.c b/sys/kern/vfs_vnops.c
index c7df6ad..4826b08 100644
--- a/sys/kern/vfs_vnops.c
+++ b/sys/kern/vfs_vnops.c
@@ -298,35 +298,42 @@ vn_close(vp, flags, file_cred, td)
 }
 
 /*
- * Sequential heuristic - detect sequential operation
+ * Heuristic to detect sequential operation.
  */
-static __inline
-int
+static int
 sequential_heuristic(struct uio *uio, struct file *fp)
 {
 
+	/*
+	 * Offset 0 is handled specially.  open() sets f_seqcount to 1 so
+	 * that the first I/O is normally considered to be slightly
+	 * sequential.  Seeking to offset 0 doesn't change sequentiality
+	 * unless previous seeks have reduced f_seqcount to 0, in which
+	 * case offset 0 is not special.
+	 */
 	if ((uio->uio_offset == 0 && fp->f_seqcount > 0) ||
 	    uio->uio_offset == fp->f_nextoff) {
 		/*
-		 * XXX we assume that the filesystem block size is
-		 * the default.  Not true, but still gives us a pretty
-		 * good indicator of how sequential the read operations
-		 * are.
+		 * f_seqcount is in units of fixed-size blocks so that it
+		 * depends mainly on the amount of sequential I/O and not
+		 * much on the number of sequential I/O's.  The fixed size
+		 * of 16384 is hard-coded here since it is (not quite) just
+		 * a magic size that works well here.  This size is more
+		 * closely related to the best I/O size for real disks than
+		 * to any block size used by software.
 		 */
-		fp->f_seqcount += (uio->uio_resid + BKVASIZE - 1) / BKVASIZE;
+		fp->f_seqcount += howmany(uio->uio_resid, 16384);
 		if (fp->f_seqcount > IO_SEQMAX)
 			fp->f_seqcount = IO_SEQMAX;
-		return(fp->f_seqcount << IO_SEQSHIFT);
+		return (fp->f_seqcount << IO_SEQSHIFT);
 	}
 
-	/*
-	 * Not sequential, quick draw-down of seqcount
-	 */
+	/* Not sequential.  Quickly draw-down sequentiality. */
 	if (fp->f_seqcount > 1)
 		fp->f_seqcount = 1;
 	else
 		fp->f_seqcount = 0;
-	return(0);
+	return (0);
 }
 
 /*