diff options
| -rw-r--r-- | sys/kern/vfs_vnops.c | 33 |
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); } /* |
