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In sequential_heuristic():

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- 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.
This commit is contained in:
Bruce Evans 2008-01-05 08:54:51 +00:00
parent 4113f8d741
commit 9283848511
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=175106
1 changed files with 20 additions and 13 deletions
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33
sys/kern/vfs_vnops.c
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@ -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);
}
/*