Change default VFS timestamp precision?

[John Baldwin]

On Tuesday, December 16, 2014 1:51:09 pm Warner Losh wrote:
> 
> > On Dec 16, 2014, at 11:48 AM, John Baldwin <jhb at freebsd.org> wrote:
> > 
> > We still ship with vfs.timestamp_precision=0 by default meaning that VFS
> > timestamps have a granularity of one second.  It is not unusual on modern
> > systems for multiple updates to a file or directory to occur within a single
> > second (and thus share the same effective timestamp).  This can break things
> > that depend on timestamps to know when something has changed or is stale (such
> > as make(1) or NFS clients).  On hardware that has a cheap timecounter, I we
> > should use the most-precise timestamps (vfs.timestamp_precision=3).  However,
> > I'm less sure of what to do for other cases such as i386/amd64 when not using
> > TSC, or on other platforms.  OTOH, perhaps you aren't doing lots of heavy I/O
> > access on a system with a slow timecounter (or if you are doing heavy I/O,
> > slow timecounter access won't be your bottleneck)?
> > 
> > I can think of a few options:
> > 
> > 1) Change vfs.timestamp_precision default to 3 for all systems.
> > 
> > 2) Only change vfs.timestamp_precision default to 3 for amd64/i386 using an
> >    #ifdef.
> > 
> > 3) Something else?
> > 
> > What do other folks think?
> 
> (1). If there’s a specific kernel / platform that’s slow, we can make it an option
> for those kernels.

Mm, so something like:

Index: vfs_subr.c
===================================================================
--- vfs_subr.c	(revision 275828)
+++ vfs_subr.c	(working copy)
@@ -632,7 +632,11 @@ vfs_getnewfsid(struct mount *mp)
  */
 enum { TSP_SEC, TSP_HZ, TSP_USEC, TSP_NSEC };
 
+#ifdef VFS_CHEAP_TIMESTAMPS
 static int timestamp_precision = TSP_SEC;
+#else
+static int timestamp_precision = TSP_NSEC;
+#endif
 SYSCTL_INT(_vfs, OID_AUTO, timestamp_precision, CTLFLAG_RW,
     &timestamp_precision, 0, "File timestamp precision (0: seconds, "
     "1: sec + ns accurate to 1/HZ, 2: sec + ns truncated to ms, "

where VFS_CHEAP_TIMESTAMPS becomes a new kernel config option?

(We should also probably make this a loader tunable)

-- 
John Baldwin