an easy (?) question on namecache sizing

[Konstantin Belousov]

On Thu, Nov 05, 2015 at 12:25:38PM -0800, Kirk McKusick wrote:
> > Date: Thu, 5 Nov 2015 21:56:48 +0200
> > From: Konstantin Belousov <kostikbel at gmail.com>
> > To: Kirk McKusick <mckusick at mckusick.com>
> > Subject: Re: an easy (?) question on namecache sizing
> > Cc: fs at freebsd.org
> > 
> > On Thu, Nov 05, 2015 at 10:56:55AM -0800, Kirk McKusick wrote:
> >> 
> >> I propose that we update wantfreevnodes in sysctl_update_desiredvnodes()
> >> so that it tracks the change in desiredvnodes:
> >> 
> >> Index: /sys/kern/vfs_subr.c
> >> ===================================================================
> >> --- /sys/kern/vfs_subr.c	(revision 290387)
> >> +++ /sys/kern/vfs_subr.c	(working copy)
> >> @@ -293,6 +293,7 @@
> >>  	if (old_desiredvnodes != desiredvnodes) {
> >> +		wantfreevnodes = desiredvnodes / 4;
> >>  		vfs_hash_changesize(desiredvnodes);
> >>  		cache_changesize(desiredvnodes);
> >>  	}
> >>  	return (0);
> >>  }
> >> 
> >> Otherwise bumping up desiredvnodes will be less effective than expected.
> >> 
> >> I see that Bruce has also suggested this change in his more extensive
> >> revisions.
> > 
> > I think the idea is right, but the implementation is not. Just changing
> > wantfreevnodes after desirevnodes was reduced, creates a window where an
> > other thread could see small value for desiredvnodes, but large value
> > for wantfreevnodes. Then, e.g. vlrureclaim() would go wild. IMO it should
> > ensure that the observable values are non-contradictory.
> 
> Does moving the setting of wantfreevnodes before the cache size changes
> (as redone above) close the window enough? The vlrureclaim() function
> operates slowly enough that a brief period of inconsistency seems
> unimportant. Changing desiredvnodes happens very rarely. And at the moment
> we are not correcting wantfreevnodes at all. Or am I missing some key point?

I think wantfreevnodes should be set before the cache size changes when
desiredvnodes is decreased, but kept at the place in your patch for the
increasing case.