File trees: the deeper, the weirder

[Kostik Belousov]

On Mon, Oct 30, 2006 at 04:05:19PM +0300, Yar Tikhiy wrote:
> On Sun, Oct 29, 2006 at 11:32:58AM -0500, Matt Emmerton wrote:
> > [ Restoring some OP context.]
> > 
> > > On Sun, Oct 29, 2006 at 05:07:16PM +0300, Yar Tikhiy wrote:
> > >
> > > > As for the said program, it keeps its 1 Hz pace, mostly waiting on
> > > > "vlruwk".  It's killable, after a delay.  The system doesn't show ...
> > > >
> > > > Weird, eh?  Any ideas what's going on?
> > >
> > > I would guess that you need a new vnode to create the new file, but no
> > > vnodes are obvious candidates for freeing because they all have a child
> > > directory in use. Is there some sort of vnode clearing that goes on every
> > > second if we are short of vnodes?
> > 
> > See sys/vfs_subr.c, subroutine getnewvnode().  We call msleep() if we're
> > waiting on vnodes to be created (or recycled).  And just look at the 'hz'
> > parameter passed to msleep()!
> > 
> > The calling process's mkdir() will end up waiting in getnewvnode() (in
> > "vlruwk" state) while the vnlru kernel thread does it's thing (which is to
> > recycle vnodes.)
> > 
> > Either the vnlru kernel thread has to work faster, or the caller has to
> > sleep less, in order to avoid this lock-step behaviour.
> 
> I'm afraid that, though your analysis is right, you arrive at wrong
> conclusions.  The process waits for the whole second in getnewvnode()
> because the vnlru thread cannot free as much vnodes as it wants to.
> vnlru_proc() will wake up sleepers on vnlruproc_sig (i.e.,
> getnewvnode()) only if (numvnodes <= desiredvnodes * 9 / 10).
> Whether this condition is attainable depends on vlrureclaim() (called
> from the vnlru thread) freeing vnodes at a sufficient rate.  Perhaps
> vlrureclaim() just can't keep the pace at this conditions.
> debug.vnlru_nowhere increasing is an indication of that.  Consequently,
> each getnewvnode() call sleeps 1 second, then grabs a vnode beyond
> desiredvnodes.  It's no surprise that the 1 second delays start to
> appear after approx. kern.maxvnodes directories were created.

I think that David is right. The references _from_ the directory make it immune
to vnode reclamation. Try this patch. It is very unfair for lsof.

Index: sys/kern/vfs_subr.c
===================================================================
RCS file: /usr/local/arch/ncvs/src/sys/kern/vfs_subr.c,v
retrieving revision 1.685
diff -u -r1.685 vfs_subr.c
--- sys/kern/vfs_subr.c	2 Oct 2006 07:25:58 -0000	1.685
+++ sys/kern/vfs_subr.c	30 Oct 2006 13:44:59 -0000
@@ -582,7 +582,7 @@
 		 * If it's been deconstructed already, it's still
 		 * referenced, or it exceeds the trigger, skip it.
 		 */
-		if (vp->v_usecount || !LIST_EMPTY(&(vp)->v_cache_src) ||
+		if (vp->v_usecount || /* !LIST_EMPTY(&(vp)->v_cache_src) || */
 		    (vp->v_iflag & VI_DOOMED) != 0 || (vp->v_object != NULL &&
 		    vp->v_object->resident_page_count > trigger)) {
 			VI_UNLOCK(vp);
@@ -607,7 +607,7 @@
 		 * interlock, the other thread will be unable to drop the
 		 * vnode lock before our VOP_LOCK() call fails.
 		 */
-		if (vp->v_usecount || !LIST_EMPTY(&(vp)->v_cache_src) ||
+		if (vp->v_usecount || /* !LIST_EMPTY(&(vp)->v_cache_src) || */
 		    (vp->v_object != NULL && 
 		    vp->v_object->resident_page_count > trigger)) {
 			VOP_UNLOCK(vp, LK_INTERLOCK, td);
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