Date: Sat, 18 Nov 2006 12:54:00 +0300 From: Yar Tikhiy <yar@comp.chem.msu.su> To: Kostik Belousov <kostikbel@gmail.com> Cc: David Malone <dwmalone@maths.tcd.ie>, hackers@freebsd.org Subject: Re: File trees: the deeper, the weirder Message-ID: <20061118095400.GE68439@comp.chem.msu.su> In-Reply-To: <20061030134737.GF1627@deviant.kiev.zoral.com.ua> References: <20061029140716.GA12058@comp.chem.msu.su> <20061029152227.GA11826@walton.maths.tcd.ie> <006801c6fb77$e4e30100$1200a8c0@gsicomp.on.ca> <20061030130519.GE27062@comp.chem.msu.su> <20061030134737.GF1627@deviant.kiev.zoral.com.ua>
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On Mon, Oct 30, 2006 at 03:47:37PM +0200, Kostik Belousov wrote:
> 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);
By the way, what do you think v_cache_src is for? The only two
places it is used in the kernel are in the unused function
cache_leaf_test() and this one, in vlrureclaim(). Is its main
purpose just to keep directory vnodes that are referenced by nc_dvp
in some namecache entries?
--
Yar
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