Date:	Fri, 13 Jun 2008 17:53:20 -0500
From:	"Weathers, Norman R." <Norman.R.Weathers@...ocophillips.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	"Jeff Layton" <jlayton@...chiereds.net>,
	<linux-kernel@...r.kernel.org>, <linux-nfs@...r.kernel.org>,
	"Neil Brown" <neilb@...e.de>
Subject: RE: CONFIG_DEBUG_SLAB_LEAK omits size-4096 and larger?

 

> -----Original Message-----
> From: J. Bruce Fields [mailto:bfields@...ldses.org] 
> Sent: Friday, June 13, 2008 5:04 PM
> To: Weathers, Norman R.
> Cc: Jeff Layton; linux-kernel@...r.kernel.org; 
> linux-nfs@...r.kernel.org; Neil Brown
> Subject: Re: CONFIG_DEBUG_SLAB_LEAK omits size-4096 and larger?
> 
> On Fri, Jun 13, 2008 at 04:53:31PM -0500, Weathers, Norman R. wrote:
> >  
> > 
> > > > The big one seems to be the __alloc_skb. (This is with 16 
> > > threads, and
> > > > it says that we are using up somewhere between 12 and 14 GB 
> > > of memory,
> > > > about 2 to 3 gig of that is disk cache).  If I were to 
> put anymore
> > > > threads out there, the server would become almost 
> > > unresponsive (it was
> > > > bad enough as it was).   
> > > > 
> > > > At the same time, I also noticed this:
> > > > 
> > > > skbuff_fclone_cache: 1842524 __alloc_skb+0x50/0x170
> > > > 
> > > > Don't know for sure if that is meaningful or not....
> > > 
> > > OK, so, starting at net/core/skbuff.c, this means that 
> this memory was
> > > allocated by __alloc_skb() calls with something nonzero 
> in the third
> > > ("fclone") argument.  The only such caller is alloc_skb_fclone().
> > > Callers of alloc_skb_fclone() include:
> > > 
> > > 	sk_stream_alloc_skb:
> > > 		do_tcp_sendpages
> > > 		tcp_sendmsg
> > > 		tcp_fragment
> > > 		tso_fragment
> > 
> > Interesting you should mention the tso...  We recently went 
> through and
> > turned on TSO on all of our systems, trying it out to see 
> if it helped
> > with performance...  This could be something to do with 
> that.  I can try
> > disabling the tso on all of the servers and see if that 
> helps with the
> > memory.  Actually, I think I will, and I will monitor the 
> situation.  I
> > think it might help some, but I still think there may be 
> something else
> > going on in a deep corner...
> 
> I'll plead total ignorance about TSO, and it sounds like a long
> shot--but sure, it'd be worth trying, thanks.
> 

Tried it, not for sure if I like the results yet or not...  Didn't seem
to make a huge difference, but here is something that will really make
you want to drink, the 2.6.25.4 kernel does not go into the size-4096
hell.  The largest users of slab there are the size-1024 and still the
skbuff_fclone_cache.  On a box with 16 threads, it will cache up about 5
GB of disk data, and still use about 6 GB of slab to put the information
out there (without TSO on), but at least it is not causing the disk
cache to be evicted, and it appears to be a little more responsive.  If
I up it to 32 or more threads, however, it gets very sluggish, but then
again, I am hitting it with a lot of nodes.

> > 
> > > 		tcp_mtu_probe
> > > 	tcp_send_fin
> > > 	tcp_connect
> > > 	buf_acquire:
> > > 		lots of callers in tipc code (whatever that is).
> > > 
> > > So unless you're using tipc, or you have something in 
> userspace going
> > > haywire (perhaps netstat would help rule that out?), then 
> I suppose
> > > there's something wrong with knfsd's tcp code.  Which 
> makes sense, I
> > > guess.
> > > 
> > 
> > Not for sure what tipc is either....
> > 
> > > I'd think this sort of allocation would be limited by the 
> number of
> > > sockets times the size of the send and receive buffers.
> > > svc_xprt.c:svc_check_conn_limits() claims to be limiting 
> the number of
> > > sockets to (nrthreads+3)*20.  (You aren't hitting the 
> "too many open
> > > connections" printk there, are you?)  The total buffer 
> size should be
> > > bounded by something like 4 megs.
> > > 
> > > --b.
> > > 
> > 
> > Yes, we are getting a continuous stream of the too many 
> open connections
> > scrolling across our logs.  
> 
> That's interesting!  So we should probably look more closely at the
> svc_check_conn_limits() behavior.  I wonder whether some pathological
> behavior is triggered in the case where you're constantly 
> over the limit
> it's trying to enforce.
> 
> (Remind me how many active clients you have?)
> 


We currently are hitting with somewhere around 600 to 800 nodes, but it
can go up to over 1000 nodes.  We are artificially starving with a
limited number of threads (2 to 3) right now on the older 2.6.22.14
kernel because of that memory issue (which may or may not be tso
related)...

I really want to move forward to the newer kernel, but we had an issue
where clients all of the sudden wouldn't connect, yet other clients
could, to the exact same server NFS export.  I had booted the server
into the 2.6.25.4 kernel at the time, and the other admin set us back to
the 2.6.22.14 to see if that was it.  The clients started working again,
and he left it there (he also took out my options in the exports file,
no_subtree_check and insecure).  I know that we are running over the
number of privelaged ports, and we probably need the insecure, but I am
having a hard time wrapping my self around all of the problems at
once....



> > No problems.  I feel good if I exercised some deep corner 
> of the code
> > and found something that needed flushed out, that's what 
> the experience
> > is all about, isn't it?
> 
> Yep!
> 
> --b.
> 
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