netdev - Re: ATA over ethernet swapping

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Date:	Mon, 13 Aug 2007 09:45:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pavel Machek <pavel@....cz>
Cc:	"Ed L. Cashin" <ecashin@...aid.com>,
	kernel list <linux-kernel@...r.kernel.org>, ak@...e.de,
	Netdev list <netdev@...r.kernel.org>
Subject: Re: ATA over ethernet swapping

On Thu, 2007-08-09 at 12:11 +0200, Pavel Machek wrote:
> Hi!
> 
> > I've been working on this for quite some time. And should post again
> > soon. Please see the patches:
> > 
> >   http://programming.kicks-ass.net/kernel-patches/vm_deadlock/current/
> > 
> > For now it requires one uses SLUB, I hope that SLAB will go away (will
> > save me the trouble of adding support) and I guess I ought to do SLOB
> > some time (if that does stay).
> > 
> > You'd need the first 22 patches of that series, and then call
> > sk_set_memalloc(sk) on the proper socket, and do some fiddling with the
> > reconnect logic. See nfs-swapfile.patch for examples.
> 
> What do you use for testing? I set up ata over ethernet... swapping
> over that should deadlock w/o your patches.
> 
> But I'm able to compile kernel (-j 10) on 128MB machine, and I tried
> cat /dev/zero | grep foo to exhaust memory... and could not reproduce
> the deadlock. Should I pingflood? Tweak down ammount of atomic memory
> avaialable to make deadlocks easier to reproduce?

I usually test swap over NFS in the following manner, I setup a regular
inet service on the machine (apache or a bunch of ncat sockets piping to
files or something) and run a heavy workload on the machine (128M):
2*64M file backed thrashers and 2*64M anonymous thrashers. Then I start
clients for the regular inet service, wait for a bit, and shut down the
NFS server.

This makes the machine grind to a halt, I then restart the NFS server,
wait for it to reconnect and the client to come alive again.

Without the last few swap-over-NFS patches this last bit - getting back
out of that situation - never happens.

The basic idea is to make connectivity to the machine where swap traffic
goes very hard (pull a cable, cleanly shut down the server) and to keep
other network traffic pounding the machine.

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