HELP! Just 5 minutes! - kworker eating up cpu time when doing dd on 3.2.0 kernels

[Andy Whitcroft]

On Mon, Feb 06, 2012 at 02:33:07PM +0100, Stefan Bader wrote:
> On 06.02.2012 12:06, Andy Whitcroft wrote:
> > On Mon, Feb 06, 2012 at 10:53:04AM +0100, Christian Brandes wrote:
> >> Hi all!
> >>
> >> This takes just 5 minutes!!!!!
> >> Please, could someone help me!
> >>
> >> It looks like a serious bug in 3.2-Kernels.
> >>
> >> Could someone just do "dd if=/dev/zero of=/tmp/zero1.tmp bs=1024 count=13000000" on a precise system with 3.2. kernel and watch CPU usage of kworker threads going up when cache memory is filled up.
> > 
> > Nope could not reproduce it here on a machine with only 2GB of ram; this
> > is with a 3.2.0-12 Ubuntu kernel.  I see the expected kswapd0 and flush-8:0
> > using some 10% between them worst case.  Which kernel are you using,
> > what platform is triggering it.
> > 
> > -apw
> > 
> Same here with 3.2.0-14.23. About 80M/s and same kswapd0 and flush-8:0
> percentages. Using a dual core cpu.
> 
> One thing to verify as well: is /tmp part of / or a tmpfs mount? (though in that
> case one would not expect a 13G write to succeed)...

Indeed.  Also what filesystem type are you using.  My testing was with
ext4.

-apw