dirty pages (Was: Re: [PATCH] Prevent large file writeback starvation)

[Johannes Stezenbach <js@linuxtv.org>]

On Mon, Feb 06, 2006, Andrew Morton wrote:
> Mark Lord <lkml@rtr.ca> wrote:
> >
> > A simple test I do for this:
> > 
> >  $ mkdir t
> >  $ cp /usr/src/*.bz2  t    (about 400-500MB worth of kernel tar files)
> > 
> >  In another window, I do this:
> > 
> >  $ while (sleep 1); do echo -n "`date`: "; grep Dirty /proc/meminfo; done
> > 
> >  And then watch the count get large, but take virtually forever
> >  to count back down to a "safe" value.
> > 
> >  Typing "sync" causes all the Dirty pages to immediately be flushed to disk,
> >  as expected.
> 
> I've never seen that happen and I don't recall seeing any other reports of
> it, so your machine must be doing something peculiar.  I think it can
> happen if, say, an inode gets itself onto the wrong inode list, or
> incorrectly gets its dirty flag cleared.
> 
> Are you using any unusual mount options, or unusual combinations of
> filesystems, or anything like that?

I've been seeing something like this for some time, but kept
silent as I'm forced to use vmware on my Thinkpad T42p (1G RAM,
but CONFIG_NOHIGHMEM=y).
Sometimes 'sync' takes serveral seconds, even when the machine
had been idle for >15mins. I don't have laptop mode enabled.
so far I've not found a deterinistic way to reproduce this behaviour.

Anyway, I temporarily deinstalled vmware (deleted the kernel
modules and rebooted; kernel is still tainted because of madwifi
if that matters).
The behaviour I see with vmware (long 'sync' time) doesn't seem
to happen without it so far, but:

Now copying a 700MB file makes "Dirty" go up to 350MB. It then
slowly decreases to 325MB and stays there. However:

$ time sync

real	0m0.326s
user	0m0.000s
sys	0m0.280s

and output from the dirty monitor one-liner:

Mon Feb 13 14:31:43 CET 2006: Dirty:          325916 kB
Mon Feb 13 14:31:44 CET 2006: Dirty:          325916 kB
Mon Feb 13 14:31:45 CET 2006: Dirty:               4 kB
Mon Feb 13 14:31:46 CET 2006: Dirty:               8 kB


Clearly my notebook's hdd isn't that fast. ;-/
What does "Dirty" in /proc/meminfo really mean?

Kernel is 2.6.15, fs is ext3, .config etc. on request.


Johannes