Re: [PATCH 5/6] vmscan: Write out ranges of pages contiguous to the inode where possible

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 11 Jun 2010 21:44:11 +0100
Mel Gorman <mel@csn.ul.ie> wrote:

> > Well.  The main problem is that we're doing too much IO off the LRU of
> > course.
> > 
> 
> What would be considered "too much IO"?

Enough to slow things down ;)

This problem used to hurt a lot.  Since those times we've decreased the
default value of /proc/sys/vm/dirty*ratio by a lot, which surely
papered over this problem a lot.  We shouldn't forget that those ratios
_are_ tunable, after all.  If we make a change which explodes the
kernel when someone's tuned to 40% then that's a problem and we'll need
to scratch our heads over the magnitude of that problem.

As for a workload which triggers the problem on a large machine which
is tuned to 20%/10%: dunno.  If we're reliably activating pages when
dirtying them then perhaps it's no longer a problem with the default
tuning.  I'd do some testing with mem=256M though - that has a habit of
triggering weirdnesses.

btw, I'm trying to work out if zap_pte_range() really needs to run
set_page_dirty().  Didn't (pte_dirty() && !PageDirty()) pages get
themselves stamped out?