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

From: Andrew Morton
Date: Fri Jun 11 2010 - 17:35:07 EST

On Fri, 11 Jun 2010 21:44:11 +0100
Mel Gorman <mel@xxxxxxxxx> 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?

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