Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone pressure

[Rik van Riel]

On 04/21/2010 03:35 AM, Christian Ehrhardt wrote:
> Christian Ehrhardt wrote:
> > Rik van Riel wrote:
> > > On 04/20/2010 11:32 AM, Johannes Weiner wrote:
> > >
> > > > The idea is that it pans out on its own. If the workload changes, new
> > > > pages get activated and when that set grows too large, we start
> > > > shrinking
> > > > it again.
> > > >
> > > > Of course, right now this unscanned set is way too large and we can end
> > > > up wasting up to 50% of usable page cache on false active pages.
> > >
> > > Thing is, changing workloads often change back.
> > >
> > > Specifically, think of a desktop system that is doing
> > > work for the user during the day and gets backed up
> > > at night.
> > >
> > > You do not want the backup to kick the working set
> > > out of memory, because when the user returns in the
> > > morning the desktop should come back quickly after
> > > the screensaver is unlocked.
> >
> > IMHO it is fine to prevent that nightly backup job from not being
> > finished when the user arrives at morning because we didn't give him
> > some more cache - and e.g. a 30 sec transition from/to both optimized
> > states is fine.
> > But eventually I guess the point is that both behaviors are reasonable
> > to achieve - depending on the users needs.
> >
> > What we could do is combine all our thoughts we had so far:
> > a) Rik could create an experimental patch that excludes the in flight
> > pages
> > b) Johannes could create one for his suggestion to "always scan active
> > file pages but only deactivate them when the ratio is off and
> > otherwise strip buffers of clean pages"

I think you are confusing "buffer heads" with "buffers".

You can strip buffer heads off pages, but that is not
your problem.

"buffers" in /proc/meminfo stands for cached metadata,
eg. the filesystem journal, inodes, directories, etc...
Caching such metadata is legitimate, because it reduces
the number of disk seeks down the line.
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