Re: [RFC PATCH 0/2] Prioritise inodes and zones for writeback required by page reclaim

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Aug 04, 2010 at 03:56:10PM -0700, Andrew Morton wrote:
> On Wed,  4 Aug 2010 15:38:29 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
> 
> > Commenting on the series "Reduce writeback from page reclaim context V6"
> > Andrew Morton noted;
> > 
> >   direct-reclaim wants to write a dirty page because that page is in the
> >   zone which the caller wants to allocate from!  Telling the flusher threads
> >   to perform generic writeback will sometimes cause them to just gum the
> >   disk up with pages from different zones, making it even harder/slower to
> >   allocate a page from the zones we're interested in, no?
> > 
> > On the machines used to test the series, there were relatively few zones
> > and only one BDI so the scenario describes is a possibility. This series is
> > a very early prototype series aimed at mitigating the problem.
> > 
> > Patch 1 adds wakeup_flusher_threads_pages() which takes a list of pages
> > from page reclaim. Each inode belonging to a page on the list is marked
> > I_DIRTY_RECLAIM. When the flusher thread wakes, inodes with this tag are
> > unconditionally moved to the wb->b_io list for writing.
> > 
> > Patch 2 notes that writing back inodes does not necessarily write back
> > pages belonging to the zone page reclaim is concerned with. In response, it
> > adds a zone and counter to wb_writeback_work. As pages from the target zone
> > are written, the zone-specific counter is updated. When the flusher thread
> > then checks the zone counters if a specific zone is being targeted. While
> > more pages may be written than necessary, the assumption is that the pages
> > need cleaning eventually, the inode must be relatively old to have pages at
> > the end of the LRU, the IO will be relatively efficient due to less random
> > seeks and that pages from the target zone will still be cleaned.
> > 
> > Testing did not show any significant differences in terms of reducing dirty
> > file pages being written back but the lack of multiple BDIs and NUMA nodes in
> > the test rig is a problem. Maybe someone else has access to a more suitable
> > test rig.
> > 
> > Any comment as to the suitability for such a direction?
> 
> um.  Might work.  Isn't pretty though.
> 

No, it's not.

> But until we can demonstrate the problem or someone reports it, we
> probably have more important issues to be looking at ;) I think that a
> better approach is to try to trigger this problem as we develop and
> test reclaim. 

That's a reasonable plan as we'll know for sure if this is the right direction
or not. I'll put the patches on the back-burner for now and hopefully someone
will remember them if a bug is reported about large stalls under memory
pressure but that is specific to a machine with many nodes and many disks.

> And if we _can't_ demonstrate it, work out why the heck
> not - either the code's smarter than we thought it was or the test is
> no good.
> 

It's always possible that we won't be able to demonstrate it because the
right file pages are getting cleaned more often than not by the time
reclaim happens :/

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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