Re: [PATCH 12/14] vmscan: Do not writeback pages in direct reclaim

[Mel Gorman]

On Wed, Jul 07, 2010 at 01:03:38PM +0800, Wu Fengguang wrote:
> Hi Mel,
> 
> > Second, using systemtap, I was able to see that file-backed dirty
> > pages have a tendency to be near the end of the LRU even though they
> > are a small percentage of the overall pages in the LRU. I'm hoping
> > to figure out why this is as it would make avoiding writeback a lot
> > less controversial.
> 
> Your intuitions are correct -- the current background writeback logic
> fails to write elder inodes first. Under heavy loads the background
> writeback job may run for ever, totally ignoring the time order of
> inode->dirtied_when. This is probably why you see lots of dirty pages
> near the end of LRU.
> 

Possible. In a mail to Christoph, I asserted that writeback of elder inodes
was happening first but I obviously could be mistaken.

> Here is an old patch for fixing this. Sorry for being late. I'll
> pick up and refresh the patch series ASAP.  (I made a mistake last
> year to post too many patches at one time. I'll break them up into
> more manageable pieces.)
> 
> [PATCH 31/45] writeback: sync old inodes first in background writeback
> <https://kerneltrap.org/mailarchive/linux-fsdevel/2009/10/7/6476313>
> 

I'll check it out as an alternative to forward-flushing based on the
amount of dirty pages encountered during scanning. Thanks.

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