Re: [patch 10/21] buffer heads: Support slab defrag

From: David Chinner
Date: Mon May 19 2008 - 01:46:38 EST

On Fri, May 16, 2008 at 10:01:38AM -0700, Christoph Lameter wrote:
> On Fri, 16 May 2008, David Chinner wrote:
> > On Thu, May 15, 2008 at 10:42:15AM -0700, Christoph Lameter wrote:
> > > On Mon, 12 May 2008, David Chinner wrote:
> > >
> > > > If you are going to clean bufferheads (or pages), please clean entire
> > > > mappings via ->writepages as it leads to far superior I/O patterns
> > > > and a far higher aggregate rate of page cleaning.....
> > >
> > > That brings up another issue: Lets say I use writepages on a large file
> > > (couple of gig). How much do you want to write back?
> >
> > We're out of memory. I'd suggest write backing as much as you can
> > without blocking. e.g. treat it like pdflush and say 1024 pages, or
> > like balance_dirty_pages() and write a 'write_chunk' back from the
> > mapping (i.e. sync_writeback_pages()).
> Why are we out of memory?

Defragmentation is triggered as part of the usual memory reclaim
process. Which implies we've run out of free memory, correct?

> How do you trigger such a special writeout?

filemap_fdatawrite_range() perhaps?

> > Any of these are better from an I/O perspective than single page
> > writeback....
> But then filesystem can do tricks like writing out the surrounding areas
> as needed. The filesystem likely can estimate better how much writeout
> makes sense.

Pushing write-around into a method that is only supposed to write
the single page that is passed to it is a pretty bad abuse of the
API. Especially as we have many simple, ranged writeback methods
you could call. filemap_fdatawrite_range(), do_writepages(),
->writepages, etc.

FWIW, look at the mess of layering violations that write clustering
causes in XFS because we have to do this to keep allocation overhead
and fragmentation down to a minimum. It's a nasty hack to mitigate
the impact of the awful I/O patterns we see from the VM - suggesting
that all filesystems do this just so you don't have to call a
slightly smarter writeback primitive is insane....


Dave Chinner
Principal Engineer
SGI Australian Software Group
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