Messages in this thread | | | Date | Mon, 19 May 2008 15:45:54 +1000 | From | David Chinner <> | Subject | Re: [patch 10/21] buffer heads: Support slab defrag |
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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....
Cheers,
Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group
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