Re: [PATCH 30/45] vmscan: lumpy pageout

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Oct 07, 2009 at 09:29:24PM +0800, Wu Fengguang wrote:
> On Wed, Oct 07, 2009 at 07:37:35PM +0800, Nikita Danilov wrote:
> > 2009/10/7 Wu Fengguang <fengguang.wu@intel.com>:
> > > On Wed, Oct 07, 2009 at 06:38:37PM +0800, Nikita Danilov wrote:
> > >> Hello,
> > >>
> > 
> > [...]
> > 
> > >
> > > Glad to know about your experiences :) Interestingly I started with
> > > ->writepage() and then switch to ->writepages() because filesystems
> > > behave better with the latter (i.e. less file fragmentation).
> > 
> > By the way, why is your patch doing
> > 
> >         ->writepage(page->index);
> >         generic_writepages(page->index + 1, LUMPY_PAGEOUT_PAGES - 1);
> > 
> > instead of
> > 
> >         generic_writepages(page->index, LUMPY_PAGEOUT_PAGES);
> > 
> > ? Is this because of the difficulties with passing back page specific
> > errors from generic_writepages()?
> 
> Yes. It's possible to tell write_cache_pages() to return
> AOP_WRITEPAGE_ACTIVATE. Other ->writepages() don't have to deal with
> this because their ->writepage() won't return AOP_WRITEPAGE_ACTIVATE
> at all.
> 
> But it is going to be ugly to specialize the first locked page in
> every ->writepages() functions..
> 
> > >
> > > I'd like to just ignore the shmem case, by adding a
> > > bdi_cap_writeback_dirty() check. Because clustered writing to swap
> > > device may be a less gain.
> > 
> > Or you can just call try_to_unmap() from shmem_writepage() when
> > wbc->for_reclaim is true.
> 
> Hmm, it's more comfortable to stay away from shmem for the initial version.
> But feel free to submit a patch for it in future :)
> 
> > > Page filtering should also be possible in write_cache_pages().  But
> > > what do you mean by "hard-to-fix races against inode reclamation"?
> > 
> > vmscan.c pageout path doesn't take a reference on inode, so the
> > instant ->writepage() releases lock on the page, the inode can be
> > freed.
> 
> Ah, then we could just do igrab() if inode_lock is not locked?
> A bit ugly though.

Since writepage() can sleep, we don't need to worry about inode_lock.

Here is the updated patch.

Thanks,
Fengguang
---

vmscan: lumpy pageout

When pageout a dirty page, try to piggy back more consecutive dirty
pages (up to 512KB) to improve IO efficiency.

Only ext3/reiserfs which don't have its own aops->writepages are
supported in this initial version.

CC: Dave Chinner <david@fromorbit.com>
CC: Nikita Danilov <danilov@gmail.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/page-writeback.c |   12 ++++++++++++
 mm/vmscan.c         |   16 ++++++++++++++++
 2 files changed, 28 insertions(+)

--- linux.orig/mm/vmscan.c	2009-10-07 21:39:13.000000000 +0800
+++ linux/mm/vmscan.c	2009-10-07 21:39:57.000000000 +0800
@@ -344,6 +344,8 @@ typedef enum {
 	PAGE_CLEAN,
 } pageout_t;
 
+#define LUMPY_PAGEOUT_PAGES	(512 * 1024 / PAGE_CACHE_SIZE)
+
 /*
  * pageout is called by shrink_page_list() for each dirty page.
  * Calls ->writepage().
@@ -399,16 +401,30 @@ static pageout_t pageout(struct page *pa
 			.for_reclaim = 1,
 		};
 
+		igrab(mapping->host);
 		SetPageReclaim(page);
 		res = mapping->a_ops->writepage(page, &wbc);
 		if (res < 0)
 			handle_write_error(mapping, page, res);
 		if (res == AOP_WRITEPAGE_ACTIVATE) {
 			ClearPageReclaim(page);
+			iput(mapping->host);
 			return PAGE_ACTIVATE;
 		}
 
 		/*
+		 * only write_cache_pages() supports for_reclaim for now
+		 * ignore shmem for now, thanks to Nikita.
+		 */
+		if (bdi_cap_writeback_dirty(mapping->backing_dev_info) &&
+		    !mapping->a_ops->writepages) {
+			wbc.range_start = (page->index + 1) << PAGE_CACHE_SHIFT;
+			wbc.nr_to_write = LUMPY_PAGEOUT_PAGES - 1;
+			generic_writepages(mapping, &wbc);
+			iput(mapping->host);
+		}
+
+		/*
 		 * Wait on writeback if requested to. This happens when
 		 * direct reclaiming a large contiguous area and the
 		 * first attempt to free a range of pages fails.
--- linux.orig/mm/page-writeback.c	2009-10-07 21:39:13.000000000 +0800
+++ linux/mm/page-writeback.c	2009-10-07 21:39:14.000000000 +0800
@@ -805,6 +805,11 @@ int write_cache_pages(struct address_spa
 				break;
 			}
 
+			if (wbc->for_reclaim && done_index != page->index) {
+				done = 1;
+				break;
+			}
+
 			if (nr_to_write != wbc->nr_to_write &&
 			    done_index + WB_SEGMENT_DIST < page->index &&
 			    --wbc->nr_segments <= 0) {
@@ -846,6 +851,13 @@ continue_unlock:
 			if (!clear_page_dirty_for_io(page))
 				goto continue_unlock;
 
+			/*
+			 * active and unevictable pages will be checked at
+			 * rotate time
+			 */
+			if (wbc->for_reclaim)
+				SetPageReclaim(page);
+
 			ret = (*writepage)(page, wbc, data);
 			if (unlikely(ret)) {
 				if (ret == AOP_WRITEPAGE_ACTIVATE) {
--
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