Re: [PATCH] mm/readahead.c: update the LRU positions of in-corepages, too

[Chris Frost]

On Sun, Jan 31, 2010 at 10:31:42PM +0800, Wu Fengguang wrote:
> On Tue, Jan 26, 2010 at 09:32:17PM +0800, Wu Fengguang wrote:
> > On Mon, Jan 25, 2010 at 03:36:35PM -0700, Chris Frost wrote:
> > > I changed Wu's patch to add a PageLRU() guard that I believe is required
> > > and optimized zone lock acquisition to only unlock and lock at zone changes.
> > > This optimization seems to provide a 10-20% system time improvement for
> > > some of my GIMP benchmarks and no improvement for other benchmarks.
> 
> I feel very uncomfortable about this put_page() inside zone->lru_lock. 
> (might deadlock: put_page() conditionally takes zone->lru_lock again)
> 
> If you really want the optimization, can we do it like this?

Sorry that I was slow to respond. (I was out of town.)

Thanks for catching __page_cache_release() locking the zone.
I think staying simple for now sounds good. The below locks
and unlocks the zone for each page. Look good?

---
readahead: retain inactive lru pages to be accessed soon
From: Chris Frost <frost@xxxxxxxxxxx>

Ensure that cached pages in the inactive list are not prematurely evicted;
move such pages to lru head when they are covered by
- in-kernel heuristic readahead
- an posix_fadvise(POSIX_FADV_WILLNEED) hint from an application

Before this patch, pages already in core may be evicted before the
pages covered by the same prefetch scan but that were not yet in core.
Many small read requests may be forced on the disk because of this
behavior.

In particular, posix_fadvise(... POSIX_FADV_WILLNEED) on an in-core page
has no effect on the page's location in the LRU list, even if it is the
next victim on the inactive list.

This change helps address the performance problems we encountered
while modifying SQLite and the GIMP to use large file prefetching.
Overall these prefetching techniques improved the runtime of large
benchmarks by 10-17x for these applications. More in the publication
_Reducing Seek Overhead with Application-Directed Prefetching_ in
USENIX ATC 2009 and at http://libprefetch.cs.ucla.edu/.

Signed-off-by: Chris Frost <frost@xxxxxxxxxxx>
Signed-off-by: Steve VanDeBogart <vandebo@xxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 readahead.c |   44 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/mm/readahead.c b/mm/readahead.c
index aa1aa23..c615f96 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -9,7 +9,9 @@
 
 #include <linux/kernel.h>
 #include <linux/fs.h>
+#include <linux/memcontrol.h>
 #include <linux/mm.h>
+#include <linux/mm_inline.h>
 #include <linux/module.h>
 #include <linux/blkdev.h>
 #include <linux/backing-dev.h>
@@ -133,6 +135,40 @@ out:
 }
 
 /*
+ * The file range is expected to be accessed in near future.  Move pages
+ * (possibly in inactive lru tail) to lru head, so that they are retained
+ * in memory for some reasonable time.
+ */
+static void retain_inactive_pages(struct address_space *mapping,
+				  pgoff_t index, int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++) {
+		struct page *page;
+		struct zone *zone;
+
+		page = find_get_page(mapping, index + i);
+		if (!page)
+			continue;
+		zone = page_zone(page);
+		spin_lock_irq(&zone->lru_lock);
+
+		if (PageLRU(page) &&
+			!PageActive(page) &&
+			!PageUnevictable(page)) {
+			int lru = page_lru_base_type(page);
+
+			del_page_from_lru_list(zone, page, lru);
+			add_page_to_lru_list(zone, page, lru);
+		}
+
+		spin_unlock_irq(&zone->lru_lock);
+		put_page(page);
+	}
+}
+
+/*
  * __do_page_cache_readahead() actually reads a chunk of disk.  It allocates all
  * the pages first, then submits them all for I/O. This avoids the very bad
  * behaviour which would occur if page allocations are causing VM writeback.
@@ -184,6 +220,14 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 	}
 
 	/*
+	 * Normally readahead will auto stop on cached segments, so we won't
+	 * hit many cached pages. If it does happen, bring the inactive pages
+	 * adjecent to the newly prefetched ones(if any).
+	 */
+	if (ret < nr_to_read)
+		retain_inactive_pages(mapping, offset, page_idx);
+
+	/*
 	 * Now start the IO.  We ignore I/O errors - if the page is not
 	 * uptodate then the caller will launch readpage again, and
 	 * will then handle the error.

-- 
Chris Frost
http://www.frostnet.net/chris/
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