Re: [PATCH v2] fadvise: move active pages to inactive list with POSIX_FADV_DONTNEED

On Fri, Jun 24, 2011 at 07:06:42AM +0900, Minchan Kim wrote:

Hi Andrea,
Sorry for late response.
These day, I have no time to see the LKML.

On Thu, Jun 23, 2011 at 3:36 PM, Andrea Righi [off-list ref] wrote:

quoted

There were some reported problems in the past about trashing page cache
when a backup software (i.e., rsync) touches a huge amount of pages (see
for example [1]).

This problem has been almost fixed by the Minchan Kim's patch [2] and a
proper use of fadvise() in the backup software. For example this patch
set [3] has been proposed for inclusion in rsync.

However, there can be still other similar trashing problems: when the
backup software reads all the source files, some of them may be part of
the actual working set of the system. When a
posix_fadvise(POSIX_FADV_DONTNEED) is performed _all_ pages are evicted
from pagecache, both the working set and the use-once pages touched only
by the backup software.

Agreed. It's rather aggressive.

quoted

With the following solution when posix_fadvise(POSIX_FADV_DONTNEED) is
called for an active page instead of removing it from the page cache it
is added to the tail of the inactive list. Otherwise, if it's already in
the inactive list the page is removed from the page cache.

In this way if the backup was the only user of a page, that page will
be immediately removed from the page cache by calling
posix_fadvise(POSIX_FADV_DONTNEED). If the page was also touched by
other processes it'll be moved to the inactive list, having another
chance of being re-added to the working set, or simply reclaimed when
memory is needed.

Testcase:

 - create a 1GB file called "zero"
 - run md5sum zero to read all the pages in page cache (this is to
   simulate the user activity on this file)
 - run "rsync zero zero_copy" (rsync is patched with [3])
 - re-run md5sum zero (user activity on the working set) and measure
   the time to complete this command

The test has been performed using 3.0.0-rc4 vanilla and with this patch
applied (3.0.0-rc4-fadvise).

Results:
                 avg elapsed time      block:block_bio_queue
 3.0.0-rc4                  4.127s                      8,214
 3.0.0-rc4-fadvise          2.146s                          0

Great!

quoted

In the first case the file is evicted from page cache completely and we
must re-read it from the disk. In the second case the file is still in
page cache (in the inactive list) and we don't need any other additional
I/O operation.

[1] http://marc.info/?l=rsync&m=128885034930933&w=2
[2] https://lkml.org/lkml/2011/2/20/57
[3] http://lists.samba.org/archive/rsync/2010-November/025827.html

ChangeLog v1 -> v2:
 - fix comment in invalidate_mapping_pages()

Acked-by: Rik van Riel <redacted>
Reviewed-by: KOSAKI Motohiro <redacted>
Signed-off-by: Andrea Righi <redacted>
---
 mm/swap.c     |    9 +++++----
 mm/truncate.c |   10 +++++++---
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 3a442f1..fc8bb76 100644
--- a/mm/swap.c
+++ b/mm/swap.c

@@ -411,10 +411,11 @@ void add_page_to_unevictable_list(struct page *page)
 *
 * 1. active, mapped page -> none
 * 2. active, dirty/writeback page -> inactive, head, PG_reclaim
- * 3. inactive, mapped page -> none
- * 4. inactive, dirty/writeback page -> inactive, head, PG_reclaim
- * 5. inactive, clean -> inactive, tail
- * 6. Others -> none
+ * 3. active, clean -> inactive, tail
+ * 4. inactive, mapped page -> none
+ * 5. inactive, dirty/writeback page -> inactive, head, PG_reclaim
+ * 6. inactive, clean -> inactive, tail
+ * 7. Others -> none

Nitpick.
I would like to put together them by on line as rather than adding another line.
5, [in]active, clean-> inactive, tail.
I guess it's more easy to understand.

Agreed.

If you want to put it in another line, please change below comment, too.
"In 5, why it moves inactive's head.."

Oh right. I'd put both on a single line anyway, as you suggested.

quoted

 *
 * In 4, why it moves inactive's head, the VM expects the page would
 * be write it out by flusher threads as this is much more effective

diff --git a/mm/truncate.c b/mm/truncate.c
index 3a29a61..a36af48 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c

@@ -357,11 +357,15 @@ unsigned long invalidate_mapping_pages(struct address_space *mapping,
                       if (lock_failed)
                               continue;

-                       ret = invalidate_inode_page(page);

I would like to add comment.
"Invalidation of active page is rather aggressive as we can't make
sure it's not a working set of other processes.
deactivate_page would move it into inactive's tail so the page will
have a chance to activate again if other processes
touch it. otherwise, it would be reclaimed simply".

OK.

quoted

+                       if (PageActive(page))
+                               ret = 0;
+                       else
+                               ret = invalidate_inode_page(page);

You have to change description of invalidate_mapping_pages.

 * invalidate_mapping_pages() will not block on IO activity. It will not
 * invalidate pages which are dirty, locked, under writeback, mapped into
 * pagetables or on active lru.

Correct.

quoted

                       unlock_page(page);
                       /*
-                        * Invalidation is a hint that the page is no longer
-                        * of interest and try to speed up its reclaim.
+                        * Invalidation of an inactive page is a hint that the
+                        * page is no longer of interest and try to speed up
+                        * its reclaim.
                        */
                       if (!ret)
                               deactivate_page(page);
--
1.7.4.1

Otherwise, Looks good to me.

Acked-by: Minchan Kim <redacted>

-- 
Kind regards,
Minchan Kim

Thanks for the review.

I'll add all your comments and post a new version.

-Andrea

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