On 23/06/11 14:57, Andrea Righi wrote:

On Thu, Jun 23, 2011 at 12:14:21PM +0100, Padraig Brady wrote:

quoted

On 22/06/11 22:51, Andrea Righi 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.

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

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

Signed-off-by: Andrea Righi <redacted>

Hmm, What if you do want to evict it from the cache for testing purposes?
Perhaps this functionality should be associated with POSIX_FADV_NOREUSE?
dd has been recently modified to support invalidating the cache for a file,
and it uses POSIX_FADV_DONTNEED for that.
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f311553

I don't have any objection to associate POSIX_FADV_NOREUSE to this
functionality. Actually maintaining a specific functionality to drop
file cache pages can be useful, indeed.

However, I'm not sure if POSIX_FADV_NOREUSE or POSIX_FADV_DONTNEED
either are suitable.

According to the standard:

 POSIX_FADV_NOREUSE = data will be accessed only once
 POSIX_FADV_DONTNEED = data will not be accessed in the near future

So, associating the "drop the page cache" semantic sounds like an
implementation detail and applications shouldn't implicitly rely on this
behaviour.

Well the "standard" really is what has been implemented up to now.
POSIX_FADV_NOREUSE currently does nothing so, associating this
new behavior with it seems less problematic for user space.
Also the names fit pretty well I think.

  POSIX_FADV_DONTNEED = drop if possible
  POSIX_FADV_NOREUSE = current app won't reuse so reduce cache eligibility

cheers,
Padraig.

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