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

["Pádraig Brady" <P@draigBrady.com>]

On 22/06/11 22:51, Andrea Righi wrote:
> 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 <andrea@betterlinux.com>

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

cheers,
Padraig.

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