On Mon, Jun 27, 2011 at 11:17:51AM +0100, Padraig Brady wrote:

On 27/06/11 08:11, Andrea Righi wrote:

quoted

On Mon, Jun 27, 2011 at 12:04:41PM +0900, KOSAKI Motohiro wrote:

quoted

(2011/06/24 22:49), 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_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.

A previous proposal [4] tried to resolve this problem being less
agressive in invalidating active pages, moving them to the inactive list
intead of just evict them from the page cache.

However, this approach changed completely the old behavior of
invalidate_mapping_pages(), that is not only used by fadvise.

The new solution maps POSIX_FADV_NOREUSE to the less-agressive page
invalidation policy.

With POSIX_FADV_NOREUSE active pages are moved to the tail of the
inactive list, and pages in the inactive list are just removed from page
cache. Pages mapped by other processes or unevictable pages are not
touched at all.

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_FADV_NOREUSE.
If the page was also touched by other tasks 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.

In conclusion, now userspace applications that want to drop some page
cache pages can choose between the following advices:

 POSIX_FADV_DONTNEED = drop page cache if possible
 POSIX_FADV_NOREUSE = reduce page cache eligibility

Eeek.

Your POSIX_FADV_NOREUSE is very different from POSIX definition.
POSIX says,

       POSIX_FADV_NOREUSE
              Specifies that the application expects to access the specified data once  and  then
              not reuse it thereafter.

IfI understand correctly, it designed for calling _before_ data access
and to be expected may prevent lru activation. But your NORESE is designed
for calling _after_ data access. Big difference might makes a chance of
portability issue.

You're right. NOREUSE is designed to implement drop behind policy.

Hmm fair enough.
NOREUSE is meant for specifying you _will_ need the data _once_

Isn't this what rsync actually wants though?
I.E. to specify NOREUSE for the file up front
so it would drop from cache automatically as processed,
(if not already in cache).

I realize that would be a more invasive patch.

quoted

I'll post a new patch that will plug this logic in DONTNEED (like the
presious version), but without breaking the old /proc/sys/vm/drop_caches
behavior.

But will that break existing apps (running as root) that expect DONTNEED
to drop cache for a _file_.  Perhaps posix_fadvise() is meant to have
process rather than system scope, but that has not been the case until now.

The actual problem I think is that apps expect that DONTNEED can be used
to drop cache, but this is not written anywhere in the POSIX standard.

I would also like to have both functionalities: 1) be sure to drop page
cache pages (now there's only a system-wide knob to do this:
/proc/sys/vm/drop_caches), 2) give an advice to the kernel that I will
not reuse some pages in the future.

The standard can only provide 2). If we also want 1) at the file
granularity, I think we'd need to introduce something linux specific to
avoid having portability problems.

-Andrea

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