Re: [PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE

[KOSAKI Motohiro]

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





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