Message-ID: <4E086F51.50403@draigBrady.com>
Date: Mon, 27 Jun 2011 12:53:53 +0100
From: =?ISO-8859-1?Q?P=E1draig_Brady?= <P@draigBrady.com>
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To: Andrea Righi <andrea@betterlinux.com>
CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        akpm@linux-foundation.org, minchan.kim@gmail.com, riel@redhat.com,
        peterz@infradead.org, hannes@cmpxchg.org,
        kamezawa.hiroyu@jp.fujitsu.com, aarcange@redhat.com, hughd@google.com,
        jamesjer@betterlinux.com, marcus@bluehost.com, matt@bluehost.com,
        tytso@mit.edu, shaohua.li@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE
References: <1308923350-7932-1-git-send-email-andrea@betterlinux.com> <4E07F349.2040900@jp.fujitsu.com> <20110627071139.GC1247@thinkpad> <4E0858CF.6070808@draigBrady.com> <20110627102933.GA1282@thinkpad>
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On 27/06/11 11:29, Andrea Righi wrote:
> 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.

True, though Linux is the reference for posix_fadvise() implementations,
given its lack of support on other platforms.

So just to summarize for _my_ reference.
You're changing DONTNEED to mean "drop if !PageActive()".
I.E. according to http://linux-mm.org/PageReplacementDesign
"drop if files only accessed once".

This will mean that there is no way currently to
remove a particular file from the cache on linux.
Hopefully that won't affect any of:
http://codesearch.google.com/#search/&q=POSIX_FADV_DONTNEED

Ideally I'd like cache functions for:
 DROP, ADD, ADD if space?
which could correspond to:
 DONTNEED, WILLNEED, NOREUSE
but what we're going for are these somewhat overlapping functions:
 DROP if used once?, ADD, ADD if space

cheers,
P?draig.

? Not implemented yet.

? Hopefully there are no access patterns a single
process can do to make a PageActive as that would
probably not be desired in relation to "Drop if used once"
functionality.
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