linux-kernel - Re: [PATCH v3 0/2] fadvise: support POSIX_FADV

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Date:	Mon, 27 Jun 2011 12:53:53 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Andrea Righi <andrea@...terlinux.com>
CC:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	akpm@...ux-foundation.org, minchan.kim@...il.com, riel@...hat.com,
	peterz@...radead.org, hannes@...xchg.org,
	kamezawa.hiroyu@...fujitsu.com, aarcange@...hat.com,
	hughd@...gle.com, jamesjer@...terlinux.com, marcus@...ehost.com,
	matt@...ehost.com, tytso@....edu, shaohua.li@...el.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] fadvise: support POSIX_FADV_NOREUSE

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 space¹
which could correspond to:
 DONTNEED, WILLNEED, NOREUSE
but what we're going for are these somewhat overlapping functions:
 DROP if used once², ADD, ADD if space

cheers,
Pádraig.

¹ Not implemented yet.

² 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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