linux-kernel - Re: [ck] Re: 2.6.20-ck1

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Date:	Sun, 18 Feb 2007 01:41:07 +0100
From:	"Radoslaw Szkodzinski" <astralstorm@...il.com>
To:	"Andrew Morton" <akpm@...ux-foundation.org>
Cc:	"Con Kolivas" <kernel@...ivas.org>,
	"Chuck Ebbert" <cebbert@...hat.com>,
	"michael chang" <thenewme91@...il.com>,
	"ck mailing list" <ck@....kolivas.org>,
	"linux kernel mailing list" <linux-kernel@...r.kernel.org>
Subject: Re: [ck] Re: 2.6.20-ck1

On 2/18/07, Andrew Morton <akpm@...ux-foundation.org> wrote:
> Generally, the penalties for getting this stuff wrong are very very high:
> orders of magnitude slowdowns in the right situations.  Which I suspect
> will make any system-wide knob ultimately unsuccessful.
>

Yes, they were. Now, it's an extremely light and well-tuned patch.
kprefetchd should only run on a totally idle system now.

> The ideal way of getting this *right* is to change every application in the
> world to get smart about using sync_page_range() and/or posix_fadvise(),
> then to add a set of command-line options to each application in the world
> so the user can control its pagecache handling.

We don't live in a perfect world. :-)

> Obviously that isn't practical.  But what _could_ be done is to put these
> pagecache smarts into glibc's read() and write() code.  So the user can do:
>
>         MAX_PAGECACHE=4M MAX_DIRTY_PAGECACHE=2M rsync foo bar
>
> This will provide pagecache control for pretty much every application.  It
> has limitations (fork+exec behaviour??) but will be useful.

Not too useful for interactive applications with unpredictable memory
consumption behaviour, where swap-prefetch still helps.

> A kernel-based solution might use new rlimits, but would not be as flexible
> or successful as a libc-based one, I suspect.
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