From owner-freebsd-arch@FreeBSD.ORG  Wed Nov  9 06:18:16 2011
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: freebsd-arch@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id EFE771065670;
	Wed,  9 Nov 2011 06:18:16 +0000 (UTC)
	(envelope-from tim@kientzle.com)
Received: from monday.kientzle.com (99-115-135-74.uvs.sntcca.sbcglobal.net
	[99.115.135.74])
	by mx1.freebsd.org (Postfix) with ESMTP id C988A8FC08;
	Wed,  9 Nov 2011 06:18:16 +0000 (UTC)
Received: (from root@localhost)
	by monday.kientzle.com (8.14.4/8.14.4) id pA96I8OO088574;
	Wed, 9 Nov 2011 06:18:08 GMT (envelope-from tim@kientzle.com)
Received: from [192.168.2.119] (CiscoE3000 [192.168.1.65])
	by kientzle.com with SMTP id vzw4keh838gh4n74yzvdazjisn;
	Wed, 09 Nov 2011 06:18:08 +0000 (UTC)
	(envelope-from tim@kientzle.com)
Mime-Version: 1.0 (Apple Message framework v1251.1)
Content-Type: text/plain; charset=us-ascii
From: Tim Kientzle <tim@kientzle.com>
In-Reply-To: <20111109043512.GT6110@elvis.mu.org>
Date: Tue, 8 Nov 2011 22:18:07 -0800
Content-Transfer-Encoding: 7bit
Message-Id: <3D0BF37D-0C31-4509-A231-F4D1F81472D8@kientzle.com>
References: <201110281426.00013.jhb@freebsd.org> <4EB2C9DD.9090606@FreeBSD.org>
	<20111104160319.GD6110@elvis.mu.org>
	<201111080800.32717.jhb@freebsd.org>
	<20111109033504.GS6110@elvis.mu.org>
	<840E509B-0D63-41C2-B26A-31655F1D42C2@kientzle.com>
	<20111109043512.GT6110@elvis.mu.org>
To: Alfred Perlstein <alfred@freebsd.org>
X-Mailer: Apple Mail (2.1251.1)
Cc: Bruce Cran <bruce@cran.org.uk>, Ed Schouten <ed@80386.nl>,
	freebsd-arch@freebsd.org, Jilles Tjoelker <jilles@stack.nl>,
	arch@freebsd.org
Subject: Re: [PATCH] fadvise(2) system call
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 09 Nov 2011 06:18:17 -0000

On Nov 8, 2011, at 8:35 PM, Alfred Perlstein wrote:

> A query to sysctl, perhaps 'vfs.bufspace' (I haven't bothered to
> look at the actual correct sysctl node to query) would give a real
> indication of the amount of buffer size there is.

The kernel knows the size of the file and knows how
much buffer cache there is.  So the kernel already knows
whether the file will fit.

>  If the file to
> extract is larger than that size, it would be pretty obvious to use
> the "will not need" flag for file access.

It's not at all obvious.

If I have 1GB of cache and I'm going
to generate and then read back a 2GB file,
the best strategy is to hold the first
1GB in cache.

If I'm going to write the file and it will never be
read back, then the best strategy is to not
cache any of it.

Sometimes, a program knows which of
these is likely, but if it doesn't know, it shouldn't
say.

Tim