Date: Wed, 9 Nov 2011 03:08:49 -0800 From: Alfred Perlstein <alfred@freebsd.org> To: Tim Kientzle <tim@kientzle.com> 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 Message-ID: <20111109110849.GV6110@elvis.mu.org> In-Reply-To: <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> <3D0BF37D-0C31-4509-A231-F4D1F81472D8@kientzle.com>
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http://bit.ly/hwm4GC * Tim Kientzle <tim@kientzle.com> [111108 22:18] wrote: > 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 -- - Alfred Perlstein .- VMOA #5191, 03 vmax, 92 gs500, 85 ch250, 07 zx10 .- FreeBSD committer
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