Date: Thu, 26 Jun 2003 20:49:44 -0700 From: Jon Mini <mini@freebsd.org> To: "D. J. Bernstein" <djb@cr.yp.to> Cc: freebsd-performance@freebsd.org Subject: Re: ten thousand small processes Message-ID: <20030627034944.GP55678@elvis.mu.org> In-Reply-To: <20030626025029.71392.qmail@cr.yp.to> References: <20030625060629.51087.qmail@cr.yp.to> <20030625023621.N17881-100000@mail.chesapeake.net> <20030625094301.56349.qmail@cr.yp.to> <20030626025029.71392.qmail@cr.yp.to>
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I am aware of what you say, but it doesn't not change the
inaccuracy of your original statement.
D. J. Bernstein [djb@cr.yp.to] wrote :
> Jon Mini writes:
> > I'm sorry, but you are way off here. First of all, caches are *much
> > larger* than the size of the processes you are talking about.
>
> I'm sorry, but you are being misled by a naive model of CPU performance.
> On a typical Pentium in our department, the following program becomes
> three times faster when SPACING is changed from 4096 to 128:
>
> #define SPACING 4096
> char data[8 * SPACING];
> main()
> {
> int i;
> for (i = 0;i < 10000000;++i) {
> data[0] = data[SPACING];
> data[2 * SPACING] = data[3 * SPACING];
> data[4 * SPACING] = data[5 * SPACING];
> data[6 * SPACING] = data[7 * SPACING];
> }
> }
>
> >From an asm programmer's perspective, when FreeBSD decides to spread a
> small program's variables between
>
> * the beginning of a data page,
> * the beginning of a bss page,
> * the beginning of a malloc mmap page,
> * the beginning of a heap page,
> * the beginning of the next heap page,
> * the beginning of yet another heap page,
>
> et cetera, it is actively trying (with varying degrees of success) to
> damage cache performance in exactly the same way that this program does.
>
> ---D. J. Bernstein, Associate Professor, Department of Mathematics,
> Statistics, and Computer Science, University of Illinois at Chicago
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--
Jonathan Mini <mini@freebsd.org>
http://www.freebsd.org/
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