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Date: Sat, 6 May 2000 00:22:03 -0500
From: Dan Nelson <dnelson@emsphone.com>
To: Jean-Marc Zucconi <jmz@FreeBSD.ORG>
Cc: current@FreeBSD.ORG
Subject: Re: Can someone explain this?
Message-ID: <20000506002203.A6363@dan.emsphone.com>
References: <200005060030.RAA11795@freefall.freebsd.org>
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In the last episode (May 05), Jean-Marc Zucconi said:
> Here is something I don't understand:
> 
> $ sh -c  '/usr/bin/time  ./a.out'
>         2.40 real         2.38 user         0.01 sys
> $ /usr/bin/time  ./a.out
>         7.19 real         7.19 user         0.00 sys
> 
> The same program is 3 times slower in the second case. The effect is
> systematic but depends on the program being run. I have seen inverse
> behavior with another program. Using time -l, I note that this seems
> to be related with a higher value of 'involuntary context switches'
> (3 times more switches in the slower case).

It has to do with your stack.  Calling the program via /bin/sh sets up
your environment differently, so your program's stack starts at a
different place.  Try running this:

main (int argc, char **argv)
{
    int i;
    double x=2, y=2, z=2;
    printf ("%p\n",&i);
    for (i = 0; i < 10000000; i++) z = y*x;
    return 0;
}

Run this commandline:

STR= ; export STR ; while : ; do ; STR=z$STR ; /usr/bin/time ./a,out ; done

And watch your execution time flip flop every 4 runs.

Here are some bits from the gcc infopage explaining your options if you
want consistant speed from programs using doubles:

`-mpreferred-stack-boundary=NUM'
     Attempt to keep the stack boundary aligned to a 2 raised to NUM
     byte boundary.  If `-mpreferred-stack-boundary' is not specified,
     the default is 4 (16 bytes or 128 bits).
     The stack is required to be aligned on a 4 byte boundary.  On
     Pentium and PentiumPro, `double' and `long double' values should be
     aligned to an 8 byte boundary (see `-malign-double') or suffer
     significant run time performance penalties.  On Pentium III, the
     Streaming SIMD Extention (SSE) data type `__m128' suffers similar
     penalties if it is not 16 byte aligned.

`-mno-align-double'
     Control whether GCC aligns `double', `long double', and `long
     long' variables on a two word boundary or a one word boundary.
     Aligning `double' variables on a two word boundary will produce
     code that runs somewhat faster on a `Pentium' at the expense of
     more memory.

     *Warning:* if you use the `-malign-double' switch, structures
     containing the above types will be aligned differently than the
     published application binary interface specifications for the 386.


-- 
	Dan Nelson
	dnelson@emsphone.com


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