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> In-Reply-To: <200005060030.RAA11795@freefall.freebsd.org>; from "Jean-Marc Zucconi" on Fri May 5 17:30:52 GMT 2000 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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