On Wed, Apr 1, 2015 at 4:33 PM, Martin Mitáš <m...@morous.org> wrote:
>
>
> Dne 1. 4. 2015 v 10:13 Mattias Engdegård napsal(a):
> > 1 apr 2015 kl. 05.35 skrev Dongsheng Song <dongsheng.s...@gmail.com>:
> >
> >> In my testing, getenv() is very fast.
> >>
> >> *) unset PRINTF_EXPONENT_DIGITS
> >>
> >> preheat 10000 times, then perform 1000000 times (use 4.67777 seconds)
> >> getenv cost: 4.67777 us
> >>
> >> *) set PRINTF_EXPONENT_DIGITS=3
> >> preheat 10000 times, then perform 1000000 times (use 3.41991 seconds)
> >> getenv: 3.41991 us
> >
> > 4 µs is a lot on a modern CPU, and an unacceptable overhead for a basic
> library function such as sprintf.
>
> If I assume getenv() iterates over complete environment in most cases (when
> PRINTF_EXPONENT_DIGITS is not set), then there is probably much worse
> overhead
> in a real world programs which is not likely covered in a trivial test
> program.
>
> The function getenv() shall replace data in CPU cache, which are likely
> useful
> for the ongoing program computation, with a lot of junk (the complete
> process
> environment).
>
> Morous
>
I'm certainly not opposed your patch, but a bit curious why getenv() slow
your program. we can always simplify a real world program to a test program
to discourse the performance issue.
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