Re: Weird performance problem

[Greg Chicares]

On 2010-07-14 14:52Z, Leandro Lucarella wrote:
> Long story short, I have a program that when executed from a shell it
> takes about 3 seconds to complete, but when executed via make, it takes
> 5 seconds.

I trivially modified your very clear testcase here:

> http://www.llucax.com.ar/blog/blog/post/-7a56a111

to run a unit-test program of my own, but didn't see any penalty:

$echo 'all:' > Makefile
$echo -e '\t$C' >> Makefile
$make C="./elapsed_time ./zero_test-a"
./elapsed_time ./zero_test -a
Elapsed time: 43 milliseconds
$./elapsed_time ./zero_test -a
Elapsed time: 43 milliseconds

> Is there anything make does to the environment (in a general sense, not
> only environment variables) before executing commands that could cause
> a difference like this?

To diagnose this further, you might try echoing the precise time
 - before you start 'make'
 - just before running your test program
 - just after  running your test program
 - right after 'make' ends

My first guess would be that the difference you observe is shell
startup time. I realize that you have evidence that argues against
that (because only one test program seems to be affected). But you
have already measured running your program directly:
  /usr/bin/time -f%e ./voronoi -n 30000
and it wouldn't hurt to run
  sh -c '/usr/bin/time -f%e ./voronoi -n 30000'
just to see whether that takes remarkably longer.

If you have defined a $SHELL variable in the environment, 'make'
will use that instead of '/bin/sh'.