fork/exec performance problem?

[Johan Holmberg via Digitalmars-d]

Hi!

I'm trying to write a simple D program to emulate "parallel -u -jN", ie.
running a number of commands in parallel to take advantage of a multicore
machine (I'm testing on a 24-core Ubuntu machine).

I have written almost equivalent programs in C++ and D, and hoped that they
should run equally fast. But the performance of the D version degrades when
the number of commands increase, and I don't understand why. Maybe I'm
using D incorrectly? Or is it the garbage collector that kicks in (even if
I hope I don't allocate much memory after the initial setup)?

My first testcase consisted of a file with 85000 C/C++ compilation
commands, to be run 24 in parallel. Most source files are really small
(different modules in the runtime library of a C/C++ compiler for embedded
development, built in different flavors).

If I invoke the D program 9 times with around 10000 (85000/9 to be exact)
commands each time, it performs almost on par with the C++ version. But
with all 85000 files in one invokation, the D version takes 1.5 times as
long (6min 30s --> 10min).

My programs (C++ and D) are really simple:

1) read all commands from STDIN into an array in the program
2) iterate over the array and keep N programs running at all times
3) start new programs with "fork/exec"
4) wait for finished programs with "waitpid"

If I compare the start of a 85000-run and a 10000-run, the 85000-run is
slower right from the start. I don't understand why? The only difference
must be that 85000-run has allocated a bigger array.

My D program can be viewed at:


https://bitbucket.org/holmberg556/examples/src/79ef65e389346e9957c535b77201a829af9c62f2/parallel_exec/parallel_exec_dlang.d

Any help would be appreciated.

/Johan Holmberg
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