19 Apr
2020
19 Apr
'20
6:33 p.m.
Hi,
Thanks for the response. I decided to look into the isuse a little bit, and
it seems like the problem is mainly in apply_rec() in bddop.c. I was able
to rewrite it into a stack-based version, which seems to have fixed the
crashing (though postprocessing still takes forever, which is the original
result I was expecting, anyway). I also tried my new version on the command
from that issue you linked (
https://gitlab.lrde.epita.fr/spot/spot/-/issues/396), and that also works
now, though it takes about 4-5 hours to finish.
It's probably not the ideal implementation, but I attached a patch file
containing the fix, if you are interested. It seems to work (all the tests
pass), and it's about the same speed in most cases.
Thanks,
Reed
Le lun. 30 mars 2020 à 08:56, Alexandre Duret-Lutz <adl(a)lrde.epita.fr> a
écrit :
On Mon, Mar 30, 2020 at 2:15 PM Reed Oei
<reedoei2(a)illinois.edu> wrote:
Hello,
I am encountering a bug where Spot segfaults when I try to postprocess a
relatively large automaton. I have uploaded the file to
http://reedoei.com/files/smaller-inner.aut.zip; it has 113930 states and
1537155 edges. I tried to come up with a smaller example but was having
trouble doing so. Below is the sequence of commands, using the Python
bindings, to reproduce this bug for me:
import spot
A = spot.automaton('smaller-inner.aut')
B = A.postprocess('BA', 'Deterministic', 'Low') # Crashes here
B.save('postprocessed.aut')
I also tried using various other others for postprocess such as Small
and Low, or
Any and High (because Any and Low/Medium does nothing).
Hi Reed,
This is a HUGE automaton to pass to postprocess.
Running
% autfilt --det --low smaller-inner.aut
did not finish within 20 minutes, so I interrupted it and noticed the
code was busy in the simulation-based reduction code.
The simulation-based reduction is notoriously slow: it uses a
BDD-based representation that requires one variable per equivalence
classe (maybe the number of states), and when too many variables are
used, it's possible that the garbage collection for the BuDDy library
crashes with a stack overflow (see
https://gitlab.lrde.epita.fr/spot/spot/-/issues/396 ) Maybe that's
the crash you got.
You can disable simulation-based reductions with
% autfilt -x simul=0,ba-simul=0,det-simul=0 --det --low smaller-inner.aut
but this also did not finish withing 20 minutes. When I interrupted
it it was stuck in the determinization code.
Running an exponential algorithm on an automaton with soo many states
is unlikely to finish quickly.
Running the same command without requiring a determinisation, and
without simulation-based
reductions worked and reduced the automaton to 107083 states:
% autfilt -x simul=0,ba-simul=0 --small --low smaller-inner.aut
--
Alexandre Duret-Lutz