[Haskell-cafe] regression in memory usage between 6.10 and 7.x?

Ben Franksen ben.franksen at online.de
Sat Dec 17 20:51:26 UTC 2016

In the functional pearl "A Play on Regular Expressions" by Sebastian
Fischer, Frank Huch, and Thomas Wilke, the authors explain how to
implement a Glushkov automaton in a very elegant way. They claim that
when they compile their code with ghc-6.10.4 and with -O2 optimization
they get:

1 MB total memory in use
Total time 0.06s (0.21s elapsed)

for a program that matches stdin against the equivalent (in grep -E
syntax) of


When I try their code on my machine with ghc-7.10.3 (and also -O2) I get:

ben at sarun[1]: .../play/regexfp > ghc Reg.hs -O2 -o re
[1 of 1] Compiling Main             ( Reg.hs, Reg.o )
Linking re ...
ben at sarun[1]: .../play/regexfp > ./re 500 +RTS -s
155 MB total memory in use
Total   time    1.031s  (  1.037s elapsed)

The details of the RTS report furthermore tell me that GC is responsible
for more than 80% of the runtime.

Now, I certainly have a different machine and perhaps OS than the
authors of the paper, but that can't explain a factor of 16 in runtime
and of 150 in memory use. I have double and triple checked my code for
possible mistakes. I /do/ get the expected results (same as in the paper).

I tried different numbers and found

n	total memory in use / MB
100	8
200	28
500	169
1000	698

which shows that memory consumtion is roughly quadratic in n. I tried
heap profiling but that tells me only that all the allocation is done by
the function 'shift', which I already knew (or rather guessed).

I also tried several compiler versions: 7.0.4, 7.10.3, 7.4.2, 7.6.3,
8.0.1. They all result in similar performance.

The ghc version the authors used is 6.10.4 and I don't have a package
for that compatible to my distribution.

Any hint as to where these differences might come from are appreciated.

I have attached the program for reference.

"Make it so they have to reboot after every typo." ― Scott Adams
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