| Rob Burns on Mon, 14 Dec 2015 10:13:49 +0100 |
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| Re: Memory question |
* Rob Burns [2015-12-14 04:53]:
> Is anyone able to explain or interpret the following please? I wrote the
> following simple program to test pari memory limits. It calculates powers
> of 2:
>
> pari_init(2000000000,2);
>
>
> m = gen_1;
>
> for (i=1; a<1000000; a++) {
I guess the 'i=1' should be an 'a=1'
>
> m = gmul2n(m, 1);
>
> }
>
>
> pari_close();
>
> The PARI stack overflows around a = 179000. However at this point m is only
> around 2800 bytes according to taille2.
Not surprising: after N loops, memory usage is O(N^2) [ = the sum of the
sizes of the successive 'm' whose size grows linearly ]
> Adding in some garbage collection only improves the result by a small
> amount.
You did something wrong, the following program runs to completion:
#include <pari/pari.h>
int main()
{
pari_sp av;
long a;
GEN m;
pari_init(2000000000,2);
av = avma; m = gen_1;
for (a=1; a<1000000; a++) {
m = gerepileuptoint(av, gmul2n(m, 1));
}
pari_close(); return 0;
}
And so does this one (about 3 times faster since it performs GC once in a
while):
[...]
for (a=1; a<1000000; a++) {
m = gmul2n(m, 1);
if ((a & 0xfff) == 0) m = gerepileuptoint(av, m);
}
[...]
> The same algorithm written with gmp can run up to a = 10^7 while using at
> most 1.93 GB of memory.
Presumably you wrote it in such a way that 'm' is modified in place.
Cheers,
K.B.
--
Karim Belabas, IMB (UMR 5251) Tel: (+33) (0)5 40 00 26 17
Universite de Bordeaux Fax: (+33) (0)5 40 00 69 50
351, cours de la Liberation http://www.math.u-bordeaux.fr/~kbelabas/
F-33405 Talence (France) http://pari.math.u-bordeaux.fr/ [PARI/GP]
`