Dana Jacobsen

Performance and memory use are two areas of concern for many libraries. Last December I made a number of changes in my Math::Prime::Util module to reduce memory use, and bulk88 helped really tweak some of the XS. I thought I'd pick a simple task and look at the speed and memory use of a number of solutions.

Times are with perl 5.20.0 on an Ubuntu Core2 E7500 machine. I used /usr/bin/time perl -E '...' to get the time and memory use. With this system just starting up Perl on the command line takes about 2MB.

The first two entries are simple Perl trial division routines:

# mod-6 wheel

sub isprime { my($n) = @_; return ($n >=2) if $n < 4; return if ($n%2 == 0) || ($n%3 == 0); my $sn=int(sqrt($n)); for (my $i = 5; $i <= $sn; $i += 6) { return unless $n % $i && $n % ($i+2); } 1; }

$s += isprime($_) for 1..1e7; say $s;

# Standard method, from RosettaCode
sub isprime { my($n) = @_; $n % $_ or return for 2 .. sqrt $n; $n > 1; }
$s += isprime($_) for 1..1e7; say $s;

These have essentially no memory use, but are pretty slow especially as the input increases. However, very low memory and gets the job done for small inputs.

The modules Math::GMP and Math::GMPz have calls to GMP's mpz_probab_prime_p function. They are the lowest memory of the module solutions by a small margin, but not the fastest. They shouldn't slow down much with larger inputs.

Math::GMPz has a bit clunkier interface but is faster and exports the entire GMP integer API (which makes it use a little more memory to load). The speed will differ based on the number of tests: only 3 are required for this size, but we must have at minimum 11 for 64-bit inputs and probably more to avoid false results. Math::GMP has much more object overhead though I believe this is being worked on.

Like some other modules, the result of the primality test is either 2 (definitely prime), 1 (probably prime), or 0 (definitely composite). Using the double negation is an easy and fast way to make the result either 1 or 0.

use Math::GMP; $i=Math::GMP->new(0); for (1..1e7) { $i++; $s += !!$i->probab_prime(15) } ; say $s;

use Math::GMPz; $i=Math::GMPz->new(0); for (1..1e7) { $i++; $s += !!Math::GMPz::Rmpz_probab_prime_p($i,15) }; say $s

Math::Prime::Util is my number theory module. After working on cutting down memory use it's reasonably small even with many exportable functions. It is the fastest solution as well.

By default it will load the GMP back-end if available. This uses a little more memory, but can be turned off either by not having it installed or setting the environment variable MPU_NO_GMP to a non-zero value.

use Math::Prime::Util "is_prime"; $s += !!is_prime($_) for 1..1e7; say $s;

To match the behavior of Math::Prime::FastSieve, we can precalculate the primes, making is_prime a simple bit-set lookup.

use Math::Prime::Util qw/is_prime prime_precalc/; prime_precalc(1e7); $s += !!is_prime($_) for 1..1e7; say $s;

Math::Prime::XS does mod-6 trial division in C. It's fast for small inputs, but as expected from an exponential-time algorithm, will slow down a lot with large inputs. It uses more memory than I'd expect.

use Math::Prime::XS "is_prime"; $s += is_prime($_) for 1..1e7; say $s;

Math::Primality implements the BPSW algorithm in Perl using Math::GMPz. It really is better suited for bigint inputs, being both slow and memory intensive for this simple task.

use Math::Primality "is_prime"; $s += !!is_prime($_) for 1..1e7; say $s;

Math::Pari is the Perl interface to the old Pari 2.1.7 library. It has lots of functionality, but does take up almost 1.5MB more startup memory. While PARI/GP is reasonably efficient (about 4 seconds), the module returns the boolean result as a Math::Pari object which sucks up lots of time. The double negation is a little faster than directly summing the result.

use Math::Pari qw/isprime/; $s += !!isprime($_) for 1..1e7; say $s;

Math::Prime::FastSieve takes a little different approach. It's written using Inline::CPP and sieves a range into a bit vector, after which operations (such as isprime) can be efficiently performed. That does limit its range, but the time shows it's quite fast at this operation.

use Math::Prime::FastSieve; my $sieve = Math::Prime::FastSieve::Sieve->new(1e7); $s += $sieve->isprime($_) for 1..1e7; say $s;

Lastly we come to Abigail's regex. Very popular for code golfing and showing awesome regex hacks, it occasionally is seen as a practical recommendation from people who have clearly never used it for non-toy-size inputs. It's very cool. It's also not practical for larger inputs. For this task it took 6.8 seconds and 2684k for the first 10k, but 2607 seconds and 7144k for isprime for the first 100k integers. It takes over 1 minute to verify 999,983 is prime, and for 9,999,991 I killed it after 40 minutes. Hence my estimate of over a year to finish the sum-to-10M example.

sub isprime { ('1' x shift) !~ /^1?$|^(11+?)\1+$/ } $s += isprime($_) for 1..1e7; say $s

Conclusions

If you're using Moose or a long-running process, the memory use for any of the reasonable solutions here probably doesn't matter -- use what is easy and fast. For command-line programs or processes that are spun up just for a single task, the memory use can matter. My module was hitting 9MB before I finally had enough and reduced it substantially (a big chunk of that was by having functions go straight to XS and load up the thousands of lines of PP only if required). Even for such a simple task as this we can see sizes ranging from 2MB to 6MB, with over 2MB difference even between modules.

Another subject that is important, especially for making utility scripts, is startup time. This task did not measure that, but it can also be a bottleneck especially if comparing vs. standalone C programs that have essentially no startup cost.