2005-12-11

When perl is not quite fast enough

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For functions that only calculate

This is useful for functions that calculate things with no side effects, slow functions that you often call repeatedly with the same parameters. It's not useful for functions that do things external to the program (such as generating output), nor is it good for very small, fast functions.

Can tie cache to a disk file

The hash Memoize uses is a regular perl hash. This means that you can tie the hash to a disk file. This allows Memoize to remember things across runs of your program. That way, you could use Memoize in a CGI to cache static content that you only generate on demand (but remember you'll need file locking). The first person who requests something has to wait for the generation routine, but everyone else gets it straight from the cache. You can also arrange for another program to periodically expire results from the cache.

As of 5.8 Memoize module has been assimilated into the core. Users of earlier perl can get it from CPAN.

Miscellaneous

These are quite general ideas for optimisation that aren't particularly perl specific.

Pull things out of loops

perl's hash lookups are fast. But they aren't as fast as a lexical variable. enc2xs was calling a function each time round a loop based on a hash lookup using $type as the key. The value of $type didn't change, so I pulled the lookup out above the loop into a lexical variable:

    my $type_func = $encode_types{$type};

and doing it only once was faster.

Experiment with number of arguments

Something else I found was that enc2xs was calling a function which took several arguments from a small number of places. The function contained code to set defaults if some of the arguments were not supplied. I found that the way the program ran, most of the calls passed in all the values and didn't need the defaults. Changing the function to not set defaults, and writing those defaults out explicitly where needed bought me a speed up.

Tail recursion

Tail recursion is where the last thing a function does it call itself again with slightly different arguments. It's a common idiom, and some languages can automatically optimise it away. Perl is not one of those languages. So every time a function tail recurses you have another subroutine call [not cheap - Arthur Bergman notes that it is 10 pages of C source, and will blow the instruction cache on a CPU] and re-entering that subroutine again causes more memory to be allocated to store a new set of lexical variables [also not cheap].

perl can't spot that it could just throw away the old lexicals and re-use their space, but you can, so you can save CPU and RAM by re-writing your tail recursive subroutines with loops. In general, trying to reduce recursion by replacing it with iterative algorithms should speed things up.
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