Skip to main content

Bash Inefficient Matrix Multiplication

Bash can only handle integer arithmetic. To process floats, you need to shell out to 'bc' or some other external utility. The core multiplication changes to:

    result=`echo  "$result + $m * $n" | bc`;

Boy, does that slow things down! For each multiplication, you're launching a shell, invoking a program, returning results. The result is 800 multiplications a second. Note this is running on integer matrices, there remain unresolved problems processing floats.


One obvious problem is shelling out for every multiplication. More efficient is to accumulate the expression to calculate, and just invoke the external calculator once. Now we only shell out N^2 times, instead of N^3, so increasing matrix size leads to better performance, up to a limit. the 32x32 matrix gives 18x performance compared to the simpler version. This implies the 32x reduction in shelling out is offset by the cost of building the expression. By the time we reach 100x100, overhead costs are overwhelming gains: a 100x reduction in shelling out generates only a 16x improvement in performance.


Comments

Popular posts from this blog

Creating Perl5 Objects with Moxie

Having in the previous article prepared data types for car suits and card ranks, I can now combine them to provide a playing card class, using Stevan Little's Moxie module (version 0.04, so definitely early days.) The goal is to provide an object-oriented paradigm to the Perl 5 programming language which is more sophisticated, more powerful and less verbose than manually bless()-ing hashes. To achieve that goal it needs to be faster and light-weight compared to Moose. Currently, Moxie.pm and and MOP.pm are add-on modules, but eventually, when they are more complete, when the wrinkles have been ironed out, and when they have gained acceptance and a community of users, they might be merged into the Perl core.

One significant feature of Moxie is that it reduces boilerplate code. You don't have to specify warnigns or strict. As well, the features or the perl you are using are enabled, among them say, state, signatures, and post_deref.
A Simple Moxie Class package Card { …

Perl5, Moxie and Enumurated Data Types

Moxie - a new object system for Perl5 Stevan Little created the Moose multiverse to upgrade the Perl 5 programming language's object-oriented system more in line with the wonderfull world of Perl 6. Unfortunately, it's grown into a bloated giant, which has inspired light-weight alternatives Moos, Moo, Mo, and others. Now he's trying to create a modern, efficient OO system that can become built into the language.

I've seen a few of his presentations at YAPC (Yet Another Perl Conference, now known as TPC, The Perl Conference), among them ‎p5 mop final final v5 this is the last one i promise tar gz<. So I was delighted to recently see an announcement of the module Moxie, and decided to try implementing a card game.

While the package provides some POD documentation about the main module, Moxie, it doesn't actually explain the enum package, Moxie::Enum. But delving into the tests directory reveals its secrets.
Creating an Enum package Ranks { use Moxie::Enum; …

Adventures in Autovivification

Having recently started a new job, I was exposed to old code with multi-step tests against autovivification in multi-level hashes. You get used to the code you have seen, but in a new environment it‘s irritating and jarring.
Moose does not generally have the problem, first because class structure is pre-declared, because values are accessed using accessor functions rather than directly, and because responsibility is delegated down to attributes, avoiding long chains. On the other hand, Moose has it's own overhead, so hand-rolled objects, and bare structures still have their use.
If you don‘t protect against autovivification, then mis-spelling a key, or referencing keys which haven‘t been instantiated in this instance, causes those keys to instantly come into existence.
#!/usr/bin/perl use warnings; use strict; use Data::Dump 'dump'; use 5.024; my $var = {key1 => {key2 => {key3 => 'a'}}}; say dump $var; if ( $var->{key1}{key2}{key3b}[13]{foob…