BigDecimal#log is performance-broken
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Key: JRUBY-4787
URL: http://jira.codehaus.org/browse/JRUBY-4787
Project: JRuby
Issue Type: Improvement
Components: Core Classes/Modules
Affects Versions: JRuby 1.4
Environment: Mac OS X 10.6.3
Reporter: Hans-Georg Höhne
Priority: Minor
BigDecimal#log is performance-broken for arguments x with x < 0.1 or x > 10.
This is an old issue, see for example Ruby Cookbook, O'Reilly 2006. Their fix
is several times faster for small or large arguments:
http://codeidol.com/other/rubyckbk/Numbers/Taking-Logarithms/
require "bigdecimal"
require "bigdecimal/math"
require "benchmark"
include BigMath
module BigMath
def fast_log(x, prec)
sign, fraction, power, exponent = x.split
fraction = BigDecimal("0.#{fraction}")
power = BigDecimal("#{power}")
log(fraction, prec) + (log(power, prec) * exponent)
end
end
prec = 100
Benchmark.bmbm(15) do |results|
results.report("fast_log 0.0001:") {
BigMath.fast_log(BigDecimal("0.0001"), prec) }
results.report("fast_log 0.001:") {
BigMath.fast_log(BigDecimal("0.001"), prec) }
results.report("fast_log 1000:") { BigMath.fast_log(BigDecimal("1000"),
prec) }
results.report("fast_log 10000:") {
BigMath.fast_log(BigDecimal("10000"), prec) }
results.report("log 0.0001:") { BigMath.log(BigDecimal("0.0001"), prec)
}
results.report("log 0.001:") { BigMath.log(BigDecimal("0.001"), prec) }
results.report("log 1000:") { BigMath.log(BigDecimal("1000"), prec) }
results.report("log 10000:") { BigMath.log(BigDecimal("10000"), prec) }
end
jruby 1.5.0.RC3 (ruby 1.8.7 patchlevel 249) (2010-05-04 603f15a) (Java
HotSpot(TM) 64-Bit Server VM 1.6.0_17) [x86_64-java]
user system total real
fast_log 0.0001: 0.019000 0.000000 0.019000 ( 0.018000)
fast_log 0.001: 0.030000 0.000000 0.030000 ( 0.030000)
fast_log 1000: 0.019000 0.000000 0.019000 ( 0.019000)
fast_log 10000: 0.018000 0.000000 0.018000 ( 0.019000)
log 0.0001: 7.050000 0.000000 7.050000 ( 7.050000)
log 0.001: 0.687000 0.000000 0.687000 ( 0.687000)
log 1000: 0.673000 0.000000 0.673000 ( 0.673000)
log 10000: 6.658000 0.000000 6.658000 ( 6.658000)
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