bm.rb

require 'benchmark'
require 'csv'

GEM_REPO = 'many_gems'
ENV['GEM_HOME'] = GEM_REPO
ENV['GEM_PATH'] = GEM_REPO

N = 15
MEDIAN = N / 2
Gem.clear_paths # clear all of rubygems cache
times = (100..1000).step(100).map { |i|
  Dir.chdir(GEM_REPO) { system "git checkout #{i}" }

  worst_gem = "pkg#{i - 1}"

  gem_req = N.times.map {
    start = Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond)
    Process.waitpid fork { gem worst_gem; require worst_gem; }
    Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond) - start
  }.sort[MEDIAN]

  bare_req = N.times.map {
    start = Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond)
    Process.waitpid fork { require worst_gem; }
    Process.clock_gettime(Process::CLOCK_MONOTONIC, :millisecond) - start
  }.sort[MEDIAN]
  [i, gem_req, bare_req]
}

CSV($stdout) do |csv|
  csv << ['number of gems', 'gem "foo"; require "foo"', 'require "foo"']
  times.each { |t| csv << t }
end

gemerator.rb

###
# Use this to generate many gems.
require 'rubygems/specification'
require 'fileutils'
require 'thread'

def util_spec name, version = 2, deps = nil # :yields: specification
  spec = Gem::Specification.new do |s|
    s.platform    = Gem::Platform::RUBY
    s.name        = name
    s.version     = version
    s.author      = 'A User'
    s.email       = 'example@example.com'
    s.homepage    = 'http://example.com'
    s.summary     = "this is a summary"
    s.description = "This is a test description"

    yield s if block_given?
  end

  if deps then
    # Since Hash#each is unordered in 1.8, sort the keys and iterate that
    # way so the tests are deterministic on all implementations.
    deps.keys.sort.each do |n|
      spec.add_dependency n, (deps[n] || '>= 0')
    end
  end
  spec
end

queue = Queue.new

thread_num = (ENV['P'] || 4).to_i
threads = thread_num.times.map {
  Thread.new do
    while w = queue.pop
      x = util_spec("pkg#{w}", 1, {}) { |s| s.files << "lib/pkg#{w}.rb" }
      Gem::Installer.for_spec(x).install
    end
  end
}

N = (ENV['N'] || 1_000).to_i
N.times { |i| queue << i }
thread_num.times { |i| queue << nil }
threads.each(&:join)
Dir.chdir ENV['GEM_HOME'] do
  system "git add ."
  system "git commit -am'#{N}'"
  system "git tag #{N}"
end