disable dynamic mmap threshold that leads to fragmentation

- see M_MMAP_THRESHOLD in "man mallopt"
- https://sourceware.org/bugzilla/show_bug.cgi?id=11044
- allow disabling tuning by setting JBP_NO_MALLOC_TUNING env

lib/java_buildpack/component/base_component.rb

@@ -93,8 +93,14 @@ def command_environment
 
       # Resolve the environment that's passed on the command line
       def resolve_command_environment
+        return if ENV['JBP_NO_MALLOC_TUNING'] && ENV['JBP_NO_MALLOC_TUNING'] != '0'
         # set MALLOC_ARENA_MAX by default
         @default_command_environment['MALLOC_ARENA_MAX'] = 2 unless ENV.key? 'MALLOC_ARENA_MAX'
+        # disable dynamic mmap threshold, see M_MMAP_THRESHOLD in "man mallopt"
+        @default_command_environment['MALLOC_MMAP_THRESHOLD_'] = 131_072 unless ENV.key? 'MALLOC_MMAP_THRESHOLD_'
+        @default_command_environment['MALLOC_TRIM_THRESHOLD_'] = 131_072 unless ENV.key? 'MALLOC_TRIM_THRESHOLD_'
+        @default_command_environment['MALLOC_TOP_PAD_'] = 131_072 unless ENV.key? 'MALLOC_TOP_PAD_'
+        @default_command_environment['MALLOC_MMAP_MAX_'] = 65_536 unless ENV.key? 'MALLOC_MMAP_MAX_'
       end
 
       # Downloads an item with the given name and version from the given URI, then yields the resultant file to the

spec/bin/release_spec.rb

@@ -41,4 +41,34 @@
       expect(YAML.load(stdout.string)['default_process_types']['web']).to match(/^MALLOC_ARENA_MAX=2 .*/)
     end
   end
+
+  it 'allow disabling malloc tuning',
+     app_fixture: 'integration_valid' do
+
+    ENV['JBP_NO_MALLOC_TUNING'] = '1'
+    run("bin/release #{app_dir}") do |status|
+      expect(status).to be_success
+      expect(YAML.load(stdout.string)['default_process_types']['web']).not_to match(/^MALLOC_ARENA_MAX.*/)
+    end
+  end
+
+  it 'do malloc tuning when JBP_NO_MALLOC_TUNING=0',
+     app_fixture: 'integration_valid' do
+
+    ENV['JBP_NO_MALLOC_TUNING'] = '0'
+    run("bin/release #{app_dir}") do |status|
+      expect(status).to be_success
+      expect(YAML.load(stdout.string)['default_process_types']['web']).to match(/^MALLOC_ARENA_MAX=2 .*/)
+    end
+  end
+
+  it 'when env contains value, don\'t include it to the command line',
+     app_fixture: 'integration_valid' do
+
+    ENV['MALLOC_ARENA_MAX'] = '4'
+    run("bin/release #{app_dir}") do |status|
+      expect(status).to be_success
+      expect(YAML.load(stdout.string)['default_process_types']['web']).not_to match(/^MALLOC_ARENA_MAX.*/)
+    end
+  end
 end

spec/integration_helper.rb

@@ -40,6 +40,7 @@
 
   after do
     FileUtils.rm_rf buildpack_dir
+    ENV.delete('JBP_NO_MALLOC_TUNING')
   end
 
   def run(command)