set MALLOC_ARENA_MAX to a lower dynamic value to prevent OOM

The default behavior is 8x the number of detected CPUs . As Cloud
Foundry typically uses large host machines with smaller containers,
and the Java process is unaware of the difference in allocated CPUs,
the numbers are way off. This often leads to high native memory usage,
followed by a cgroup OOM killer event.

We go with Heroku's recommendation of lowering to a
setting of 2 for small instances and grow larger linearly with memory.

References:
- cloudfoundry/java-buildpack#163
- https://devcenter.heroku.com/articles/testing-cedar-14-memory-use
- cloudfoundry/java-buildpack#320

start.py

@@ -241,7 +241,7 @@ def get_constants(metadata):
     return constants
 
 
-def set_jvm_memory(javaopts, vcap, java_version):
+def set_jvm_memory(m2ee_section, vcap, java_version):
     max_memory = os.environ.get('MEMORY_LIMIT')
 
     if max_memory:
@@ -273,6 +273,7 @@ def set_jvm_memory(javaopts, vcap, java_version):
                     heap_size,
                 )
             )
+    javaopts = m2ee_section['javaopts']
 
     javaopts.append('-Xmx%s' % heap_size)
     javaopts.append('-Xms%s' % heap_size)
@@ -284,6 +285,13 @@ def set_jvm_memory(javaopts, vcap, java_version):
 
     logger.debug('Java heap size set to %s' % heap_size)
 
+    if os.getenv('MALLOC_ARENA_MAX'):
+        logger.info('Using provided environment setting for MALLOC_ARENA_MAX')
+    else:
+        m2ee_section['custom_environment']['MALLOC_ARENA_MAX'] = str(
+            max(1, limit / 1024) * 2
+        )
+
 
 def _get_s3_specific_config(vcap_services, m2ee):
     access_key = secret = bucket = encryption_keys = key_suffix = None
@@ -671,7 +679,7 @@ def set_up_m2ee_client(vcap_data):
         m2ee.config.get_runtime_version()
     )
     set_jvm_memory(
-        m2ee.config._conf['m2ee']['javaopts'],
+        m2ee.config._conf['m2ee'],
         vcap_data,
         java_version,
     )