calculate_heap_sizes()
{
    case "`uname`" in
        Linux)
            system_memory_in_mb=`free -m | awk '/:/ {print $2;exit}'`
            system_cpu_cores=`egrep -c 'processor([[:space:]]+):.*' /proc/cpuinfo`
        ;;
        FreeBSD)
            system_memory_in_bytes=`sysctl hw.physmem | awk '{print $2}'`
            system_memory_in_mb=`expr $system_memory_in_bytes / 1024 / 1024`
            system_cpu_cores=`sysctl hw.ncpu | awk '{print $2}'`
        ;;
        SunOS)
            system_memory_in_mb=`prtconf | awk '/Memory size:/ {print $3}'`
            system_cpu_cores=`psrinfo | wc -l`
        ;;
        Darwin)
            system_memory_in_bytes=`sysctl hw.memsize | awk '{print $2}'`
            system_memory_in_mb=`expr $system_memory_in_bytes / 1024 / 1024`
            system_cpu_cores=`sysctl hw.ncpu | awk '{print $2}'`
        ;;
        *)
            # assume reasonable defaults for e.g. a modern desktop or
            # cheap server
            system_memory_in_mb="2048"
            system_cpu_cores="2"
        ;;
    esac

    # some systems like the raspberry pi don't report cores, use at least 1
    if [ "$system_cpu_cores" -lt "1" ]
    then
        system_cpu_cores="1"
    fi

    # set max heap size based on the following
    # max(min(1/2 ram, 1024MB), min(1/4 ram, 8GB))
    # calculate 1/2 ram and cap to 1024MB
    # calculate 1/4 ram and cap to 8192MB
    # pick the max
    half_system_memory_in_mb=`expr $system_memory_in_mb / 2`
    quarter_system_memory_in_mb=`expr $half_system_memory_in_mb / 2`
    if [ "$half_system_memory_in_mb" -gt "1024" ]
    then
        half_system_memory_in_mb="1024"
    fi
    if [ "$quarter_system_memory_in_mb" -gt "8192" ]
    then
        quarter_system_memory_in_mb="8192"
    fi
    if [ "$half_system_memory_in_mb" -gt "$quarter_system_memory_in_mb" ]
    then
        max_heap_size_in_mb="$half_system_memory_in_mb"
    else
        max_heap_size_in_mb="$quarter_system_memory_in_mb"
    fi
    MAX_HEAP_SIZE="${max_heap_size_in_mb}M"

    # Young gen: min(max_sensible_per_modern_cpu_core * num_cores, 1/4 * heap size)
    max_sensible_yg_per_core_in_mb="100"
    max_sensible_yg_in_mb=`expr $max_sensible_yg_per_core_in_mb "*" $system_cpu_cores`

    desired_yg_in_mb=`expr $max_heap_size_in_mb / 4`

    if [ "$desired_yg_in_mb" -gt "$max_sensible_yg_in_mb" ]
    then
        HEAP_NEWSIZE="${max_sensible_yg_in_mb}M"
    else
        HEAP_NEWSIZE="${desired_yg_in_mb}M"
    fi
}

# Determine the sort of JVM we'll be running on.
java_ver_output=`"${JAVA:-java}" -version 2>&1`
jvmver=`echo "$java_ver_output" | grep '[openjdk|java] version' | awk -F'"' 'NR==1 {print $2}'`
JVM_VERSION=${jvmver%_*}
JVM_PATCH_VERSION=${jvmver#*_}

if [ "$JVM_VERSION" \< "1.8" ] ; then
    echo "Cassandra 3.0 and later require Java 8u40 or later."
    exit 1;
fi

if [ "$JVM_VERSION" \< "1.8" ] && [ "$JVM_PATCH_VERSION" -lt 40 ] ; then
    echo "Cassandra 3.0 and later require Java 8u40 or later."
    exit 1;
fi

jvm=`echo "$java_ver_output" | grep -A 1 'java version' | awk 'NR==2 {print $1}'`
case "$jvm" in
    OpenJDK)
        JVM_VENDOR=OpenJDK
        # this will be "64-Bit" or "32-Bit"
        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $2}'`
        ;;
    "Java(TM)")
        JVM_VENDOR=Oracle
        # this will be "64-Bit" or "32-Bit"
        JVM_ARCH=`echo "$java_ver_output" | awk 'NR==3 {print $3}'`
        ;;
    *)
        # Help fill in other JVM values
        JVM_VENDOR=other
        JVM_ARCH=unknown
        ;;
esac

#GC log path has to be defined here because it needs to access CASSANDRA_HOME
JVM_OPTS="$JVM_OPTS -Xloggc:${CASSANDRA_HOME}/logs/gc.log"

# Here we create the arguments that will get passed to the jvm when
# starting cassandra.

# Read user-defined JVM options from jvm.options file
JVM_OPTS_FILE=$CASSANDRA_CONF/jvm.options
for opt in `grep "^-" $JVM_OPTS_FILE`
do
  JVM_OPTS="$JVM_OPTS $opt"
done

# Check what parameters were defined on jvm.options file to avoid conflicts
echo $JVM_OPTS | grep -q Xmn
DEFINED_XMN=$?
echo $JVM_OPTS | grep -q Xmx
DEFINED_XMX=$?
echo $JVM_OPTS | grep -q Xms
DEFINED_XMS=$?
echo $JVM_OPTS | grep -q UseConcMarkSweepGC
USING_CMS=$?
echo $JVM_OPTS | grep -q UseG1GC
USING_G1=$?

# Override these to set the amount of memory to allocate to the JVM at
# start-up. For production use you may wish to adjust this for your
# environment. MAX_HEAP_SIZE is the total amount of memory dedicated
# to the Java heap. HEAP_NEWSIZE refers to the size of the young
# generation. Both MAX_HEAP_SIZE and HEAP_NEWSIZE should be either set
# or not (if you set one, set the other).
#
# The main trade-off for the young generation is that the larger it
# is, the longer GC pause times will be. The shorter it is, the more
# expensive GC will be (usually).
#
# The example HEAP_NEWSIZE assumes a modern 8-core+ machine for decent pause
# times. If in doubt, and if you do not particularly want to tweak, go with
# 100 MB per physical CPU core.

#MAX_HEAP_SIZE="4G"
#HEAP_NEWSIZE="800M"

# Set this to control the amount of arenas per-thread in glibc
#export MALLOC_ARENA_MAX=4

# only calculate the size if it's not set manually
if [ "x$MAX_HEAP_SIZE" = "x" ] && [ "x$HEAP_NEWSIZE" = "x" -o $USING_G1 -eq 0 ]; then
    calculate_heap_sizes
elif [ "x$MAX_HEAP_SIZE" = "x" ] ||  [ "x$HEAP_NEWSIZE" = "x" -a $USING_G1 -ne 0 ]; then
    echo "please set or unset MAX_HEAP_SIZE and HEAP_NEWSIZE in pairs when using CMS GC (see cassandra-env.sh)"
    exit 1
fi

if [ "x$MALLOC_ARENA_MAX" = "x" ] ; then
    export MALLOC_ARENA_MAX=4
fi

# We only set -Xms and -Xmx if they were not defined on jvm.options file
# If defined, both Xmx and Xms should be defined together.
if [ $DEFINED_XMX -ne 0 ] && [ $DEFINED_XMS -ne 0 ]; then
     JVM_OPTS="$JVM_OPTS -Xms${MAX_HEAP_SIZE}"
     JVM_OPTS="$JVM_OPTS -Xmx${MAX_HEAP_SIZE}"
elif [ $DEFINED_XMX -ne 0 ] || [ $DEFINED_XMS -ne 0 ]; then
     echo "Please set or unset -Xmx and -Xms flags in pairs on jvm.options file."
     exit 1
fi

# We only set -Xmn flag if it was not defined in jvm.options file
# and if the CMS GC is being used
# If defined, both Xmn and Xmx should be defined together.
if [ $DEFINED_XMN -eq 0 ] && [ $DEFINED_XMX -ne 0 ]; then
    echo "Please set or unset -Xmx and -Xmn flags in pairs on jvm.options file."
    exit 1
elif [ $DEFINED_XMN -ne 0 ] && [ $USING_CMS -eq 0 ]; then
    JVM_OPTS="$JVM_OPTS -Xmn${HEAP_NEWSIZE}"
fi

if [ "$JVM_ARCH" = "64-Bit" ] && [ $USING_CMS -eq 0 ]; then
    JVM_OPTS="$JVM_OPTS -XX:+UseCondCardMark"
fi

# enable assertions.  disabling this in production will give a modest
# performance benefit (around 5%).
JVM_OPTS="$JVM_OPTS -ea"

# Per-thread stack size.
JVM_OPTS="$JVM_OPTS -Xss256k"

# Make sure all memory is faulted and zeroed on startup.
# This helps prevent soft faults in containers and makes
# transparent hugepage allocation more effective.
JVM_OPTS="$JVM_OPTS -XX:+AlwaysPreTouch"

# Biased locking does not benefit Cassandra.
JVM_OPTS="$JVM_OPTS -XX:-UseBiasedLocking"

# Larger interned string table, for gossip's benefit (CASSANDRA-6410)
JVM_OPTS="$JVM_OPTS -XX:StringTableSize=1000003"

# Enable thread-local allocation blocks and allow the JVM to automatically
# resize them at runtime.
JVM_OPTS="$JVM_OPTS -XX:+UseTLAB -XX:+ResizeTLAB"

# http://www.evanjones.ca/jvm-mmap-pause.html
JVM_OPTS="$JVM_OPTS -XX:+PerfDisableSharedMem"

# provides hints to the JIT compiler
JVM_OPTS="$JVM_OPTS -XX:CompileCommandFile=$CASSANDRA_CONF/hotspot_compiler"

# add the jamm javaagent
JVM_OPTS="$JVM_OPTS -javaagent:$CASSANDRA_HOME/lib/jamm-0.3.0.jar"

# enable thread priorities, primarily so we can give periodic tasks
# a lower priority to avoid interfering with client workload
JVM_OPTS="$JVM_OPTS -XX:+UseThreadPriorities"
# allows lowering thread priority without being root.  see
# http://tech.stolsvik.com/2010/01/linux-java-thread-priorities-workaround.html
JVM_OPTS="$JVM_OPTS -XX:ThreadPriorityPolicy=42"

# set jvm HeapDumpPath with CASSANDRA_HEAPDUMP_DIR
JVM_OPTS="$JVM_OPTS -XX:+HeapDumpOnOutOfMemoryError"
if [ "x$CASSANDRA_HEAPDUMP_DIR" != "x" ]; then
    JVM_OPTS="$JVM_OPTS -XX:HeapDumpPath=$CASSANDRA_HEAPDUMP_DIR/cassandra-`date +%s`-pid$$.hprof"
fi

# uncomment to have Cassandra JVM listen for remote debuggers/profilers on port 1414
# JVM_OPTS="$JVM_OPTS -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=1414"

# uncomment to have Cassandra JVM log internal method compilation (developers only)
# JVM_OPTS="$JVM_OPTS -XX:+UnlockDiagnosticVMOptions -XX:+LogCompilation"
# JVM_OPTS="$JVM_OPTS -XX:+UnlockCommercialFeatures -XX:+FlightRecorder"

# Prefer binding to IPv4 network intefaces (when net.ipv6.bindv6only=1). See
# http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6342561 (short version:
# comment out this entry to enable IPv6 support).
JVM_OPTS="$JVM_OPTS -Djava.net.preferIPv4Stack=true"

# jmx: metrics and administration interface
#
# add this if you're having trouble connecting:
# JVM_OPTS="$JVM_OPTS -Djava.rmi.server.hostname=<public name>"
#
# see
# https://blogs.oracle.com/jmxetc/entry/troubleshooting_connection_problems_in_jconsole
# for more on configuring JMX through firewalls, etc. (Short version:
# get it working with no firewall first.)
#
# Cassandra ships with JMX accessible *only* from localhost.
# To enable remote JMX connections, uncomment lines below
# with authentication and/or ssl enabled. See https://wiki.apache.org/cassandra/JmxSecurity
#
if [ "x$LOCAL_JMX" = "x" ]; then
    LOCAL_JMX=yes
fi

# Specifies the default port over which Cassandra will be available for
# JMX connections.
# For security reasons, you should not expose this port to the internet.  Firewall it if needed.
JMX_PORT="7199"

if [ "$LOCAL_JMX" = "yes" ]; then
  JVM_OPTS="$JVM_OPTS -Dcassandra.jmx.local.port=$JMX_PORT -XX:+DisableExplicitGC"
else
  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.port=$JMX_PORT"
  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.rmi.port=$JMX_PORT"
  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.ssl=false"
  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.authenticate=true"
  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.password.file=/etc/cassandra/jmxremote.password"
#  JVM_OPTS="$JVM_OPTS -Djavax.net.ssl.keyStore=/path/to/keystore"
#  JVM_OPTS="$JVM_OPTS -Djavax.net.ssl.keyStorePassword=<keystore-password>"
#  JVM_OPTS="$JVM_OPTS -Djavax.net.ssl.trustStore=/path/to/truststore"
#  JVM_OPTS="$JVM_OPTS -Djavax.net.ssl.trustStorePassword=<truststore-password>"
#  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.ssl.need.client.auth=true"
#  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.registry.ssl=true"
#  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.ssl.enabled.protocols=<enabled-protocols>"
#  JVM_OPTS="$JVM_OPTS -Dcom.sun.management.jmxremote.ssl.enabled.cipher.suites=<enabled-cipher-suites>"
fi

# To use mx4j, an HTML interface for JMX, add mx4j-tools.jar to the lib/
# directory.
# See http://wiki.apache.org/cassandra/Operations#Monitoring_with_MX4J
# By default mx4j listens on 0.0.0.0:8081. Uncomment the following lines
# to control its listen address and port.
#MX4J_ADDRESS="-Dmx4jaddress=127.0.0.1"
#MX4J_PORT="-Dmx4jport=8081"

# Cassandra uses SIGAR to capture OS metrics CASSANDRA-7838
# for SIGAR we have to set the java.library.path
# to the location of the native libraries.
JVM_OPTS="$JVM_OPTS -Djava.library.path=$CASSANDRA_HOME/lib/sigar-bin"

JVM_OPTS="$JVM_OPTS $MX4J_ADDRESS"
JVM_OPTS="$JVM_OPTS $MX4J_PORT"
JVM_OPTS="$JVM_OPTS $JVM_EXTRA_OPTS"

JVM_OPTS="$JVM_OPTS -javaagent:/cassandra-diagnostics/cassandra-diagnostics-core.jar"
JVM_OPTS="$JVM_OPTS -Dcassandra.diagnostics.config=cassandra-diagnostics.yml"
CLASSPATH="/cassandra-diagnostics/cassandra-diagnostics-connector.jar:$CLASSPATH"
