[SMALLFIX] Use better defaults for JVM memory sizes (#6938)

* [SMALLFIX] Use better defaults for JVM memory sizes

* [SMALLFIX] Fix comment

bin/alluxio-start.sh

@@ -54,6 +54,14 @@ ensure_dirs() {
   fi
 }
 
+# returns 1 if "$1" contains "$2", 0 otherwise.
+contains() {
+  if [[ "$1" = *"$2"* ]]; then
+    return 1
+  fi
+  return 0
+}
+
 get_env() {
   DEFAULT_LIBEXEC_DIR="${BIN}"/../libexec
   ALLUXIO_LIBEXEC_DIR=${ALLUXIO_LIBEXEC_DIR:-${DEFAULT_LIBEXEC_DIR}}
@@ -162,6 +170,12 @@ start_master() {
       ALLUXIO_SECONDARY_MASTER_JAVA_OPTS=${ALLUXIO_JAVA_OPTS}
     fi
 
+    # use a default Xmx value for the master
+    contains "${ALLUXIO_SECONDARY_MASTER_JAVA_OPTS}" "Xmx"
+    if [[ $? -eq 0 ]]; then
+      ALLUXIO_SECONDARY_MASTER_JAVA_OPTS+=" -Xmx8g "
+    fi
+
     echo "Starting secondary master @ $(hostname -f). Logging to ${ALLUXIO_LOGS_DIR}"
     (nohup "${JAVA}" -cp ${CLASSPATH} \
      ${ALLUXIO_SECONDARY_MASTER_JAVA_OPTS} \
@@ -171,6 +185,12 @@ start_master() {
       ALLUXIO_MASTER_JAVA_OPTS=${ALLUXIO_JAVA_OPTS}
     fi
 
+    # use a default Xmx value for the master
+    contains "${ALLUXIO_MASTER_JAVA_OPTS}" "Xmx"
+    if [[ $? -eq 0 ]]; then
+      ALLUXIO_MASTER_JAVA_OPTS+=" -Xmx8g "
+    fi
+
     echo "Starting master @ $(hostname -f). Logging to ${ALLUXIO_LOGS_DIR}"
     (nohup "${JAVA}" -cp ${CLASSPATH} \
      ${ALLUXIO_MASTER_JAVA_OPTS} \
@@ -209,6 +229,18 @@ start_worker() {
     ALLUXIO_WORKER_JAVA_OPTS=${ALLUXIO_JAVA_OPTS}
   fi
 
+  # use a default Xmx value for the worker
+  contains "${ALLUXIO_WORKER_JAVA_OPTS}" "Xmx"
+  if [[ $? -eq 0 ]]; then
+    ALLUXIO_WORKER_JAVA_OPTS+=" -Xmx4g "
+  fi
+
+  # use a default MaxDirectMemorySize value for the worker
+  contains "${ALLUXIO_WORKER_JAVA_OPTS}" "XX:MaxDirectMemorySize"
+  if [[ $? -eq 0 ]]; then
+    ALLUXIO_WORKER_JAVA_OPTS+=" -XX:MaxDirectMemorySize=4g "
+  fi
+
   echo "Starting worker @ $(hostname -f). Logging to ${ALLUXIO_LOGS_DIR}"
   (nohup "${JAVA}" -cp ${CLASSPATH} \
    ${ALLUXIO_WORKER_JAVA_OPTS} \