cassandra is started with Xms 1g and Xmx 2g
This is incorrect. Xms and Xmx value should be 8g

cassand+     1     0 99 04:57 ?        1-00:25:02 java -Xloggc:/var/log/cassandra/gc.log -ea -XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42 -XX:+HeapDumpOnOutOfMemoryError -Xss256k -XX:StringTableSize=1000003 -XX:+AlwaysPreTouch -XX:-UseBiasedLocking -XX:+UseTLAB -XX:+ResizeTLAB -XX:+UseNUMA -XX:+PerfDisableSharedMem -Djava.net.preferIPv4Stack=true -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -XX:+CMSParallelRemarkEnabled -XX:SurvivorRatio=8 -XX:MaxTenuringThreshold=1 -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -XX:CMSWaitDuration=10000 -XX:+CMSParallelInitialMarkEnabled -XX:+CMSEdenChunksRecordAlways -XX:+CMSClassUnloadingEnabled -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -XX:+PrintPromotionFailure -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=10M -Xms8192M -Xmx8192M -Xmn2048M -XX:CompileCommandFile=/etc/cassandra/hotspot_compiler -javaagent:/usr/share/cassandra/lib/jamm-0.3.0.jar -Dcassandra.jmx.local.port=7199 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.password.file=/etc/cassandra/jmxremote.password -Djava.library.path=/usr/share/cassandra/lib/sigar-bin -Xms1g -Xmx2g -Dcassandra.rpc_port=9160 -Dcassandra.native_transport_port=9042 -Dcassandra.ssl_storage_port=7011 -Dcassandra.storage_port=7010 -Dcassandra.jmx.local.port=7200 -Dcassandra.libjemalloc=/usr/lib/x86_64-linux-gnu/libjemalloc.so.1 -XX:OnOutOfMemoryError=kill -9 %p -Dlogback.configurationFile=logback.xml -Dcassandra.logdir=/var/log/cassandra -Dcassandra.storagedir=/var/lib/cassandra -Dcassandra-foreground=yes -cp /etc/cassandra:/usr/share/cassandra/lib/HdrHistogram-2.1.9.jar:/usr/share/cassandra/lib/ST4-4.0.8.jar:/usr/share/cassandra/lib/airline-0.6.jar:/usr/share/cassandra/lib/antlr-runtime-3.5.2.jar:/usr/share/cassandra/lib/asm-5.0.4.jar:/usr/share/cassandra/lib/caffeine-2.2.6.jar:/usr/share/cassandra/lib/cassandra-driver-core-3.0.1-shaded.jar:/usr/share/cassandra/lib/commons-cli-1.1.jar:/usr/share/cassandra/lib/commons-codec-1.9.jar:/usr/share/cassandra/lib/commons-lang3-3.1.jar:/usr/share/cassandra/lib/commons-math3-3.2.jar:/usr/share/cassandra/lib/compress-lzf-0.8.4.jar:/usr/share/cassandra/lib/concurrent-trees-2.4.0.jar:/usr/share/cassandra/lib/concurrentlinkedhashmap-lru-1.4.jar:/usr/share/cassandra/lib/disruptor-3.0.1.jar:/usr/share/cassandra/lib/ecj-4.4.2.jar:/usr/share/cassandra/lib/guava-18.0.jar:/usr/share/cassandra/lib/high-scale-lib-1.0.6.jar:/usr/share/cassandra/lib/hppc-0.5.4.jar:/usr/share/cassandra/lib/jackson-core-asl-1.9.13.jar:/usr/share/cassandra/lib/jackson-mapper-asl-1.9.13.jar:/usr/share/cassandra/lib/jamm-0.3.0.jar:/usr/share/cassandra/lib/javax.inject.jar:/usr/share/cassandra/lib/jbcrypt-0.3m.jar:/usr/share/cassandra/lib/jcl-over-slf4j-1.7.7.jar:/usr/share/cassandra/lib/jctools-core-1.2.1.jar:/usr/share/cassandra/lib/jflex-1.6.0.jar:/usr/share/cassandra/lib/jna-4.2.2.jar:/usr/share/cassandra/lib/joda-time-2.4.jar:/usr/share/cassandra/lib/json-simple-1.1.jar:/usr/share/cassandra/lib/jstackjunit-0.0.1.jar:/usr/share/cassandra/lib/libthrift-0.9.2.jar:/usr/share/cassandra/lib/log4j-over-slf4j-1.7.7.jar:/usr/share/cassandra/lib/logback-classic-1.1.3.jar:/usr/share/cassandra/lib/logback-core-1.1.3.jar:/usr/share/cassandra/lib/lz4-1.3.0.jar:/usr/share/cassandra/lib/metrics-core-3.1.0.jar:/usr/share/cassandra/lib/metrics-jvm-3.1.0.jar:/usr/share/cassandra/lib/metrics-logback-3.1.0.jar:/usr/share/cassandra/lib/netty-all-4.0.44.Final.jar:/usr/share/cassandra/lib/ohc-core-0.4.4.jar:/usr/share/cassandra/lib/ohc-core-j8-0.4.4.jar:/usr/share/cassandra/lib/reporter-config-base-3.0.3.jar:/usr/share/cassandra/lib/reporter-config3-3.0.3.jar:/usr/share/cassandra/lib/sigar-1.6.4.jar:/usr/share/cassandra/lib/slf4j-api-1.7.7.jar:/usr/share/cassandra/lib/snakeyaml-1.11.jar:/usr/share/cassandra/lib/snappy-java-1.1.1.7.jar:/usr/share/cassandra/lib/snowball-stemmer-1.3.0.581.1.jar:/usr/share/cassandra/lib/stream-2.5.2.jar:/usr/share/cassandra/lib/thrift-server-0.3.7.jar:/usr/share/cassandra/apache-cassandra-3.11.2.jar:/usr/share/cassandra/apache-cassandra-thrift-3.11.2.jar:/usr/share/cassandra/apache-cassandra.jar:/usr/share/cassandra/stress.jar: org.apache.cassandra.service.CassandraDaemon