understanding cassandra CPU overhead

gistfile1.txt

jhaddad@jhaddad-rmbp15 ~/dev$ wget https://bintray.com/artifact/download/aragozin/generic/sjk-plus-0.3.6.jarjava -jar sjk-plus-0.3.6.jar 

jhaddad@jhaddad-rmbp15 ~/dev$ java -jar sjk-plus-0.3.6.jar ttop -s localhost:7199 -o CPU -n 30
Monitoring threads ...

2015-06-20T14:17:01.194-0400 Process summary
  process cpu=7.00%
  application cpu=3.02% (user=2.72% sys=0.30%)
  other: cpu=3.98%
  heap allocation rate 914kb/s
[002776] user= 2.55% sys= 0.20% alloc=  881kb/s - RMI TCP Connection(577)-127.0.0.1
[001887] user= 0.07% sys= 0.03% alloc=   10kb/s - Thread-2
[000010] user= 0.03% sys= 0.03% alloc=   984b/s - ScheduledTasks:1
[002777] user= 0.02% sys= 0.01% alloc=  4285b/s - JMX server connection timeout 2777
[000012] user= 0.01% sys= 0.00% alloc=  3196b/s - Background_Reporter:1
[000117] user= 0.00% sys= 0.00% alloc=     0b/s - threadDeathWatcher-3-1
[000095] user= 0.01% sys= 0.00% alloc=   723b/s - HintedHandoff:1
[000093] user= 0.00% sys= 0.00% alloc=   423b/s - GossipTasks:1
[000112] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-14
[000106] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-8
[000108] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-10
[000011] user= 0.00% sys= 0.00% alloc=    52b/s - EXPIRING-MAP-REAPER:1
[000099] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-1
[000109] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-11
[000103] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-5
[000025] user= 0.00% sys= 0.00% alloc=   12kb/s - OptionalTasks:1
[000104] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-6
[000113] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-15
[000100] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-2
[000101] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-3
[000102] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-4
[000021] user= 0.00% sys= 0.00% alloc=    15b/s - PERIODIC-COMMIT-LOG-SYNCER
[000105] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-7
[000110] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-12
[000107] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-9
[000114] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-16
[000111] user= 0.00% sys= 0.00% alloc=    60b/s - nioEventLoopGroup-2-13
[000141] user= 0.00% sys= 0.00% alloc=     6b/s - HintedHandoff:2
[001914] user= 0.00% sys= 0.00% alloc=     6b/s - NanoTimeToCurrentTimeMillis updater
[000002] user= 0.00% sys= 0.00% alloc=     0b/s - Reference Handler