CPU Usage

Elastic Stack Elasticsearch
1

I have some question
I run 2 nodes with elasticsearch 2.1
and I run to check all proccess with curl localhost:9200/_nodes/stats?pretty=true
and I get
"os" : {
"timestamp" : 1451405974295,
"load_average" : 0.73,
"mem" : {
"total_in_bytes" : 2097991680,
"free_in_bytes" : 28123136,
"used_in_bytes" : 2069868544,
"free_percent" : 1,
"used_percent" : 99
},

but when I check use terminal use top
my cpu usage verry small only 50% when I sum all my process
In my question is why I get diffrent use curl localhost:9200/_nodes/stats?pretty=true and top

2

how much memory you assign to run elasticsearch server?

3

if I run curl -XGET 'http://localhost:9200/_cluster/stats?human&pretty' I get
"mem" : {
"heap_used" : "324.7mb",
"heap_used_in_bytes" : 340546968,
"heap_max" : "495mb",
"heap_max_in_bytes" : 519045120
},
"threads" : 79
},
I set per node ES_HEAP_SIZE=256m

My server have 2 GB