Heap size of AWS ElasticSearch cluster - elasticsearch

Question

I have an AWS ElasticSearch t2.medium instance with 2 nodes running, and hardly any load on it. Still it is crashing all the time.
I see the following graph for the metric JVMMemoryPressure:
When I go to Kibana, I see the following error message:
Questions:
Do I interpret correctly that the machines only have 64 MB of memory available, instead of the 4 GB that should be associated with this instance type? Is there another place to verify the absolute amount of heap memory, instead of on Kibana only when it is going wrong?
If so, how can I change this behavior?
If this is normal, where can I look for possible causes of ElasticSearch crashing whenever the memory footprint reaches 100%. I have only very small load on the instance.
In the logging of the instance, I see a lot of warnings, e.g. the ones below. They don't provide any clue for where to start debugging the issue.
[2018-08-15T07:36:37,021][WARN ][r.suppressed ] path: __PATH__ params:
{}
org.elasticsearch.cluster.block.ClusterBlockException: blocked by: [__PATH__ master];
at org.elasticsearch.cluster.block.ClusterBlocks.globalBlockedException(ClusterBlocks.java:165) ~[elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.action.bulk.TransportBulkAction$BulkOperation.handleBlockExceptions(TransportBulkAction.java:387) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.action.bulk.TransportBulkAction$BulkOperation.doRun(TransportBulkAction.java:273) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.action.bulk.TransportBulkAction$BulkOperation$2.onTimeout(TransportBulkAction.java:421) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.cluster.ClusterStateObserver$ContextPreservingListener.onTimeout(ClusterStateObserver.java:317) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.cluster.ClusterStateObserver$ObserverClusterStateListener.onTimeout(ClusterStateObserver.java:244) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.cluster.service.ClusterApplierService$NotifyTimeout.run(ClusterApplierService.java:578) [elasticsearch-6.0.1.jar:6.0.1]
at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:569) [elasticsearch-6.0.1.jar:6.0.1]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_172]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_172]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_172]
or
[2018-08-15T07:36:37,691][WARN ][o.e.d.z.ZenDiscovery ] [U1DMgyE] not enough master nodes discovered during pinging (found [[Candidate{node={U1DMgyE}{U1DMgyE1Rn2gId2aRgRDtw}{F-tqTFGDRZaovQF8ILC44w}{__IP__}{__IP__}{__AMAZON_INTERNAL__, __AMAZON_INTERNAL__}, clusterStateVersion=207939}]], but needed [2]), pinging again
or
[2018-08-15T07:36:42,303][WARN ][o.e.t.n.Netty4Transport ] [U1DMgyE] write and flush on the network layer failed (channel: [id: 0x385d3b63, __PATH__ ! __PATH__])
java.io.IOException: Connection reset by peer
at sun.nio.ch.FileDispatcherImpl.writev0(Native Method) ~[?:1.8.0_172]
at sun.nio.ch.SocketDispatcher.writev(SocketDispatcher.java:51) ~[?:1.8.0_172]
at sun.nio.ch.IOUtil.write(IOUtil.java:148) ~[?:1.8.0_172]
at sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:504) ~[?:1.8.0_172]
at io.netty.channel.socket.nio.NioSocketChannel.doWrite(NioSocketChannel.java:432) ~[netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.AbstractChannel$AbstractUnsafe.flush0(AbstractChannel.java:856) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.nio.AbstractNioChannel$AbstractNioUnsafe.forceFlush(AbstractNioChannel.java:368) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:638) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeysPlain(NioEventLoop.java:544) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:498) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:458) [netty-transport-4.1.13.Final.jar:4.1.13.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858) [netty-common-4.1.13.Final.jar:4.1.13.Final]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_172]

Answer 1

I have learned that that number is incorrect. I don't know where it is coming from. To get the correct memory usage, one runs the following query:
GET "<es_url>:9200/_nodes/stats"

Answer 2

If you're looking for only memory usage, use GET /"<es_url>:9200/_cat/nodes?h=heap* - it gives a more readable response like below.
{
"payload": [
{
"heap.current": "4.1gb",
"heap.max": "15.9gb",
"heap.percent": "25"
},
{
"heap.current": "3.9gb",
"heap.max": "15.9gb",
"heap.percent": "24"
},
...
}
_nodes/stats is elaborate with all other details also, though.