How to change Logstash's default max heap size

To change Logstash's default max heap size, do as follows:

LS_HEAP_SIZE=4g ./bin/logstash -f generator.conf

You can check if it works with `jps -v` as follows:

$ jps -v
15582 Main -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -Djava.awt.headless=true -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -XX:+HeapDumpOnOutOfMemoryError -Xmx4g -Xss2048k -Djffi.boot.library.path=/home/izeye/programs/logstash-2.3.4/vendor/jruby/lib/jni -XX:+UseParNewGC -XX:+UseConcMarkSweepGC -Djava.awt.headless=true -XX:CMSInitiatingOccupancyFraction=75 -XX:+UseCMSInitiatingOccupancyOnly -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/home/izeye/programs/logstash-2.3.4/heapdump.hprof -Xbootclasspath/a:/home/izeye/programs/logstash-2.3.4/vendor/jruby/lib/jruby.jar -Djruby.home=/home/izeye/programs/logstash-2.3.4/vendor/jruby -Djruby.lib=/home/izeye/programs/logstash-2.3.4/vendor/jruby/lib -Djruby.script=jruby -Djruby.shell=/bin/sh
15646 Jps -Dapplication.home=/home/izeye/programs/jdk1.8.0_45 -Xms8m
$

You can see `-Xmx4g` (ie. 4GB).

Reference:
https://www.elastic.co/guide/en/logstash/current/command-line-flags.html