Where is rest of the memory?

[amitavmohanty01 (Amitav Mohanty)]

I am trying to install Logstash 7.4.0 from the RPM. I have a 6.8.3 RPM already installed and the process is running. I am getting the following error:

Downloading packages:
Delta RPMs disabled because /usr/bin/applydeltarpm not installed.
logstash-7.4.0.rpm                                                                                                                                                                                                   | 166 MB  00:00:01     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Updating   : 1:logstash-7.4.0-1.noarch                                                                                                                                                                                                1/2 
warning: /etc/logstash/jvm.options created as /etc/logstash/jvm.options.rpmnew
warning: /etc/logstash/log4j2.properties created as /etc/logstash/log4j2.properties.rpmnew
Using provided startup.options file: /etc/logstash/startup.options
Errno::ENOMEM: Cannot allocate memory - systemctl
            spawn at org/jruby/RubyProcess.java:1567
            spawn at org/jruby/RubyKernel.java:1646
        popen_run at /usr/share/logstash/vendor/jruby/lib/ruby/stdlib/open3.rb:202
           popen3 at /usr/share/logstash/vendor/jruby/lib/ruby/stdlib/open3.rb:98
          execute at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/detector.rb:74
   detect_systemd at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/detector.rb:29
  detect_platform at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/detector.rb:24
           detect at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/detector.rb:18
   setup_defaults at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/cli.rb:153
          execute at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/cli.rb:119
              run at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:67
              run at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/pleaserun-0.0.30/lib/pleaserun/cli.rb:114
              run at /usr/share/logstash/vendor/bundle/jruby/2.5.0/gems/clamp-0.6.5/lib/clamp/command.rb:132
           <main> at /usr/share/logstash/lib/systeminstall/pleasewrap.rb:12
Unable to install system startup script for Logstash.
  Cleanup    : 1:logstash-6.8.3-1.noarch                                                                                                                                                                                                2/2 
  Verifying  : 1:logstash-7.4.0-1.noarch                                                                                                                                                                                                1/2 
  Verifying  : 1:logstash-6.8.3-1.noarch  

To fix it, I had to stop the process and install the package.

I have allocated 50% of RAM to logstash. So, I assumed rest should be available for the system. Total RAM is 8 gb. So, please suggest why there is no memory left.