Logstash file descriptors keep increasing

i'am using logstash version logstash 6.4.3. The problem is the number of file descriptors that keeps increasing and each time i restart logstash it gets back to normal then increases again.
Is that normal behavior ?

Maybe. What does your configuration look like?

My logstash.yml is like this:

path.data: /var/lib/logstash
config.reload.automatic: true
config.reload.interval: 300s
path.logs: /var/log/logstash

and since i'am getting syslog logs, my conf for syslog is:

    input {
    syslog {
        port => 5040
    }
}
  output {
    elasticsearch {
        hosts => ["<ip1>:9200", "<ip2>:9200", "<ip3>:9200"]
        index => "srv-syslog-%{+YYYY.MM.dd}"
    }
}