In case you're using logstash 5.6.x from elastic, version 5.6.9 is released with logstash-filter-grok 4.0.3. This one fixes a bad memory leak that was a cause for frequent logstash crashes since logstash 5.5.6. Reference: https://github.com/logstash-plugins/logstash-filter-grok/issues/135

I hope this is now again a decent logstash 5.x release. I've heard some rumours that the 6.x versions is also a bit plagued by memory leaks.