Tools for finding solr performance issues

I think you should start by reviewing Solr Performance Factors on the Solr Wiki. You can also use the tool SolrMeter to help with stress testing your changes.

If you are looking for long-term monitoring you may also look into tools/services like Scalable Performance Monitoring (SPM) from my company, Sematext or others. http://sematext.com/spm/index.html

This service will expose request rates, latency, all Solr cache info, JVM memory, GC, CPU, load, disk and network IO, etc.

We eat our own dog food and use this tool to monitor Solr performance of Solr instances behind search-lucene.com and search-hadoop.com/ and we use it regularly with our customers when we need to help them around Solr performance tuning.

If you don't want "proper" performance monitoring a la above, then you can use tools like vmstat, iostat, sar, top, jstack, etc. for troubleshooting Solr performance, assuming you run it under UNIX.