[Reminder for myself] Performance Monitoring

Determine the points where requests are waiting or slow

If system is busy, deny request at the earliest entry point (e.g. if database is busy then stop accepting requests at the web server)

If under load stop accepting requests and

• process pending requests or
• drop them all or
• restart application 

If adjusting system make sure you're not just moving the load to some other component (e.g from CPU->HDD)

Things to check for

Java:

• Pools size
• Heap utilization
• Garbe collection
• Cache hit ratio
• Messaging systems

Communication:

• Network I/O (bytes r/w, throughput)

OS:

• Disk I/O
• CPU usage
• Memory usage
• Threads usage