I am looking to understand precisely how to use the load average and cpu usage on a Red hat machine that hosts a Tomcat 8 only. After looking on the net, I concluded the following assertions. Are the assertions right ? I am deeply sure of the first one since it comes from official Tomcat documentation. And I am confused about which processes could be in uninterruptible sleep.
1) Tomcat uses a thread to process a request, the maximum number of used threads is defined by Tomcat configuration ( see Tomcat documentation )
2) Oracle JVM works with native threads only since JRE 1.3 ( See JVM and threads I did not find an Oracle reference for this point)
3) Linux’s run queue contains processes and threads (id native threads) the same way ( See Linux Load Averages: Solving the Mystery and Wikipedia )
4) Load average provides the average number of process/threads in the run queue ( See Linux Journal )
5) On a Linux machine running a Tomcat only, the load average provides almost the average number of requests.
6) On Linux the load average count process/threads in state running, runnable, and uninterruptible sleep ( See Linux Journal )
7) Process/threads in uninterruptible sleep are waiting for disks I/O, non interruptible locks, network I/O ( See Redhat documenationt that includes network I/O and Linux Load Averages: Solving the Mystery that does not include network I/0 )
The point 7 is not coherent with the reference See Linux Journal that says that “In fact, it is precisely the CPU load that is measured, because load averages do not include any processes or threads waiting on I/O, networking, databases or anything else not demanding the CPU. “.
I understood that if a process reads swap it is in uninterruptible sleep but if it reads a file on internal disk, nfs folder or a SAN bay, is it in uninterruptible sleep ? Red hat documentation listed network, if a process requests a resource on the network is it in uninterruptible sleep ?