CPU usage of Java + JDBC

While testing my webapp under load I got following top excerpt under Linux:

 PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
3964 nobody    20   0 4965m 622m 6048 S  8.5 11.0   6:02.49 java 
1985 mysql     20   0  294m 125m 3804 S  2.1  2.2   0:05.39 /usr/sbin/mysqld

I need an explanation on java's %CPU column. As I understand it, during web request one of java's thread is performing some pure Java logic, consuming some o开发者_开发百科f CPU time (let it be 5ms). Then it connects to database via JDBC, sends some sql query and waits, say 10ms, for response. These 10ms are obviously counted as /usr/sbin/mysqld CPU usage. Then java thread resumes it's operation and finishes, consuming another 20ms and amounting to 5+10+20=30ms total execution time.

And the newbie question is: don't you think that these db-related 10ms are counted twice: one time as java thread waiting for db to process the query and second time as database CPU usage itself? What don't I understand here?

When a process is simply waiting on a socket it will not normally be using significant amounts of CPU. Basically, the kernel knows that it is in a waiting state and will not schedule it to run - except for little things like signal handling, which can be used e.g. to run the garbage collector regularly.

(OK, if there is a lot of garbage to collect, that "little thing" might not be so little.)