How 'expensive' is it to execute jstack on a running JVM?

Question

I'm considering whipping up a script to

1. run once a minute (or every five minutes)
2. run jstack against a running JVM in production
3. parse the jstack output and tally up things I'm interested in
4. export the results for graphing 24/365 by a centralized Cacti installation on another server

But I've no clue as to how expensive or invasive jstack is on a running JVM. How expensive is it to execute jstack on a running JVM? Am I setting myself up for a world of hurt?

Answer 1

Depending on the number of threads and the size of your heap, jstack could possibly be quite expensive. JStack is meant for troubleshooting and wasn't designed for stats gathering. It might be better to use some form on instrumentation or expose a JMX interface to get the information you want directly, rather than have to parse stack traces.

Answer 2

Measure. One of the time variants (/usr/bin/time I believe) has a -p option which allows you to see the cpu resources used:

ravn:~ ravn$ /usr/bin/time -p echo Hello World
Hello World
real         0.32
user         0.00
sys          0.00
ravn:~ ravn$ 

This means that it took 0.32 seconds wall time, using 0.00 seconds of cpu time in user space and 0.00 seconds in kernel space.

Create a test scenario where you have a program running but not doing anything, and try comparing with the usage WITH and WITHOUT a jstack attaching e.g. every second. Then you have hard numbers and can experiment to see what would give a reasonable overhead.

My hunch would be that once every five minutes is neglectible.