This agent validates JFR traces for the CPU time sampler and
the normal sampler by comparing it with each other
and the results of calling Thread.getAllStackStraces()
(essentially my tiny-profiler.
It is used to validate the upcoming CPU time sampler (see parttimenerd_jfr_cpu_time_sampler3.
We assume that all three samplers should produce the same result when walking the stack at exactly the same time.
The minimal sampling interval for JFR is 1ms, so the events are somewhat spaced. But this is not a problem:
To check if a sample s is valid, we get the sample of the
same thread from the other sampler that lies directly before (a) and the one
directly after our sample (b).
There should than be a significant overlap between the common prefix of a and b and
our sample s (many of the top frames should be the same).
This is especially true if our other sampler has a small sampling interval.
The idea is now to compare all three samplers with each other and report the cases where no overlap is found, which should be mostly errors. We also report the cases where the overlap is very small compared to the total stack depth.
Truncated traces are ignored for simplicity.
We implemented this as a Java agent, that runs the sampling alongside the application.
We can use this to compute the effective sampling intervals for each sampler and thread, giving the median relative difference, to hopefully show that the CPU time sampler closer adheres to the sampling interval.
Instrument N percent of all methods with entry and exit and use this for validation.
Required JDK 17 or newer.
mvn packageRun
java -javaagent:target/trace-validation ...to start the validation.
See java -javaagent:target/trace-validation=help for options.
Example: For running it with renaissance, use
test -e renaissance.jar || wget https://github.com/renaissance-benchmarks/renaissance/releases/download/v0.15.0/renaissance-gpl-0.15.0.jar -O renaissance.jar
java -javaagent:target/validation-agent.jar -jar renaissance.jar allMIT, Copyright 2024 SAP SE or an SAP affiliate company, Johannes Bechberger and factor analyzer contributors
This project is a prototype of the SapMachine team at SAP SE.