what's the difference between sampler and profiler in JvisualVM?

Question

I found there are two plug-in in JvisualVM, one is sampler and another is profiler.

I also found they have a similar UI, however the results have a big difference, so what's the difference meaning for them?

And why they have a big difference?

Answer 1

In general:

A profiler is running all the time, so it gives you the complete call stack; at any given point in time.

A sampler only takes "snapshots" at distinct point in times.

Thing is: when you "profile" everything, then that slows down your JVM significantly; and it creates enormous amounts of data within a few seconds. Think about: the profiler will write down each and any method invocation that takes place!

So typically, you initially use a sampler, when you have "no idea" what is going on within your application. And then you just hope that the samples tell you something; like "hey, within our 10 000 samples, we are in that one method most of the time, why is that?" But as soon as you have a better understanding what you are "hunting" for, you would try to do a full profiler run in order to capture the whole call chain that leads into some method.

And then there is some "middle ground" - where you profile "everything" but exclude things. In other words: most profilers allow you to say "do not profile methods in classes in this or that package". But of course - excluding packages/hierarchies only makes sense when you already have a pretty good feeling which direction you intend to investigate.