RFR: 8134963 [Newtests] New tests for G1 remembered set up and down

[Thomas Schatzl]

Hi Dmitry,

On Fri, 2015-11-13 at 18:15 +0300, Dmitry Fazunenko wrote:
> Hi everyone,
> 
> Can I have a couple of reviews for the new test for G1 Remembered Set:
> 
> https://bugs.openjdk.java.net/browse/JDK-8134963
> http://cr.openjdk.java.net/~dfazunen/8134963/
> 
> The test creates a lot of cross region references (making RSet to grow 
> up) and then clear that references up.
> The test doesn't check any particular assertions, it just expects that 
> there will be no crash.

Some comments:

  - the test does not stress the remembered set operations well:

     - runtime is too short. In my test runs, not a single remembered
set cleanup occurs, i.e. the remembered set will never shrink.

     - in general, due to the short runtime, g1 seems to be warming up.

     - the test does not stress the need for remsets being MT safe at
all. I.e. the main program uses one thread.

  - accidentally the test has been run in some jprt jobs. I got a few
out-of-native memory errors running it (that may be due to some other
changes in that build though).

So I think while the test may be correct, it just does not seem to do a
good job on its task.

I frequently use a test based on a contribution years ago (well,
basically a verbatim copy) when working on remembered sets (see the
attachment at
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2012-July/004740.html), running it for a few minutes. Of course, it is a lot more demanding than the test in this change. Maybe it can be scaled back to a useful size, and potentially the object size used there varied a little.

Thanks,
  Thomas