RFR (S): 8165292: The gc+task logging is repeated a lot, decreasing the usefulness of -Xlog:gc*=info

[Thomas Schatzl]

Hi all,

  can I have reviews for this small change that fixes log spamming of
the form

[26.584s][info][gc,task ] GC(15) GC Workers: using 18 out of 18 
[26.585s][info][gc,task ] GC(15) GC Workers: using 18 out of 18 

that is repeated like 10 times per gc. This kind of clogs the
PrintGCDetails output.

This log message originates from WorkGang::update_active_workers(),
every time the number of active workers is updated. Since with jdk9
this occurs quite often, you get a lot of these messages.

Looking at the code, there are actually two concerns here: first,
notifying the user that the (default) number of a worker for the
workgang has changed due to policy, second some apparent dev message
notifying of any change of the number of workers.

We want the first kind of message at relatively high log level, but not
necessarily the second.

So the solution is to add a info level message at the places where the
code consciously updates the number of default threads (once per
gc/marking), and move the developer messages about any change to the
number of workers to trace level.

I added a few (imo) out-of-scope CRs to improve the developer messages
(JDK-8165449) and make sure that every time we run a gang task a useful
message is printed (i.e. unify the message, because at the moment this
is at the discretion of the developer when adding a new gang
task), JDK-8165450.

CR:
https://bugs.openjdk.java.net/browse/JDK-8165292
Webrev:
http://cr.openjdk.java.net/~tschatzl/8165292/webrev/
Testing:
jprt, test case

Thanks,
  THomas