High termination times pre-concurrent cycle in G1

[Thomas Schatzl]

Hi William,

On Mon, 2016-09-05 at 12:02 +0200, William Good wrote:
> Hi Thomas,
> 
> Attached is a log file from a run with this morning's tip. I don't
> see any change unfortunately.

  hmm. I talked to somebody else about it, and looked a bit at the Akka
code for this and related bugs: the reason why it does not help may
just be because the problem is the data structure itself, not the gc.

I.e. the AbstractNodeQueue from Akka seems to be a linked list of
nodes, probably rather very deep and with low fan-out. So there may be
not a lot of work that can actually be done in parallel.

Additionally, the references into young gen really cause significant
work.

As opposed to the case that JDK-8152438 fixes, that is, if a worker
thread does have a lot of work that could be taken over by other
threads available, but does not share it.

If that is indeed the case, jdk9 will not help a lot if anything. If I
look at another referenced bug [1], AbstractNodeQueue seems kind of a
"textbook" linked list.

Fixing the application seems to be the easiest option here.

Thanks for your effort in pinning this down,
  Thomas

[1] https://github.com/akka/akka/issues/17547

> I'll try with jdk9 tip later today since you mention there are more
> fixes there.
> 
> configure summary looked like this, I think everything here is in
> order:
> 
> Configuration summary:
> * Debug level:    release
> * JDK variant:    normal
> * JVM variants:   server
> * OpenJDK target: OS: linux, CPU architecture: x86, address length:
> 64
> 
> Tools summary:
> * Boot JDK:       java version "1.8.0_101" Java(TM) SE Runtime
> Environment (build 1.8.0_101-b13) Java HotSpot(TM) 64-Bit Server VM
> (build 25.101-b13, mixed mode)  (at /var/tmp/jdk1.8.0_101)
> * C Compiler:     gcc (GCC) 4.8.5 20150623 (Red Hat-4) version 4.8.5
> (at /usr/bin/gcc)
> * C++ Compiler:   g++ (GCC) 4.8.5 20150623 (Red Hat-4) version 4.8.5
> (at /usr/bin/g++)

Yes, that's fine.

Thanks,
  Thomas