RFR (S): [increase HeapBaseMinAddress for G1] 8012265: VM often crashes on solaris with a lot of memory

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?

http://cr.openjdk.java.net/~brutisso/8012265/webrev.00/

Sending this to the broad hotspot list since it affects compressed oops, 
which is a VM wide concern.

Background:

The G1 GC uses malloc extensively to allocate auxiliary data structures. 
On Solaris malloc requires a consecutive chunk of memory for the C-heap. 
It picks this chunk in the low addresses so when hotspot maps the Java 
heap in the low addresses to get compressed oops we limit the available 
memory for the C-heap on Solaris.

For compressed oops the Java heap is mapped at HeapBaseMinAddress, which 
by default is 2GB on all platforms except Solaris x86 where it is 256MB. 
If we have a large Solaris x86 we get many GC worker threads and many G1 
heap regions. All of them requiring substantial C-heap allocations.

The combination of a limited C-heap and a lot of mallocs for G1 may 
cause us to run out of memory on large Solaris x86 machines, even with 
our default settings.

The long term solution is to change G1 to be more efficient about its 
memory consumption and preferably use mapped memory instead of malloced 
memory. This is a lot of work, but will definitely have to be done.

A short term solution to reduce the risk of running out of memory is to 
increase the default value of HeapBaseMinAddress for G1. That is what my 
proposed patch does.

This will reduce the maximum possible heap sizes for compressed oops 
without a heap base. Shifted compressed oops will be reduced from almost 
32g to 31g and unshifted compressed oops from almost 4g to 3g. Still 
more than on any other platform where HeapBaseMinAddress=2g by default. 
It will only affect G1 and it can be overridden by explicitly setting 
HeapBaseMinAddress on the command line.

I've filed this enhancement in JBS to track the work to change back the 
default value:

JDK-8016505: G1: Revert back to use HeapBaseMinAddress=256m on Solaris x86

Thanks,
Bengt