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

[Vladimir Kozlov]

Looks  good.

Vladimir

On 6/12/13 11:58 PM, Bengt Rutisson wrote:
>
> 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