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

[Coleen Phillimore]

Looks good to me too.
Coleen

On 06/13/2013 09:07 AM, Mikael Gerdin wrote:
> Bengt,
>
> On 2013-06-13 08:58, Bengt Rutisson wrote:
>>
>> Hi everyone,
>>
>> Could I have a couple of reviews for this change?
>>
>> http://cr.openjdk.java.net/~brutisso/8012265/webrev.00/
>
> this looks good to me.
>
> /Mikael
>
>>
>> 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