RFR: 8224820: ZGC: Support discontiguous heap reservations

[Stefan Karlsson]

Looks good.

StefanK

On 2019-09-17 12:05, Per Liden wrote:
> Hi,
> 
> Discussed with Erik off-line. In light of things like future Windows 
> support, we probably don't want to make the reservation code shared, 
> instead just move it to os/posix.
> 
> Here's a proposed webrev for doing just that:
> 
> http://cr.openjdk.java.net/~pliden/8224820/webrev.0
> 
> /Per
> 
> On 9/5/19 3:51 PM, Per Liden wrote:
>> On 8/9/19 11:59 AM, Erik Österlund wrote:
>>> Hi,
>>>
>>> Today ZGC reserves a huge chunk of virtual address space when the JVM 
>>> starts. This typically succeeds because we grab the VA before anyone 
>>> else has time to do so. But if some ASLR library or something was to 
>>> grab a tiny part of the desired VA, ZGC can't start. We should 
>>> support discontiguous heap reservations to support this.
>>>
>>> On linux, by default, this does not happen. But on OS X, it does 
>>> happen relatively frequently. So we need to fix this to allow a mac 
>>> port.
>>>
>>> This patch implements a recursive algorithm for finding holes at 2MB 
>>> granularities in the normally contiguous reservation when 
>>> initializing the heap, removing them from our VA.
>>>
>>> This patch depends on 8224815, which depends on 8229189 and 8229278. 
>>> They are all out for review.
>>>
>>> Webrev:
>>> http://cr.openjdk.java.net/~eosterlund/8224820/webrev.00/
>>
>> Looks good. However, please pass down max_capacity from ZPageAllocator 
>> to ZVirtualMemory's constructor, instead of reading MaxHeapSize.
>>
>> cheers,
>> Per
>>
>>>
>>> Bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8224820
>>>
>>> Thanks,
>>> /Erik