RFR: 8210064: ZGC: Introduce ZConcurrentRootsIterator for scanning a subset of strong IN_NATIVE roots concurrently

[Per Liden]

Hi Erik,

On 09/04/2018 11:39 AM, Erik Österlund wrote:
> Hi,
> 
> Coleen pushed 8210155 before I pushed this. So now I need to wrap the 
> CLDG iterations in a CLDG lock.
> I made a patch that adds the lock and relaxes the locking assert 
> appropriately so that the thread invoking the worker gang can take the 
> lock. This implies that the worker threads will not own the lock, but 
> that is okay.
> 
> Incremental:
> http://cr.openjdk.java.net/~eosterlund/8210064/webrev.01_02/
> 
> Full:
> http://cr.openjdk.java.net/~eosterlund/8210064/webrev.02/

Looks good. Just one small thing. It seems the old 
assert_locked_or_safepoint() can now be expressed as 
assert_locked_or_safepoint_weak() plus some extra conditions.

/Per

> 
> Thanks,
> /Erik
> 
> On 2018-08-31 10:37, Erik Österlund wrote:
>> Hi Per,
>>
>> On 2018-08-31 09:57, Per Liden wrote:
>>> Hi Erik,
>>>
>>> On 08/30/2018 10:46 AM, Erik Österlund wrote:
>>>> Hi,
>>>>
>>>> We now have enough load barriers to support scanning of CLDs and JNI 
>>>> handles concurrently. I propose to do that and move these root sets 
>>>> out from ZRootsIterator, and hence the GC pause. They will be 
>>>> scanned during concurrent marking (and heap iteration), but no 
>>>> longer during relocation.
>>>>
>>>> I still perform ClassLoaderDataGraph::clear_claimed_marks() in the 
>>>> pause because it seems cheap. But it can be moved out of the pause 
>>>> when Coleen gets her new cool CLDG lock in.
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~eosterlund/8210064/webrev.00/
>>>
>>> I have some minor requests. Instead of listing them all, I attached a 
>>> patch which addresses those.
>>>
>>> The main thing is that I don't think ZDriver should know about 
>>> "concurrent roots", just that it's doing "mark" or "mark continue", 
>>> so I suggest we turn that into a "bool initial" argument to mark() 
>>> instead of exposing a mark_concurrent_roots() function.
>>
>> Sure, that makes sense.
>>
>> Thanks for the review!
>>
>> /Erik
>>
>>> The other things are minor style adjustments.
>>> /Per
>>>
>>>>
>>>> Bug:
>>>> https://bugs.openjdk.java.net/browse/JDK-8210064
>>>>
>>>> Tested through hs-tier1-3.
>>>>
>>>> Thanks,
>>>> /Erik
>>
>