SA for ZGC

[Stefan Karlsson]

Hi Yasumasa,

Here's my first stab at adding the ZGC load barries to the SA:

http://cr.openjdk.java.net/~stefank/zgc/zSALoadBarriers/webrev.01/

I find it unfortunate that we have to duplicate the C++ code into Java, 
to get this to work. But it is what it is.

Some comments about the code:

1) Some of the constants are Linux-specific and there's no platform 
abstraction in place.

2) ZOop.to_address subverts the type system so that we can get from 
OopHandles and longs to Addresses. This seems to be frowned upon if you 
read the SA code. Maybe there are more correct ways to implement this?

3) I use the ZGC weak barrier, since the SA agent only needs to be able 
to access the Objects, not mark them. Which is what the weak barriers 
provide.

4) I see that you used VMObjectFactory.newObject, I skipped that part, 
but we can think about unifying our patches.

5) I had to invent a way to read global variables from HotSpot. See the 
usage of ZGlobalsForVMStructs. Maybe there's a more correct way to do this?

I stress tested the thread dumping part that threw exceptions for me 
earlier by running SPECjbb2005:
$ java -XX:+UseZGC -Xlog:gc -cp jbb.jar:check.jar spec.jbb.JBBmain

and then continuously dumping the threads:
$ while true; do echo -e "verbose true\nthreads" | jhsdb -J-ea clhsdb 
--pid=`jps | grep JBBmain | awk '{print $1}'`; done


Cheers,
StefanK

On 2018-01-09 13:59, Yasumasa Suenaga wrote:
> Hi Stefan,
> 
> 
>> Let me try to create an initial prototype for load barriers in the SA 
>> case and see where this takes us.
> 
> Sure!
> 
> 
>> FYI i also found that compressed oops are not fully supported in the SA:
>> https://bugs.openjdk.java.net/browse/JDK-8194737
> 
> Thanks, I didn't know that.
> I hope this issue will be fixed soon :-)
> 
> 
> Yasumasa
> 
> 
> On 2018/01/09 18:36, Stefan Karlsson wrote:
>> Hi Yasumasa,
>>
>> Thanks for your contribution!
>>
>> I tested your patch together with jhsdb hsdb and immediately hit 
>> problems because of the lack of load barriers in the SA code. Let me 
>> try to create an initial prototype for load barriers in the SA case 
>> and see where this takes us.
>>
>> FYI i also found that compressed oops are not fully supported in the SA:
>> https://bugs.openjdk.java.net/browse/JDK-8194737
>>
>> Thanks,
>> StefanK
>>
>> On 2017-12-21 23:38, Yasumasa Suenaga wrote:
>>> Thanks Per!
>>>
>>> I'm waiting for comments and sponsorship.
>>>
>>>
>>> Yasumasa
>>>
>>>
>>> On 2017/12/22 4:33, Per Liden wrote:
>>>> Hi Yasumasa,
>>>>
>>>> On 2017-12-21 09:40, Yasumasa Suenaga wrote:
>>>>> Hi all,
>>>>>
>>>>> I checked SA implementation for ZGC, but it is not yet available.
>>>>> For example, we can see WrongTypeException when CLHSDB `universe`
>>>>> command is executed.
>>>>>
>>>>> As the first step, I propose to implement ZCollectedHeap and 
>>>>> related classes for
>>>>>   SA as this webrev:
>>>>>
>>>>>    http://cr.openjdk.java.net/~ysuenaga/z/sa-universe/
>>>>>
>>>>> After applying this patch, we can use `universe` command on CLHSDB.
>>>>> (I followed `VM.info` jcmd output format)
>>>>
>>>> Thanks! Most of us working on ZGC are currently on vacation, but 
>>>> we'll have a closer look at the patch after the holidays.
>>>>
>>>> cheers,
>>>> Per
>>>>
>>>>>
>>>>> Of course, it is not all. We need to work more for it.
>>>>>
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Yasumasa
>>>>>