RFR(s): 8234086: VM operation can be simplified

[Robbin Ehn]

Hi David, thanks for having a look.

On 11/19/19 2:27 PM, David Holmes wrote:
> Hi Robbin,
> 
> On 19/11/2019 9:05 pm, Robbin Ehn wrote:
>> Hi all, please review.
> 
> Preliminary comments ... so ...
> 
> +class VM_Operation : public StackObj {
>    public:
>     enum Mode {
>       _safepoint,       // blocking,        safepoint, vm_op C-heap allocated
> -    _no_safepoint,    // blocking,     no safepoint, vm_op C-Heap allocated
> -    _concurrent,      // non-blocking, no safepoint, vm_op C-Heap allocated
> -    _async_safepoint  // non-blocking,    safepoint, vm_op C-Heap allocated
> +    _no_safepoint    // blocking,     no safepoint, vm_op C-Heap allocated
>     };
> 
> 
> You are basically getting rid of concurrent and async_safepoint VM op 
> capability. Okay. But you're also making all VM ops StackObj so all those "VM op 
> C-heap allocated" comments are no longer correct. Also many of the comments 
> around the VM ops you have changed from async to sync,and from C-heap to 
> stackobj, are also no longer correct.
> 
> Please update.

Not sure if I found all:
http://cr.openjdk.java.net/~rehn/8234086/v2/inc/webrev/index.html
http://cr.openjdk.java.net/~rehn/8234086/v2/full/webrev/index.html

Thanks, Robbin

> 
> Thanks,
> David
> -----
> 
>> CMS was the last real user of the more advantage features of VM operation.
>> VM operation can be simplified to always be an stack object and thus either be
>> of safepoint or no safepoint type.
>>
>> VM_EnableBiasedLocking is executed once by watcher thread, if needed (default 
>> not used). Making it synchrone doesn't matter.
>> VM_ThreadStop is executed by a JavaThread, that thread should stop for the 
>> safepoint anyways, no real point in not stopping direct.
>> VM_ScavengeMonitors is only used to trigger a safepoint cleanup, the VM op is 
>> not needed. Arguably this thread should actually stop here, since we are about 
>> to safepoint.
>>
>> There is also a small cleanup in vmThread.cpp where an unused method is removed.
>> And the extra safepoint is removed:
>> "// We want to make sure that we get to a safepoint regularly"
>> No we don't :)
>>
>> Issue:
>> https://bugs.openjdk.java.net/browse/JDK-8234086
>> Change-set:
>> http://cr.openjdk.java.net/~rehn/8234086/v1/webrev/index.html
>>
>> Tested scavenge manually, passes t1-2.
>>
>> Thanks, Robbin