RFR 8042668: GC Support for shared heap ranges in CDS (RE: JDK-8059092)

Tom Benson tom.benson at oracle.com
Tue Jun 2 11:39:28 UTC 2015


Hi,
An updated webrev addressing the comments from Per and Bengt is 
athttp://cr.openjdk.java.net/~brutisso/8042668/webrev.01/ .
I also updated the notes in the JBS entry to reflect the name changes.
Tom

On 6/1/2015 11:22 AM, Tom Benson wrote:
> Hi Per,
> Thanks very much for the review.
>
> On 6/1/2015 10:35 AM, Per Liden wrote:
>> Hi Tom,
>>
>> On 2015-05-29 23:30, Tom Benson wrote:
>>> Hi,
>>> Please review these changes for JDK-8042668, which constitute the GC
>>> support for JDK-8059092 for storing interned strings in CDS archives
>>> (JEP 250).  The RFR for JDK-8059092 was recently posted by Jiangli 
>>> Zhou,
>>> and it would be best if overall comments could go to that thread, with
>>> GC-specific comments here.
>>>
>>> JBS:   https://bugs.openjdk.java.net/browse/JDK-8042668
>>> Webrev: http://cr.openjdk.java.net/~brutisso/8042668/webrev.00/
>>
>>
>> Maybe it's just me, but the concept of "recording" feels a bit 
>> strange in this context. May I suggest that we remove the "record" 
>> and "recording" part of the names and instead just call it an 
>> "archive" that we can allocate in? Something like:
>>
>> class G1ArchiveAllocator ... {
>>   HeapWord* mem_allocate(...);
>>
>>   void finalize(...);
>>
>>   ...
>> };
>>
>> class G1CollectedHeap ... {
>>   void begin_archive_mem_allocate();
>>
>>   bool is_archive_mem_allocation_too_large(...);
>>
>>   HeapWord* archive_mem_allocate(...);
>>
>>   void end_archive_mem_allocate(...);
>>
>>   ...
>> };
>
> Hmmm....   Yes, I guess the name "RecordingAllocator" does show the 
> evolution of the design, more than the ultimate use.  It was named 
> "recording" because it allowed a way to keep track of the recorded 
> ranges, in contrast with an earlier design that allocated a block of 
> memory up front.   I'm fine with changing this to an ArchiveAllocator 
> as you suggest, if I hear no objections.
>
>>
>>
>> g1CollectedHeap.cpp
>> -------------------
>>
>> * In G1CollectedHeap::end_record_alloc_range(), shouldn't we delete 
>> the allocator as the last step?
>>
>
> Yes.  I think I made that change at one point and then removed it for 
> some reason, which may be gone.  I'll re-make it.
>
>
>> * I guess this change could be skipped, as it makes the comment 
>> slightly malformed.
>>
>> -        // We ignore humongous regions, we left the humongous set 
>> unchanged
>> +        // We ignore humongous regions.
>> +        // We left the humongous set unchanged,
>>
>
> OK.
>
>>
>> g1Allocator.hpp
>> ---------------
>>
>> + class G1RecordingAllocator : public CHeapObj<mtGC> {
>> +   friend class VMStructs;
>>
>> You could skip this friend declaration, since it's not accessed by 
>> VMStructs. Only needed if the class is exposed in the SA.
>>
>>
>
> OK.  Thanks,
> Tom
>
>> cheers,
>> /Per
>>
>>>
>>> These changes add a new "archive" region type to G1.  The description
>>> field in JDK-8042668 contains an "Implementation Notes" section which
>>> describes components of the design, and should be useful for a code
>>> review.   The overview:
>>>
>>>     "Archive" regions are G1 regions that are not modifiable by GC,
>>>     being neither scavenged nor compacted, or even marked in the object
>>>     header. They can contain no pointers to non-archive heap regions,
>>>     and object headers point to shared CDS metaspace (though this last
>>>     point is not enforced by G1). Thus, they allow the underlying
>>>     hardware pages to be shared among multiple JVM instances.
>>>
>>>     In short, a dump-time run (using -Xshare:dump) will allocate space
>>>     in the Java heap for the strings which are to be shared, copy the
>>>     string objects and arrays to that space, and then archive the 
>>> entire
>>>     address range in the CDS archive. At restore-time (using
>>>     -Xshare:on), that same heap range will be allocated at JVM init
>>>     time, and the archived data will be mmap'ed into it. GC must treat
>>>     the range as 'pinned,' never moving or writing to any objects 
>>> within
>>>     it, so that cross-JVM sharing will work.
>>>
>>> Testing:  All testing for JDK-8059092 included this code. Manual
>>> testing with prototype calls to the new GC support was performed before
>>> integration, along with JPRT and benchmark runs.
>>>
>>> Performance:  The GC changes had no significant impact on SpecJBB, JVM,
>>> or Dacapo benchmarks, run on x64 Linux.  However, a small (~1%) 
>>> increase
>>> in Full GC times was seen in tests when the shared string support was
>>> not in use, when runs are configured to encounter them.   When shared
>>> strings ARE in use, the impact could be as high as 5% for a likely
>>> worst-case.   Please see the JBS entry for a discussion.
>>>
>>> Thanks,
>>> Tom
>>>
>



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