RFR 8042668: GC Support for shared heap ranges in CDS (RE: JDK-8059092)
Jiangli Zhou
jiangli.zhou at oracle.com
Tue Jun 2 19:33:12 UTC 2015
Here is the updated runtime webrev reflects the name changes: http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/
Thanks,
JIangli
On Jun 2, 2015, at 4:39 AM, Tom Benson <tom.benson at oracle.com> wrote:
> 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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