RFR 8042668: GC Support for shared heap ranges in CDS (RE: JDK-8059092)
Jiangli Zhou
jiangli.zhou at oracle.com
Sun Jun 7 21:15:27 UTC 2015
Hi Coleen,
Thank you for review! Here is an updated webrev, http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.03/.
On Jun 5, 2015, at 3:43 PM, Coleen Phillimore <coleen.phillimore at oracle.com> wrote:
> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/src/share/vm/classfile/javaClasses.hpp.udiff.html
>
> + string->obj_field_put_raw(value_offset, (oop)buffer);
>
> Do you need the oop cast since objArrayOop is a subclass of oop?
Removed.
>
> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/src/share/vm/classfile/stringTable.cpp.udiff.html
>
> Can you change the name lookup_dynamic to lookup_runtime() instead? I think invokedynamic when I see this or some other dynamic sort of thing. It's just the runtime string table, right? Or lookup_in_main_table() which is longer but it's mostly hidden.
I rename the function to lookup_in_main_table(). It sounds good to me.
>
> + oop s = (oop)(bucket->literal());
>
>
> Is this cast unnecessary since literal() is an oop in this table?
Removed.
>
>
> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/src/share/vm/gc/g1/g1StringDedupThread.cpp.udiff.html
>
> Can you add a comment why you are deduplicating the shared strings? and when this is happening? Is this at startup to prime the deduplication table?
I added following comments:
// The CDS archive does not include the string dedupication table. Only the string
// table is saved in the archive. The shared strings from CDS archive need to be
// added to the string dedupication table before deduplication occurs. That is
// done in the begining of the G1StringDedupThread (see G1StringDedupThread::run()
// below).
>
> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/src/share/vm/memory/filemap.cpp.udiff.html
>
> + buf = _header->region_addr(i);
>
>
> Can you make this statement intialize buf (move the type declaration to this line).
Done.
>
> + addr = _header->region_addr(i);
>
>
> Same with adde.
Done.
>
> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.02/src/share/vm/memory/filemap.hpp.udiff.html
>
> + int _narrow_oop_shift; // compressed oop encoding shift
> + uintx _max_heap_size; // java max heap size during dumping
> + Universe::NARROW_OOP_MODE _narrow_oop_mode;
>
> Can you make _narrow_oop_mode not line up with the comments?
I thought that would look neater. ;) I removed the extra spaces before _narrow_oop_mode and also added a comment for the field.
>
> This whole change looks really good. My comments are minor.
Thanks!
Jiangli
>
> The name change from record to archive looks a lot better!
>
> Thanks,
> Coleen
>
>
> On 6/2/15 3:33 PM, Jiangli Zhou wrote:
>> 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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