RFR JDK-8059092: Store Interned Strings in CDS Archives

Bengt Rutisson bengt.rutisson at oracle.com
Mon Jun 1 08:34:55 UTC 2015


Sorry. Sent this reply in the wrong email thread. Will re-post in the 
correct one.

Bengt


On 01/06/15 10:05, Bengt Rutisson wrote:
>
> Hi Tom,
>
> Thanks for providing the great summary that eased the review process. 
> This change looks good to me.
>
> Some minor comments:
>
> g1Allocator.cpp
>
>  214   assert(_bottom >= _allocation_region->bottom(), "inconsistent 
> allocation state");
>  215   assert(_max <= _allocation_region->end(), "inconsistent 
> allocation state");
>  216   assert(_bottom <= old_top && old_top <= _max, "inconsistent 
> allocation state");
>
> Maybe add err_msg() to see what the values actually were?
>
>
> g1CollectedHeap.cpp
>
>  965 bool
>  966 G1CollectedHeap::check_archive_addresses()
>
> We usually don’t have a linefeed after the return type. So, this 
> should be:
>
>  965 bool G1CollectedHeap::check_archive_addresses()
>
> Same for G1CollectedHeap::alloc_archive_regions() and 
> G1CollectedHeap::fill_archive_regions()
>
>
> g1MarkSweep.cpp
>
>  306 void G1MarkSweep::enable_archive_object_check() {
>  307   if (!_archive_check_enabled) {
>
> Shouldn’t this be called exactly once, either from 
> G1RecordingAllocator::create_allocator() or 
> G1CollectedHeap::alloc_archive_regions()? In that case maybe the “if” 
> should be changed to an assert or guarantee that 
> !_archive_check_enabled holds.
>
> Thanks,
> Bengt
>
>
> On 29/05/15 21:39, Jiangli Zhou wrote:
>> Greetings,
>>
>> Please review the changes for supporting interned String objects and 
>> the underling character arrays in the CDS shared archive. The webrevs 
>> listed below only include the runtime changes. The webrev for GC 
>> specific changes will be sent out by Tom Benson shortly.
>>
>> JEP
>> https://bugs.openjdk.java.net/browse/JDK-8059092
>>
>> Webrev
>> hotspot : 
>> http://cr.openjdk.java.net/~jiangli/8059092/webrev_hotspot.01/index.html
>> whitebox: 
>> http://cr.openjdk.java.net/~jiangli/8059092/webrev_wb.01/index.html
>>
>> Summary
>> The shared strings support is added on top of the basic CDS function 
>> that enables the archiving of the interned String objects and 
>> String’s underlying ‘value' array objects from the java heap. During 
>> CDS dump time, all Strings referenced from the string table is copied 
>> into a special region in java heap, which is at the top of the dump 
>> time java heap. The special region is referred as the string space. 
>> While copying the String related objets, a compact table is created 
>> for storing the references to the copied Strings. Both the compact 
>> table and the string space are written into the CDS archive with the 
>> rest of the class metadata during dumping. The compact table for 
>> shared strings uses the same format as the shared symbol table in CDS 
>> archive and shares implementations.
>>
>> At runtime, the string space is mapped at the same offset from the 
>> narrow oop encoding base as dump time within the java heap. That 
>> allows the shared strings to be ‘partially’ relocatable, which means 
>> the runtime java heap could be at different address location and have 
>> different size from the dump time java heap as long as the same 
>> narrow oop encoding can be used. If the narrow oop encoding changes 
>> due to the large difference between the dump-time and runtime heap 
>> sizes, the shared string space from the CDS archive is ignored and 
>> not mapped to the VM address space.
>>
>> The mapped string space is an ‘archive’ region in the java heap. All 
>> shared objects residing within the region are not collected or 
>> forwarded by GC. GC activities never write to the memory pages that 
>> are mapped as the shared string space. The identity hash of shared 
>> objects in the string space are pre-computed during CDS archive dump 
>> time. The only possible ‘write’ to the shared string space at runtime 
>> is from synchronization on the shared objects. That allows majority 
>> or all mapped string memory to be sharable between different VM 
>> processes.
>>
>> Only 64-bit process is supported for shared strings due to the 
>> dependency on the narrow oop support. Windows is not supported 
>> currently.
>>
>> Performance Results
>> Memory
>> Tested using about 3M of string data for memory measurement. Memory 
>> results were measured using linux ps_mem tool.
>> No Shared String
>>   Private  +   Shared  =  RAM used    Program   Saving
>>   28.0 MiB + 110.5 KiB =  28.1 MiB    java
>>   31.5 MiB +  12.6 MiB =  44.2 MiB    java (2)
>>   47.2 MiB +  12.7 MiB =  59.9 MiB    java (3)
>>   63.2 MiB +  12.8 MiB =  76.1 MiB    java (4)
>>   78.0 MiB +  12.9 MiB =  90.8 MiB    java (5)
>>
>> With Shared String
>>   27.6 MiB + 111.5 KiB =  27.7 MiB    java           0.4M
>>   23.7 MiB +  16.3 MiB =  40.0 MiB    java (2)      4.2M
>>   35.3 MiB +  16.4 MiB =  51.7 MiB    java (3)      8.2M
>>   48.3 MiB +  16.5 MiB =  64.8 MiB    java (4)    11.3M
>> 60.6 MiB+ 16.5 MiB= 77.2MiB java(5) 13.6M
>>
>> Runtime Performance
>> Tested on isolated linux-x64 machine.
>> SpecJVM98
>> ============================================================================== 
>>
>> logs.specjvm.before2:
>>    Benchmark           Samples        Mean     Stdev Geomean Weight
>>    specjvm98                10      603.39     23.25
>> ============================================================================== 
>>
>> logs.specjvm.after2:
>>    Benchmark           Samples        Mean     Stdev   %Diff P  
>> Significant
>>    specjvm98                10      604.89     10.85    0.25 
>> 0.856            *
>> ============================================================================== 
>>
>>
>> No performance degradation shown in specjvm.
>>
>> Testing
>> Tested with:
>> Developed unit tests
>> JPRT
>> Full QA test cycle: vm.gc, vm.runtime, nsk.sysdict, vm.metaspace, 
>> vm.quick, JCK: vm, lang, api, KS-24hrs, runThese
>> Thanks,
>> Jiangli
>>
>



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