Review request: 8026391: The Metachunk header wastes memory - 8026392: Metachunks and Metablocks are using a too large alignment

[Stefan Karlsson]

On 10/15/13 10:01 PM, Coleen Phillimore wrote:
>
> This is good, so we align metachunks to 8 byte alignment rather than 
> 16 in 64 bit?
> #define ARENA_AMALLOC_ALIGNMENT (2*BytesPerWord)

Yes. Just like we did with the PermGen.

thanks,
StefanK
>
> Coleen
>
> On 10/15/2013 02:11 PM, Stefan Karlsson wrote:
>> From an offline discussion with Jon, I decided to change the following:
>>
>> $ hg diff
>> diff -r 8e2b57a0132c src/share/vm/memory/metachunk.cpp
>> --- a/src/share/vm/memory/metachunk.cpp    Tue Oct 15 14:32:20 2013 
>> +0200
>> +++ b/src/share/vm/memory/metachunk.cpp    Tue Oct 15 20:07:51 2013 
>> +0200
>> @@ -37,8 +37,8 @@
>>    // so that 64 bit types get correctly aligned.
>>    const size_t alignment = 8;
>>
>> +  // Make sure that the Klass alignment also agree.
>>    STATIC_ASSERT(alignment == (size_t)KlassAlignmentInBytes);
>> -  STATIC_ASSERT(alignment == (size_t)BytesPerLong);
>>
>>    return alignment;
>>  }
>>
>> StefanK
>>
>> On 10/14/13 1:58 PM, Stefan Karlsson wrote:
>>> Please, review these two patches to remove some of the wastages 
>>> introduced in the metaspace.
>>>
>>>
>>> 8026391: The Metachunk header wastes memory
>>>  http://cr.openjdk.java.net/~stefank/8026391/webrev.00/
>>>  https://bugs.openjdk.java.net/browse/JDK-8026391
>>>
>>> There are a couple of fields in Metachunk that store already known 
>>> information, like the beginning and the end of the Metachunk. I 
>>> replaced those fields with code instead.
>>>
>>> To make this easier, I extracted some duplicated code in Metablock 
>>> and Metachunk into a super class named Metamem. The extracted code 
>>> is purely needed to allow these objects to be placed on the FreeList 
>>> and in the BinaryTreeDictionary.
>>>
>>>
>>> 8026392: Metachunks and Metablocks are using a too large alignment
>>>  http://cr.openjdk.java.net/~stefank/8026392/webrev.00/
>>>  https://bugs.openjdk.java.net/browse/JDK-8026392
>>>
>>> The metaspace code was using ARENA_ALIGN to align the Metablocks and 
>>> Metachunk header. This forced all allocations of metadata to be 16 
>>> bytes aligned. This patch patch changes the restriction to be 8 
>>> bytes instead, since that's the restriction needed for 64 bit types 
>>> like long, double, pointers and klass pointers,
>>>
>>>
>>> Testing: Added unit test for Metachunk. JPRT (before splitting of 
>>> the patches)
>>>
>>> thanks,
>>> StefanK
>>
>