RFR 8145628: hotspot metadata classes shouldn't use HeapWordSize or heap related macros like align_object_size

[Chris Plummer]

It seems the allocators always align the size up to at least a 64-bit 
boundary, so doesn't that make it pointless to attempt to save memory by 
keeping the allocation request size word aligned instead of 64-bit aligned?

Chris

On 1/31/16 4:18 PM, David Holmes wrote:
> Hi Coleen,
>
> I think what Chris was referring to was the CDS compaction  work - 
> which has since been abandoned. To be honest it has been so long since 
> I was working on this that I can't recall the details. At one point 
> Ioi commented how all MSO's were allocated with 8-byte alignment which 
> was unnecessary, and that we could do better and account for it in the 
> size() method. He also noted if we somehow messed up the alignment 
> when doing this that it should be quickly detectable on sparc.
>
> These current changes will affect the apparent wasted space in the 
> archive as the expected usage would be based on size() while the 
> actual usage would be determined by the allocator.
>
> Ioi was really the best person to comment-on/review this.
>
> David
> -----
>
> On 31/01/2016 12:27 AM, Coleen Phillimore wrote:
>>
>>
>> On 1/29/16 2:20 PM, Coleen Phillimore wrote:
>>>
>>> Thanks Chris,
>>>
>>> On 1/29/16 2:15 PM, Chris Plummer wrote:
>>>> Hi Coleen,
>>>>
>>>> On 1/28/16 7:31 PM, Coleen Phillimore wrote:
>>>>>
>>>>> Hi Chris,
>>>>>
>>>>> I made a few extra changes because of your question that I didn't
>>>>> answer below, a few HeapWordSize became wordSize.  I apologize that
>>>>> I don't know how to create incremental webrevs. See discussion below.
>>>>>
>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8145628.02/
>>>>>
>>>>> On 1/28/16 4:52 PM, Chris Plummer wrote:
>>>>>> On 1/28/16 1:41 PM, Coleen Phillimore wrote:
>>>>>>>
>>>>>>> Thank you, Chris for looking at this change.
>>>>>>>
>>>>>>> On 1/28/16 4:24 PM, Chris Plummer wrote:
>>>>>>>> Hi Coleen,
>>>>>>>>
>>>>>>>> Can you do some testing with ObjectAlignmentInBytes set to
>>>>>>>> something other than 8?
>>>>>>>
>>>>>>> Okay, I can run one of the testsets with that.  I verified it in
>>>>>>> the debugger mostly.
>>>>>>>>
>>>>>>>> Someone from GC team should apply your patch, grep for
>>>>>>>> align_object_size(), and confirm that the ones you didn't change
>>>>>>>> are correct. I gave a quick look and they look right to me, but I
>>>>>>>> wasn't always certain if object alignment was appropriate in all
>>>>>>>> cases.
>>>>>>>
>>>>>>> thanks - this is why I'd changed the align_object_size to
>>>>>>> align_heap_object_size before testing and changed it back, to
>>>>>>> verify that I didn't miss any.
>>>>>>>>
>>>>>>>> I see some remaining HeapWordSize references that are suspect,
>>>>>>>> like in Array.java and bytecodeTracer.cpp. I didn't go through
>>>>>>>> all of them since there are about 428. Do they need closer
>>>>>>>> inspection?
>>>>>> ??? Any comment?
>>>>>
>>>>> Actually, I tried to get a lot of HeapWordSize in the metadata but
>>>>> the primary focus of the change, despite the title, was to fix
>>>>> align_object_size wasn't used on metadata.
>>>> ok.
>>>>> That said a quick look at the instances of HeapWordSize led to some
>>>>> that weren't in the heap.  I didn't look in Array.java because it's
>>>>> in the SA which isn't maintainable anyway, but I changed a few.
>>>>> There were very few that were not referring to objects in the Java
>>>>> heap. bytecodeTracer was one and there were a couple in 
>>>>> metaspace.cpp.
>>>> Ok. If you think there may be more, or a more thorough analysis is
>>>> needed, perhaps just file a bug to get the rest later.
>>>
>>> From my look yesterday, there aren't a lot of HeapWordSize left. There
>>> are probably still a lot of HeapWord* casts for things that aren't in
>>> the Java heap.  This is a bigger cleanup that might not make sense to
>>> do in one change, but maybe in incremental changes to related code.
>>>
>>>>
>>>> As for reviewing your incremental changes, as long as it was just
>>>> more changes of HeapWordSize to wordSize, I'm sure they are fine.
>>>> (And yes, I did see that the removal of Symbol size alignment was
>>>> also added).
>>>
>>> Good, thanks.
>>>
>>>>>
>>>>> The bad news is that's more code to review.   See above webrev link.
>>>>>
>>>>>>>>
>>>>>>>> align_metadata_offset() is not used. It can be removed.
>>>>>>>
>>>>>>> Okay, I'll remove it.  That's a good idea.
>>>>>>>
>>>>>>>>
>>>>>>>> Shouldn't align_metadata_size()  align to 64-bit like
>>>>>>>> align_object_size() did, and not align to word size? Isn't that
>>>>>>>> what we agreed to? Have you tested CDS? David had concerns about
>>>>>>>> the InstanceKlass::size() not returning the same aligned size as
>>>>>>>> Metachunk::object_alignment().
>>>>>>>
>>>>>>> I ran the CDS tests but I could test some more with CDS. We don't
>>>>>>> want to force the size of objects to be 64 bit (especially Symbol)
>>>>>>> because Metachunk::object_alignment() is 64 bits.
>>>>>> Do you mean "just" because? I wasn't necessarily suggesting that
>>>>>> all metadata be 64-bit aligned. However, the ones that have their
>>>>>> allocation size 64-bit aligned should be. I think David's concern
>>>>>> is that he wrote code that computes how much memory is needed for
>>>>>> the archive, and it uses size() for that. If the Metachunk
>>>>>> allocator allocates more than size() due to the 64-bit alignment of
>>>>>> Metachunk::object_alignment(), then he will underestimate the size.
>>>>>> You'll need to double check with David to see if I got this right.
>>>>>
>>>>> I don't know what code this is but yes, it would be wrong.  It also
>>>>> would be wrong if there's any other alignment gaps or space in
>>>>> metaspace chunks because chunks themselves have an allocation
>>>>> granularity.
>>>>>
>>>>> It could be changed back by changing the function
>>>>> align_metaspace_size from 1 to WordsPerLong if you wanted to.
>>>>>
>>>>> I fixed Symbol so that it didn't call align_metaspace_size if this
>>>>> change is needed in the future.
>>>>>
>>>>> I was trying to limit the size of this change to correct
>>>>> align_object_size for metadata.
>>>> Well, there a few issues being addressed by fixing align_object_size.
>>>> Using align_object_size was incorrect from a code purity standpoint
>>>> (it was used on values unrelated to java objects), and was also
>>>> incorrect when ObjectAlignmentInBytes was not 8. This was the main
>>>> motivation for making this change.
>>>
>>> Exactly. This was higher priority because it was wrong.
>>>>
>>>> The 3rd issue is that align_object_size by default was doing 8 byte
>>>> alignment, and this wastes memory on 32-bit. However, as I mentioned
>>>> there may be some dependencies on this 8 byte alignment due to the
>>>> metaspace allocator doing 8 byte alignment. If you can get David to
>>>> say he's ok with just 4-byte size alignment on 32-bit, then I'm ok
>>>> with this change. Otherwise I think maybe you should stay with 8 byte
>>>> alignment (including symbols), and file a bug to someday change it to
>>>> word alignment, and have the metaspace allocator require that you
>>>> pass in alignment requirements.
>>>
>>> Okay, I can see what David says but I wouldn't change Symbol back.
>>> That's mostly unrelated to metadata storage and I can get 32 bit
>>> packing for symbols on 32 bit platforms.  It probably saves more space
>>> than the other more invasive ideas that we've had.
>>
>> This is reviewed now.  If David wants metadata sizing to change back to
>> 64 bits on 32 bit platforms, it's a one line change.  I'm going to push
>> it to get the rest in.
>> Thanks,
>> Coleen
>>>
>>> Thanks,
>>> Coleen
>>>
>>>>>
>>>>> Thanks for looking at this in detail.
>>>> No problem. Thanks for cleaning this up.
>>>>
>>>> Chris
>>>>>
>>>>> Coleen
>>>>>
>>>>>
>>>>>>> Unfortunately, with the latter, metadata is never aligned on 32
>>>>>>> bit boundaries for 32 bit platforms, but to fix this, we have to
>>>>>>> pass a minimum_alignment parameter to Metaspace::allocate()
>>>>>>> because the alignment is not a function of the size of the object
>>>>>>> but what is required from its nonstatic data members.
>>>>>> Correct.
>>>>>>>   I found MethodCounters, Klass (and subclasses) and ConstantPool
>>>>>>> has such alignment constraints. Not sizing metadata to 64 bit
>>>>>>> sizes is a start for making this change.
>>>>>> I agree with that, but just wanted to point out why David may be
>>>>>> concerned with this change.
>>>>>>>>
>>>>>>>> instanceKlass.hpp: Need to fix the following comment:
>>>>>>>>
>>>>>>>>   97   // sizeof(OopMapBlock) in HeapWords.
>>>>>>> Fixed, Thanks!
>>>>>> thanks,
>>>>>>
>>>>>> Chris
>>>>>>>
>>>>>>> Coleen
>>>>>>>
>>>>>>>>
>>>>>>>> thanks,
>>>>>>>>
>>>>>>>> Chris
>>>>>>>>
>>>>>>>>
>>>>>>>> On 1/27/16 10:27 AM, Coleen Phillimore wrote:
>>>>>>>>> Summary: Use align_metadata_size, align_metadata_offset and
>>>>>>>>> is_metadata_aligned for metadata rather
>>>>>>>>> than align_object_size, etc.  Use wordSize rather than
>>>>>>>>> HeapWordSize for metadata.  Use align_ptr_up
>>>>>>>>> rather than align_pointer_up (all the related functions are ptr).
>>>>>>>>>
>>>>>>>>> Ran RBT quick tests on all platforms along with Chris's Plummers
>>>>>>>>> change for 8143608, ran jtreg hotspot tests and
>>>>>>>>> nsk.sajdi.testlist co-located tests because there are SA
>>>>>>>>> changes.   Reran subset of this after merging.
>>>>>>>>>
>>>>>>>>> I have a script to update copyrights on commit. It's not a big
>>>>>>>>> change, just mostly boring.  See the bug comments for more
>>>>>>>>> details about the change.
>>>>>>>>>
>>>>>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8145628.01/
>>>>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8145628
>>>>>>>>>
>>>>>>>>> thanks,
>>>>>>>>> Coleen
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>