GH-1038 Trim object memory for ArrowBuf

[lriggs]

What's Changed

A significant number of ArrowBuf and BufferLedger objects are created during certain workloads. Saving several bytes per instance could add up to significant memory savings and reduced memory allocation expense and garbage collection.

The id field, which was a sequential value used when logging object information, is replaced with an identity hash code. This should still allow enough information for debugging without the memory overhead. There may be possible duplicate values but it shouldn't matter for logging purposes.

Atomic fields can be replaced by a primitive and a static updater which saves several bytes per instance.

ArrowBuf

Component	Before	After	Savings
idGenerator (static)	AtomicLong	Removed	24 bytes globally
id field (per instance)	long (8 bytes)	Removed	8 bytes per instance
getId()	Returns id field	Returns System.identityHashCode(this)	—

BufferLedger

Component	Before	After	Savings
LEDGER_ID_GENERATOR (static)	AtomicLong	Removed	24 bytes globally
ledgerId (per instance)	long (8 bytes)	Removed	8 bytes per instance
bufRefCnt	AtomicInteger (24 bytes)	volatile int + static updater	20 bytes per instance

Total Savings

Scale	ArrowBuf	BufferLedger	Combined
100K	800 KB	2.8 MB	~3.6 MB
1M	8 MB	28 MB	~36 MB
10M	80 MB	280 MB	~360 MB

Benchmarking

I ran the added benchmark before and after the metadata trimming.

Metadata Trimmed

Benchmark	Mode	Score	Error	Units
MemoryFootprintBenchmarks.measureAllocationPerformance	avgt	456.831	± 36.059	us/op
MemoryFootprintBenchmarks.measureArrowBufMemoryFootprint	ss	161.085	± 35.596	ms/op
Created 100000 ArrowBuf instances. Heap memory used	sum	35631520 bytes (33.98 MB)	0	bytes
Average memory per ArrowBuf	sum	356.32 bytes	0	bytes

Previous Object Layout

Benchmark	Mode	Score	Error	Units
MemoryFootprintBenchmarks.measureAllocationPerformance	avgt	466.171	± 16.233	us/op
MemoryFootprintBenchmarks.measureArrowBufMemoryFootprint	ss	176.790	± 17.943	ms/op
Created 100000 ArrowBuf instances. Heap memory used	sum	38817480 bytes (37.02 MB)	0	bytes
Average memory per ArrowBuf	sum	388.17 bytes	0	bytes

Closes #1038.