FEAT: Multiple regions support

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FEAT: Multiple regions support

Rationale

• Increasing concurrency (there is no need for heavy-weight synchronization between appends to different regions)
• Logical division of entires
• Grouping objects of different sizes together (can improve performance for concurrent append - appending similarly sized entires will result in bigger overlap for memcpy)

Description

• Persistent allocator
  • Store alloc/free actions on PMEM (which allow rebuilding the state), do compaction regularly
• Synchronization between iterators and region_free needed
  • Use delayed free / garbage collection?
• Region discovery TBD (optional)
  • Named regions?
  • Root region?

API Changes

No changes - region allocate and free is already supported.

Implementation details

Most of the code is ready to handle multiple regions. There are a few global variables which should be made per-region (move to region_runtime), e.g. region_lock from region_runtimes_map

First steps

• trivial persistent allocator based on free list
• no synchronization for iterators and region_free
• no region discovery (only iterators)

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Next steps:

• assign timestamp for each allocation (can be easily done if each allocation/free is logged as an entry in dedicated region). This will allow using region allocations with appends in a single transaction
• allow iterating over regions in order of allocation (based on timestamps)