I add ChunkAllocator in this CL to put unused memory chunk to a chunk
pool other than return it to system allocator. Now we only change
MemPool's chunk allocation and free to this.
And two configuration are introduduced too. 'chunk_reserved_bytes_limit'
is the limit of how many bytes this chunk pool can reserve in total and
its default value is 2147483648(2GB). 'use_mmap_allocate_chunk': if
chunk is allocated via mmap and default value is false.
And in my test case with default configuration a simple like
"select * from table limit 10", this can improve throughput from 280 QPS
to to 650 QPS. And when I config 'chunk_reserved_bytes_limit' to 0,
which means this is disabled, the throughput is the same with origin's.
support multiple key ranges in RowwiseIterator and StorageReadOptions
remove unused fields and member functions in RowBlock and ColumnData
read num_rows_per_block from short key index footer
BetaRowsetWriter is used to write rowset in V2 segment format.
This PR contains several interface changes
1. Rowset.make_snapshot() is renamed to `link_files_to` because hard links are also useful in copy task, linked schema change, etc
2. Rowset.copy_files_to_path() is renamed to `copy_files_to` to be consistent with other names
3. RowsetWriter.mem_pool() is removed because not all rowset writers use MemPool
4. RowsetWriter.garbage_collection() is removed because it's not used by clients
5. SegmentGroup's make_snapshot() is removed because link_segments_to_path() provides similar functionality
We create a new segment format for BetaRowset. New format merge
data file and index file into one file. And we create a new format
for short key index. In origin code index is stored in format like
RowCusor which is not efficient to compare. Now we encode multiple
column into binary, and we assure that this binary is sorted same
with the key columns.
