Set lower-bound on dynamic level sizes

[ajkr]

Changed dynamic leveling to stop setting the base level's size bound below max_bytes_for_level_base.

Behavior for config where max_bytes_for_level_base == level0_file_num_compaction_trigger * write_buffer_size and same amount of data in L0 and base-level:

• Before Level-based L0->L0 compaction #2027, compaction scoring would favor base-level due to dividing by size smaller than max_bytes_for_level_base.
• After Level-based L0->L0 compaction #2027, L0 and Lbase get equal scores. The disadvantage is L0 is often compacted before reaching the num files trigger since write_buffer_size can be bigger than the dynamically chosen base-level size. This increased write-amp.
• After this diff, L0 and Lbase still get equal scores. Now it takes level0_file_num_compaction_trigger files of size write_buffer_size to trigger L0 compaction by size, fixing the write-amp problem above.

Test Plan:

command: ./db_bench -benchmarks=fillrandom -options_file=/home/andrewkr/myrocks-options-modified -statistics -stats_per_interval=1 -stats_interval=10000000 -num=500000000

• Before Level-based L0->L0 compaction #2027
  • Throughput: 19.7 MB/s
  • W-Amp: 8.8
  • rocksdb.stall.micros: 46676943
• After Level-based L0->L0 compaction #2027
  • Throughput: 19.9 MB/s
  • W-Amp: 10.0
  • rocksdb.stall.micros: 21486605
• After this diff
  • Throughput: 19.8 MB/s
  • W-Amp: 8.6
  • rocksdb.stall.micros: 761554

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[ajkr]

ping, I've been running fillrandom benchmarks on larger DBs (2B keys) and am finding this has some small benefit to stalling, throughput, and write-amp. I also think it's more intuitive to have the base level's size bound be exactly max_bytes_for_level_base, rather than a constantly-changing number less than that. Especially since we guide people to set max_bytes_for_level_base = L0 size, this diff is useful as it prevents the LSM from assuming an hourglass shape.

[ajkr]

ping @siying, can we commit today or tomorrow?

[siying]

@ajkr yes go ahead.