Summary:
Dummy cache size of 1MB is too large for small block sizes. Our GetDefaultCacheShardBits() use min_shard_size = 512L * 1024L to determine number of shards, so 1MB will excceeds the size of the whole shard and make the cache excceeds the budget.
Change it to 256KB accordingly.
There shouldn't be obvious performance impact, since inserting a cache entry every 256KB of memtable inserts is still infrequently enough.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5175

Differential Revision: D14954289

Pulled By: siying

fbshipit-source-id: 2c275255c1ac3992174e06529e44c55538325c94