add comment to SimCache to estimate actual capacity

Summary: as title

Test Plan: make all check

Reviewers: yiwu

Reviewed By: yiwu

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D62493

include/rocksdb/utilities/sim_cache.h

@@ -23,6 +23,14 @@ class SimCache;
 // to predict block cache hit rate without actually allocating the memory. It
 // can help users tune their current block cache size, and determine how
 // efficient they are using the memory.
+//
+// Since GetSimCapacity() returns the capacity for simulutation, it differs from
+// actual memory usage, which can be estimated as:
+// sim_capacity * entry_size / (entry_size + block_size),
+// where 76 <= entry_size <= 104,
+// BlockBasedTableOptions.block_size = 4096 by default but is configurable,
+// Therefore, generally the actual memory overhead of SimCache is Less than
+// sim_capacity * 2%
 extern std::shared_ptr<SimCache> NewSimCache(
     std::shared_ptr<Cache> cache, size_t sim_capacity, int num_shard_bits,
     std::shared_ptr<Statistics> stats = nullptr);