RSS goes up when limit max_open_files from 1000 to 100

[979357361]

I try to limit the memory usage of RocksDB, here are some settings:

rocksdb_block_based_options_set_cache_index_and_filter_blocks(table_option, true);
rocksdb_block_based_options_set_pin_top_level_index_and_filter(table_option, true);
rocksdb_block_based_options_set_pin_l0_filter_and_index_blocks_in_cache(table_option, true);
rocksdb_block_based_options_set_cache_index_and_filter_blocks_with_high_priority(table_option, true);

rocksdb_block_based_options_set_index_type(table_option, rocksdb_block_based_table_index_type_two_level_index_search);
rocksdb_block_based_options_set_partition_filters(table_option, true);
rocksdb_block_based_options_set_metadata_block_size(table_option, 4096);

rocksdb_block_based_options_set_filter_policy(table_option, rocksdb_filterpolicy_create_bloom_full(10.0));
rocksdb_block_based_options_set_optimize_filters_for_memory(table_option, true);

the hyper clock cache is set to 7928MB, I think limit max_open_files to a smaller value could help,
when max_open_files=1000, stat shows:

         mem_table_total 15MB,
         mem_table_readers_total 1708690B,
         block_cache_total 7927MB,

rocksdb.estimate-table-readers-mem: 2067
Resident Set Size: 8352 MB

after I set max_open_files=100, Resident Set Size: 8352 MB goes up to 8550MB

There are three doubts:

1. why limit max_open_files from 1000 to 100 let RSS goes up 200MB?
2. the memory collected by RocksDB genarally has a big gap with RSS, how can I estimate the memory used by rocksdb?
3. stats from rocksdb.estimate-table-readers-mem is not the same as MemoryUtil::kTableReadersTotal, from source code they should be the same?

is there any guideline to tell me how to calculate the memory usage and how to limit it?
Any help is welcomed, thank you!