db_bench: Add table-readers-mem benchmark (#575)

This benchmark prints the memory estimated to be used by the table readers.
Excluding blocks in block cache.

HISTORY.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+### Enhancements
+* db_bench: add estimate-table-readers-mem benchmark which prints these stats.
+
 ## Fig v2.5.0 (06/14/2023)
 Based on RocksDB 8.1.1
 

tools/db_bench_tool.cc

@@ -298,6 +298,8 @@ DEFINE_string(
     "\twaitforcompaction - pause until compaction is (probably) done\n"
     "\tflush - flush the memtable\n"
     "\tstats       -- Print DB stats\n"
+    "\ttable-readers-mem    -- Print table readers memory. excluding memory "
+    "used in block cache\n"
     "\tresetstats  -- Reset DB stats\n"
     "\tlevelstats  -- Print the number of files and bytes per level\n"
     "\tmemstats  -- Print memtable stats\n"
@@ -3963,6 +3965,9 @@ class Benchmark {
         PrintStats("rocksdb.block-cache-entry-stats");
       } else if (name == "stats") {
         PrintStats("rocksdb.stats");
+      } else if (name == "table-readers-mem") {
+        fprintf(stdout, "table-readers-mem");
+        PrintStats("rocksdb.estimate-table-readers-mem");
       } else if (name == "resetstats") {
         ResetStats();
       } else if (name == "verify") {