Description of problem:

While testing "nsslapd-subtree-rename-switch=off" there was noticeable performance improvement.  The current implementation generates the DN dynamically by using the entryrdn attribute in the database record, and then obtaining the parent portion of the DN from other database indexes.  

Disabling subtree-rename (nsslapd-subtree-rename-switch=off) improves throughput/response_time performance especially when the number of clients increases.

The range of improvement on highest load (high number of clients) ranges up to ~10%


subtree-rename=OFF 
----------------------------------------------------
Workers         Clients           SRCH/s        GAIN
15              40              142302.868      + 0%
15              45              157062.354      + 4%    
15              50              165993.278      +12%
15              55              168967.106      +15%

20              40              133749.551      + 2%
20              45              148529.026      + 0%
20              50              150216.182      + 1%
20              55              114247.201      +13%

25              40              123818.029      + 1%
25              45              141934.678      + 1%
25              50              153096.810      + 0%
25              55              109355.695      +10%


subtree-rename=ON
----------------------------------------------------
Workers         Clients         SRCH/s
15              40              142297.329
15              45              150931.420
15              50              147570.526
15              55              146769.262

20              40              130967.392
20              45              148365.746
20              50              148903.385
20              55              121286.399

25              40              122408.771
25              45              140797.527
25              50              151812.222
25              55               98683.023


We could also maintain the existing operational attribute "entrydn" and only use it for DN "generation" on searches.  It would need to be updated on adds, and modrdns.  There could also be an impact on tombstones queries as well.