Description of problem:
 It looks like dtablesize is being set to the maxfiledescriptors soft limit. 

 This problem was found while migrating from RHEL 7 IDM to RHEL 8 IDM at large client site. I came up at first with systemd override for maxfiledescriptors with value 65535 that resolved issues at client site. Though after further investigation was able to see the issue was not the value of the maxfiledescriptors. But that dtablesize was being set to the soft limit. 


Version-Release number of selected component (if applicable):
     Affects both RHEL 8 & RHEL 9.


How reproducible:

It looks like dtablesize is being set to the maxfiledescriptors soft limit. 

Problem:
=======
     389ds is not setting dtablesize properly based when systemd is setting the maxfiledescriptors with it's default settings.
     dtablesize stays 1024 which causes massive slowdown once you hit around 950 connection. Basically queries are very slow. Also can't
     go above around ~970 concurrent connections.

      It looks like dtablesize is being set to the soft limit when it needs to be set to the actual limit. 

      So with no changes settings you see running commands:
                systemctl show dirsrv@<INSTANCE> |grep -i Limitnofile
                -----------------------------------------------------------------------------
                - RHEL 8 (Directory Server)
                      LimitNOFILE=262144
                      LimitNOFILESoft=1024      < ---- Notice

                - RHEL 9 (IDM system)
                     LimitNOFILE=524288
                     LimitNOFILESoft=1024        <--- Notice

               dsconf  <instance> config get nsslapd-maxdescriptors
               --------------------------------------------------------------------------
                - RHEL 8 
                      nsslapd-maxdescriptors: 262144

                - RHEL 9
                       nsslapd-maxdescriptors: 524288


                - 
 (THE PROBLEM)     dsconf <instance> monitor server |grep dtablesize 
                                   --------------------------------------------------------------------
                                     - RHEL 8
                                            dtablesize: 1024

                                     - RHEL 9
                                            dtablesize: 1024


WorkAround:
==========
        Once in place connections can go very very high. Testing showed 7,000+ at client site. Also queries very fast.

        Looks like this works because the override sets both hard and soft limit. So dtablesize gets a properly.
         
         Workaround
         -----------------
         - RHEL 8
                    mkdir -p /etc/systemd/system/dirsrv@.service.d/
                    cat > /etc/systemd/system/dirsrv@.service.d/filelimts.conf << EOF
                    [Service]
                    LimitNOFILE=262144
                    EOF
                  
                    systemctl daemon-reload
                    systemctl restart dirsrv@<instance>
                      
          - RHEL 9
                    mkdir -p /etc/systemd/system/dirsrv@.service.d/
                    cat > /etc/systemd/system/dirsrv@.service.d/filelimts.conf << EOF
                    [Service]
                    LimitNOFILE=524288
                    EOF
                  
                    systemctl daemon-reload
                    systemctl restart dirsrv@<instance>       
          
               systemctl show dirsrv@<INSTANCE> |grep -i Limitnofile
                -----------------------------------------------------------------------------
                - RHEL 8 (Directory Server)
                          LimitNOFILE=262144
                          LimitNOFILESoft=262144    <---- THIS !!
 
                - RHEL 9 (IDM system)
                           LimitNOFILE=524288             
                           LimitNOFILESoft=524288   <-- THIS !!
                           
                    
               dsconf  <instance> config get nsslapd-maxdescriptors
               --------------------------------------------------------------------------
                - RHEL 8 
                      nsslapd-maxdescriptors: 262144

                - RHEL 9
                       nsslapd-maxdescriptors: 524288

              
                dsconf <instance> monitor server |grep dtablesize 
                --------------------------------------------------------------------
                - RHEL 8
                         dtablesize:  262144

                 - RHEL 9
                         dtablesize: 524288



Possible FIX
==========
     Looks like the fix would be to have dtablesize set to maxfiledescriptors or the hard maxfiledescriptor limit.