why my virtual memory still increasing when openldap is running?

openldap: 2.0.17 ; OS: RH9.0

my virtual memory still increasing , no matter what operation i did, such as search/add/modify.

the top result is:

Mem: 3608380k av, 3218344k used, 390036k free, 0k shrd, 82776k buff 2062984k actv, 56724k in_d, 97036k in_c Swap: 3124560k av, 5312k used, 3119248k free 2868164k cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND 9415 root 15 0 122M 122M 1156 S 3.7 3.4 1:22 3 slapd and it quickly to 500M, when the system's memory near to 10M, openldap could not work properly