I'm afraid to increase the cachesize in DB_CONFIG:
ch_realloc means the system ran out of memory. Increasing the DB_CONFIG cachesize will run you out of memory more quickly.
The best thing for you to do would be to upgrade to OpenLDAP 2.4.33 and switch to using back-mdb.
If you are not capable of doing that, I would suggest you use tcmalloc as an alternative memory allocator to glibc, and reduce your entry cachesize and idlcachesize. If you can reclaim enough space, you should increase your DB_CONFIG cachesize, as that is the most important element of performance with back-bdb/hdb.
--Quanah -- Quanah Gibson-Mount Sr. Member of Technical Staff Zimbra, Inc A Division of VMware, Inc. -------------------- Zimbra :: the leader in open source messaging and collaboration