What is your entry cache size? I suggest you set it to something small, like 100, during the slapcat test.
It was 5000
I've tried 100 and ulimit mem at 1024000
after about 50 mins of slapcat:
1600 MB ca. MEM allocated coredump (1691966556 bytes)
output ldif ca. 20% of full DB :(
Paolo Rossi