Server crashes

Dear All, I have a big problem with my freeRadius server the log file each less than a minute logs that the server can't allocate memory Error: FATAL: Thread create failed: Cannot allocate memory I have this configuration start_servers = 20 max_servers = 400 min_spare_servers = 30 max_spare_servers = 60 max_requests_per_server = 0 i have 4G memory , this is the Top result Mem: 4090068k av, 420312k used, 3669756k free, 0k shrd, 15408k buff i got about 3G free memory , how comes that the radius can't allocate memory !!