Close sometime takes a long time

Why does close sometime take a long time? mdb_env_close (in sync7320902242508427276.so) + 69 [0x112e37d35] mdb_env_close0 (in sync7320902242508427276.so) + 248 [0x112e37948] close (in libsystem_kernel.dylib) + 10 [0x7fff8bf900c2] Usually, happens when I have killed the process the last time. System: Mac OS X Thanks, Venkat Murty