Shared Memory Not released

Dear Informix experts, We are currently have Informix Dynamic Online running on our HP T500 machine and recently have shared memory problem which is only known to be solved by brining down and up the database instance(DBA's said so). It appears that shared memory used by threads cannot be released even after application is close. I would highly appreciate if anyone can provide me some hints or point me to the correct direction. Thanks. Following is a log from onstat -m command:- Message Log File: /d2/informix/online.log 13:20:49 size of resident + virtual segments 162562048 + 362864640 > 512000000 total allowed by configuration parameter SHMTOTAL 13:20:49 out of virtual shared memory 13:20:49 Assert Failed: WARNING! No memory available for page cleaners 13:20:49 Who: Thread(43, flush_sub(0), cadf2444, 3) 13:20:49 Results: OnLine may be unable to complete a checkpoint 13:20:49 Action: Make more virtual memory available to OnLine 13:20:49 See Also: /tempfile/af.3e5ec3 13:20:49 size of resident + virtual segments 162562048 + 362864640 > 512000000 total allowed by configuration parameter SHMTOTAL 13:20:49 out of virtual shared memory 13:20:49 Assert Failed: WARNING! No memory available for page cleaners 13:20:49 Who: Thread(43, flush_sub(0), cadf2444, 3) 13:20:49 Results: OnLine may be unable to complete a checkpoint 13:20:49 Action: Make more virtual memory available to OnLine