We found warning in log of ASE like this:
Space available in the log segment has fallen critically low in database 'myDB'. All future modifications to this database will be suspended until the log is successfully dumped and space becomes available.

But we have already set "trunc log on chkpt" to true when create out application, and we checked database after such a warning , found no dead-lock or suspend database operation.
by the way ,we have no operation big enough to exhaust even 2% space of log segment.

It seems checkpoint task did not execute correctly to truncate transaction log.
who can tell me any other reason could cause such a warning ?