When a UDF hits a memory limit, it calls RuntimeState::SetMemLimitExceeded() which sets the query status, but it has no way of returning status directly. It relies on the caller checking status periodically.
HdfsTableSink::Send() checks for errors by calling RuntimeState::CheckQueryState() once at the beginning. If it is evaluating a UDF and that UDF hits the memory limit, it will need to process the whole RowBatch before it aborts the query. This could be 1024 rows and each row may hit a memory limit in that UDF. Other locations that process UDFs may be processing considerably more rows.
There are two general approaches:
- Code locations should check for status more frequently and thus abort faster after a RuntimeState::SetMemLImitExceeded() call.
- RuntimeState::SetMemLimitExceeded() should be substantially cheaper, allowing the rows to be processed faster.
RuntimeState::SetMemLimitExceeded() currently calls MemTracker::MemLimitExceeded() unconditionally. It then checks to see if it should update query_status_ (i.e. query_status_ is currently ok). Then it logs this error. This is wasteful, because MemTracker::MemLimitExceeded() is not a cheap function, and this is flooding the log for each row. RuntimeState::SetMemLimitExceeded() should check status before running MemTracker::MemoryLimitExceeded(). If query_status_ is already not ok, it can avoid the cost of the dump and logging.