This is a warning message. It means that:
The active logs has transactions in it that are committed but not
externalized.  That's what a dirty page is: a change has been processed in
it, the change has been committed but the page has not been externalized
back in its physical location on disk.

The transaction is in the log file but the log file has to remain active
until all trans. have been either rolled back or committed AND externalized.

SOFTMAX causes some externalization to happen when it is hit. Default value
100% of a log file size.
NUM_IOCLEANERS is a parm, default value 1, that determines the number of
processes that can be dispatched to clean, meaning externalize, data pages
to their location.  They're like janitors; if you don't have enough of them
in the school, the classroom stay dirty too long and you can't use them.
SOFTMAX is one of the foremen who dispatches the janitors.

HTH,  Pierre.