FreeSpaceMap hashtable out of memory - Mailing list pgsql-hackers

Hi,
Using PG under Cygwin we having following error message during INSERT
INTO
"FreeSpaceMap hashtable out of memory".

What does that mean?
And if for a moment step out of knowledge 'PG under Cygwin', what in
general
this message is about and more important how to fix it?

Thank you.