-1/0 virtualtransaction

Hi -

Does anyone have any pointers on what a virtualtransaction of '-1/0' means?

I'm using SSI and an example is

locktype | database | relation | page | tuple | virtualxid |
transactionid | classid | objid | objsubid | virtualtransaction | pid |
mode | granted | fastpath
----------+----------+----------+---------+-------+------------+---------------+---------+-------+----------+--------------------+-----+------------+---------+----------
page | 16384 | 468238 | 19216 | | |
| | | | -1/0 | | SIReadLock |
t | f

This is incredibly hard to search for.

I see these for page, tuple and (perhaps a clue?) one relation, and I'm
seeing the page and tuple locks accumulate over time. Eventually this
results in an "out of shared memory" error.

Any help is appreciated.

Thanks,
Mike