Dear All
possible problem in QueryIterDistinct.requestSubCancel()
We have been having problems with timeouts on some of our
production servers. On certain queries, timeouts of some
tens of seconds do not appear to fire, no results are
generated by the query, and the query continues to
consume a CPU. If left long enough (more than an hour)
the query finally terminates in a NullPointerExplosion.
We have been able to replicate the issue using generated
data and a slimmed-down query with four triples and
DISTINCT.
Investigation with the debugger led us to QueryIterDistinct's
requestSubCancel method. Unlike other requestSubCancel
declarations, its body is not empty and it specifically
super.closes the current iterator. Further, following the
chain of cancel-requestCancel methods from the
invocation of the timeout shows that the cancels stop
here.
We think that the cancellation protocol is satisfied
by having QueryIterDistinct.requestSubCancel have
an empty body, but we're not sure we've understood
it completely. Certainly removing requestSubCancel's
body removed the presenting problem, with the query
timing out cleanly.
I'll submit a JIRA with the data generator and
query when I've clreaned it up a bit, but I
thought a heads-up would be a good idea (and maybe
our reading of the cancel protocol is worng ...)
Chris
--
"He could not weigh up which was worse and so tried not to think about either."
/The Spellgrinder's Apprentice/
Epimorphics Ltd, http://www.epimorphics.com
Registered address: Court Lodge, 105 High Street, Portishead, Bristol BS20 6PT
Epimorphics Ltd. is a limited company registered in England (number 7016688)
