[Monetdb-developers] GDKupgradevarheap allocate/copy inconsistency

Isidor Zeuner monetdb at quidecco.de
Fri Jan 22 16:15:07 CET 2010


> > 
> > [1] https://sourceforge.net/tracker/?func=detail&aid=2936616&group_id=56967&atid=482468
> 
> I saw the tracker item, and was wondering about it.
> 
> What did you do to get into the situation?  In other words, how can I
> reproduce?
> 

Unfortunately, I don't have an isolated failing case yet. I am
continuously sending queries (mostly SELECT, INSERT and DELETE) using
MonetDB-SQL, many of which are grouped inside large
transactions. After initializing the database, it takes a while for
the issue to occur, and I'm not yet sure what triggers it.

I am currently adding instrumentation code around the code
communicating with Mapi, which will hopefully get me some reproducible
sessions for this and the other issue I am seeing

> I don't think BATs created in VIEWcreate_ should end up in
> GDKupgradevarheap.  Those BATs are views, and I would think views are
> read-only, and read-only BATs should not end up here.
> 

Yes, intuitively it seemed a bit unusual when I thought about it, but
then again I am not much used to the MonetDB internals yet.

I had found GDKupgradevarheap being supplied with a capacity less than
heap.size >> shift (it was capacity == 256, heap.size == 512 and shift
== 0), and was wondering where it came from. So I added assert
statements to all locations modifying capacity, shift or the heap
allocation, which errored out in VIEWcreate_. It happened about the
same time where I would have expected the GDKupgradevarheap issue to
occur, so I found it most likely to be related.

Of course the problematic BAT might still come from another source, so
if you say views won't reach GDKupgradevarheap, I'll probably
go for another run without the assertions in VIEWcreate_. On the other
hand, will it suffice to put assert(!isVIEW(b)) statements in front of
the GDKupgradevarheap calls in the code to verify?

> The BAT capacity is the number of elements that can be stored in the
> BAT, i.e. it is exactly the size of the heap (apart from the
> multiplication with the width of the elements).  However, this may break
> down in the case of views.  When I wrote the code, I wasn't thinking of
> views, since I didn't expect them to end up here, and I still think they
> shouldn't.
> 
> Having said this, it is probably possible, perhaps even equivalent, to
> use the size as you suggested instead of the capacity -- as long as
> views are left out of the equation.
> 

So, as views aren't supposed to get there anyway, capacity is
redundant in that function, right?

Best regards,

Isidor Zeuner




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