Missing CHECK_FOR_INTERRUPTS in hash joins
I ran into a case where a hash join took a really long time to respond
to a cancel request --- long enough that I gave up and kill -9'd it,
because its memory usage was also growing to the point where the kernel
would likely soon choose to do that for me. The culprit seems to be
that there's no CHECK_FOR_INTERRUPTS anywhere in this loop in
ExecHashJoinNewBatch():
while ((slot = ExecHashJoinGetSavedTuple(hjstate,
innerFile,
&hashvalue,
hjstate->hj_HashTupleSlot)))
{
/*
* NOTE: some tuples may be sent to future batches. Also, it is
* possible for hashtable->nbatch to be increased here!
*/
ExecHashTableInsert(hashtable, slot, hashvalue);
}
so that if you try to cancel while it's sucking a really large batch file
into memory, you lose. (In the pathological case I was checking, the
batch file was many gigabytes in size, and had certainly never all been
resident earlier.)
Adding a C.F.I. inside this loop is the most straightforward fix, but
I am leaning towards adding one in ExecHashJoinGetSavedTuple instead,
because that would also ensure that all successful paths through
ExecHashJoinOuterGetTuple will do a C.F.I. somewhere, and it seems good
for that to be consistent. The other possibility is to put one inside
ExecHashTableInsert, but the only other caller of that doesn't really need
it, since it's relying on ExecProcNode to do one.
regards, tom lane
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On Wed, Feb 15, 2017 at 2:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Adding a C.F.I. inside this loop is the most straightforward fix, but
I am leaning towards adding one in ExecHashJoinGetSavedTuple instead,
because that would also ensure that all successful paths through
ExecHashJoinOuterGetTuple will do a C.F.I. somewhere, and it seems good
for that to be consistent. The other possibility is to put one inside
ExecHashTableInsert, but the only other caller of that doesn't really need
it, since it's relying on ExecProcNode to do one.
Would it also make sense to put one in the loop in
ExecHashIncreaseNumBatches (or perhaps
ExecHashJoinSaveTuple for symmetry with the above)? Otherwise you
might have to wait for a few hundred MB of tuples to be written out
which could be slow if IO is somehow overloaded.
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Thomas Munro
http://www.enterprisedb.com
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Thomas Munro <thomas.munro@enterprisedb.com> writes:
On Wed, Feb 15, 2017 at 2:22 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
Adding a C.F.I. inside this loop is the most straightforward fix, but
I am leaning towards adding one in ExecHashJoinGetSavedTuple instead,
Would it also make sense to put one in the loop in
ExecHashIncreaseNumBatches (or perhaps
ExecHashJoinSaveTuple for symmetry with the above)? Otherwise you
might have to wait for a few hundred MB of tuples to be written out
which could be slow if IO is somehow overloaded.
Mmm, good point. I think in that case the C.F.I. had better be in
the loop in ExecHashIncreaseNumBatches, because if you were unlucky
the loop might not take the ExecHashJoinSaveTuple path for a long time.
Looking around at other callers of ExecHashJoinSaveTuple, the only one
that seems to be in need of a C.F.I. is the loop in
ExecHashRemoveNextSkewBucket, and there again there's a code path
whereby the loop doesn't call ExecHashJoinSaveTuple.
Will CFI-ify all three places.
regards, tom lane
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