Hi *,
for our project we use big transactions for application coordination.
As we need to recover from failed statements on statement level we
encapsulate
our statements with SAVEPOINT/RELEASE savepoint statements.
So far this seems to be the only way to recover from failed statements
and to
keep the enclosing transaction intact.
Things we observed:
1) For transactions with many statements we observe continuously
increasing
memory consumption in the backend.
2) Along with it the throughput decreases during the course of
statement processing. The
processing is CPU bound during that time until the transaction
finishes or rather aborts.
3) We seldom see the entire transaction commit as it is ABORTED due
to an out of memory
condition before the application can commit.
4) The backend server log shows MANY (in our case 15805) entries of
the form:
CurTransactionContext: 8192 total in 1 blocks; 8176 free (2 chunks);
16 used
This is a resource utilization of 0.2%.
Of the 128Mb allocated data size the the backend process 96% is used
for an
effective data amount of 252880 bytes.
5) Looking into the source READMEs (/backend/utils/mmgr/README,
backend/access/transam/README)
it looks like it is intentional that the backend allocates and keeps
memory for each started and commited
sub-transaction until the transaction is finally commited at top level.
Questions:
1) Is there another way to cope with failed statements without using
savepoint and without
aborting the entire transaction?
2) Would it be possible to let the parent subtransaction adopt the
state that is currently being saved
in the memory allocated for the commited child subtransaction? If
so, it would dramatically
enhance resource efficiency and possibly improve performance by
reducing adminitrative overhead.
Regards and thanks in advance,
Frank Kardel & Ansgar Seiter