Re: PostgreSql: Canceled on conflict out to old pivot

On 28/11/2023 07:41, Wirch, Eduard wrote:
> ERROR: could not serialize access due to read/write dependencies among
> transactions
>   Detail: Reason code: Canceled on identification as a pivot, with
> conflict out to old committed transaction 61866959.
>
> There is a variation of the error:
>
> PSQLException: ERROR: could not serialize access due to read/write
> dependencies among transactions
>   Detail: Reason code: Canceled on conflict out to old pivot 61940806.

Both of these errors are coming from CheckForSerializableConflictOut(),
and are indeed variations of the same kind of conflict.

> We're logging the id, begin and end of every transaction. Transaction
> 61940806 was committed without errors. The transaction responsible for
> the above error was started 40min later (and failed immediately). With
> 61866959 it is even more extreme: the first conflict error occurred 2.5h
> after 61866959 was committed.

Weird indeed. There is only one caller of
CheckForSerializableConflictOut(), and it does this:

> /*
> * Find top level xid. Bail out if xid is too early to be a conflict, or
> * if it's our own xid.
> */
> if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
> return;
> xid = SubTransGetTopmostTransaction(xid);
> if (TransactionIdPrecedes(xid, TransactionXmin))
> return;
>
> CheckForSerializableConflictOut(relation, xid, snapshot);

That check with TransactionXmin is very clear: if 'xid' precedes the
xmin of the current transaction, IOW if there were no transactions with
'xid' or older running when the current transcaction started,
CheckForSerializableConflictOut() is not called.

> The DB table access pattern is too complex to lay out here. There are
> like 20 tables that are read/written to. Transactions are usually short
> living. The longest transaction that could occur is 1 min long. My
> understanding of serializable isolation is that only overlapping
> transactions can conflict. I can be pretty sure that in the above cases
> there is no single transaction, which overlaps with 61940806 and with
> the failing transaction 40 min later.

I hate to drill on this, but are you very sure about that? I don't see
how this could happen if there are no long-running transactions. Maybe a
forgotten two-phase commit transaction? A transaction in a different
database? A developer who did "begin;" in psql and went for lunch?

> Such long running transactions
> would cause different types of errors in our system ("out of shared
> memory", "You might need to increase max_pred_locks_per_transaction").

I don't see why that would necessarily be the case, unless it's
something very specific to your application.

> Why does PostgreSql detect a conflict with a transaction which was
> committed more than 1h before? Can there be a long dependency chain
> between many short running transactions? Does the high load prevent
> Postgres from doing some clean up?

The dependencies don't chain like that, but there is a system of
"summarizing" old transactions to limit the shared memory usage. When a
transaction has dependencies on other transactions, we track those
dependencies in shared memory. But if we run short on the space reserved
for that, we summarize the dependencies, losing granularity. We lose
information of which relations/pages/tuples the xid accessed and which
transactions exactly it had a dependency on. That is safe, but can cause
false positives.

The amount of shared memory reserved for tracking the dependencies is
determined by max_pred_locks_per_transaction, so you could try
increasing that to reduce those false positives, even if you never get
the "out of shared memory" error.

--
Heikki Linnakangas
Neon (https://neon.tech)