On 06.05.2013 04:51, Mark Kirkwood wrote:
On 05/05/13 00:49, Simon Riggs wrote:
On 3 May 2013 13:41, Simon Riggs <simon@xxxxxxxxxxxxxxx> wrote:
(3) to make the check on TransactionIdIsInProgress() into a heuristic,
since we don't *need* to check that, so if we keep checking the same
xid repeatedly we can reduce the number of checks or avoid xids that
seem to be long running. That's slightly more coding than my quick
hack here but seems worth it.
I think we need both (1) and (3) but the attached patch does just (1).
This is a similar optimisation to the one I introduced for
TransactionIdIsKnownCompleted(), except this applies to repeated
checking of as yet-incomplete xids, and to bulk concurrent
transactions.
ISTM we can improve performance of TransactionIdIsInProgress() by
caching the procno of our last xid.
Mark, could you retest with both these patches? Thanks.
Thanks Simon, will do and report back.
Did anyone ever try (3) ?
I'm not sure if this the same idea as (3) above, but ISTM that HeapTupleSatisfiesMVCC doesn't actually need to call TransactionIdIsInProgress(), because it checks XidInMVCCSnapshot(). The comment at the top of tqual.c says:
Or at least, it doesn't need to call TransactionIdIsInProgress() if XidInMVCCSnapshot() returned true.
* NOTE: must check TransactionIdIsInProgress (which looks in PGXACT array)
* before TransactionIdDidCommit/TransactionIdDidAbort (which look in
* pg_clog). Otherwise we have a race condition: we might decide that a
* just-committed transaction crashed, because none of the tests succeed.
* xact.c is careful to record commit/abort in pg_clog before it unsets
* MyPgXact->xid in PGXACT array. That fixes that problem, but it also
* means there is a window where TransactionIdIsInProgress and
* TransactionIdDidCommit will both return true. If we check only
* TransactionIdDidCommit, we could consider a tuple committed when a
* later GetSnapshotData call will still think the originating transaction
* is in progress, which leads to application-level inconsistency. The
* upshot is that we gotta check TransactionIdIsInProgress first in all
* code paths, except for a few cases where we are looking at
* subtransactions of our own main transaction and so there can't be any
* race condition.
If TransactionIdIsInProgress() returns true for a given XID, then surely it was also running when the snapshot was taken (or had not even began yet). In which case the XidInMVCCSnapshot() call will also return true. Am I missing something?
There's one little problem: we currently only set the hint bits when TransactionIdIsInProgress() returns false. If we do that earlier,
But why would we do that earlier? If we never bother to call TransactionIdIsInProgress(), then we just don't set the hint bits, because we don't know what to set them to. It can't matter what order we call TransactionIdIsInProgress and TransactionIdDidCommit in if we don't call either of them at all during this invocation.
Cheers,
Jeff
