On 03.05.2013 15:41, Simon Riggs wrote:
What appears to be happening is we're spending a lot of time in
TransactionIdIsInProgress() so we can set hints and then when we find
it is still in progress we then spend more time in XidIsInSnapshot()
while we check that it is still invisible to us. Even if the
transaction we see repeatedly ends, we will still pay the cost in
XidIsInSnapshot repeatedly as we execute.
Given that code path, I would expect it to suck worse on a live system
with many sessions, and even worse with many subtransactions.
(1) A proposed fix is attached, but its only a partial one and barely tested.
Deeper fixes might be
(2) to sort the xid array if we call XidIsInSnapshot too many times
in a transaction. I don't think that is worth it, because a long
running snapshot may be examined many times, but is unlikely to see
multiple in-progress xids repeatedly. Whereas your case seems
reasonably common.
Yeah, sorting would be a waste of time most of the time.
Instead of adding a new cache field, how about just swapping the matched
XID to the beginning of the array?
Did you have some simple performance test script for this?
- Heikki
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