Hi,
On 3/2/23 8:45 PM, Jeff Davis wrote:
On Thu, 2023-03-02 at 10:20 +0100, Drouvot, Bertrand wrote:
Right, but in our case, right after the wakeup (the one due to the CV
broadcast,
aka the one that will remove it from the wait queue) we'll exit the
loop due to:
"
/* check whether we're done */
if (loc <= RecentFlushPtr)
break;
"
as the CV broadcast means that a flush/replay occurred.
But does it mean that the flush/replay advanced *enough* to be greater
than or equal to loc?
Yes I think so: loc is when we started waiting initially
and RecentFlushPtr is >= to when the broadcast has been sent.
- If it is awakened due to the CV broadcast, then we'll right after
exit the loop (see above)
...
I think that's not needed as we'd exit the loop right after we are
awakened by a CV broadcast.
See the comment here:
WalSndWaitForWal
* If this process has been taken out of the wait list, then we know
* that it has been signaled by ConditionVariableSignal (or
* ConditionVariableBroadcast), so we should return to the caller. But
* that doesn't guarantee that the exit condition is met, only that we
* ought to check it.
You seem to be arguing that in this case, it doesn't matter; that
walreceiver knows what walsender is waiting for, and will never wake it
up before it's ready. I don't think that's true, and even if it is, it
needs explanation.
What I think is that, in this particular case, we are sure that
the loop exit condition is met as we know that loc <= RecentFlushPtr.
I agree that's a good idea and that it should/would work too. I just
wanted to highlight that in this particular
case that might not be necessary to build this new API.
In this case it looks easier to add the right API than to be sure about
whether it's needed or not.
What I meant is that of course I might be wrong.
If we do not agree that the new API (in this particular case) is not needed then
I agree that building the new API is the way to go ;-) (+ it offers the
advantage to
be able to be more precise while reporting the wait event).
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
