Re: Minimal logical decoding on standbys - Mailing list pgsql-hackers

Hi,

On 3/2/23 1:40 AM, Jeff Davis wrote:
> On Wed, 2023-03-01 at 11:51 +0100, Drouvot, Bertrand wrote:
> 
>>
>> Why not "simply" call ConditionVariablePrepareToSleep() without any
>> call to ConditionVariableTimedSleep() later?
> 
> ConditionVariableSleep() re-inserts itself into the queue if it was
> previously removed. Without that, a single wakeup could remove it from
> the wait queue, and the effects of ConditionVariablePrepareToSleep()
> would be lost.

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.

So I don't see any issue in this particular case (as we are removed from the queue
but we'll not have to wait anymore).

> 
>> In that case the walsender will be put in the wait queue (thanks to
>> ConditionVariablePrepareToSleep())
>> and will be waked up by the event on the socket, the timeout or the
>> CV broadcast
> 
> I believe it will only be awakened once, and if it enters WalSndWait()
> again, future ConditionVariableBroadcast/Signal() calls won't wake it
> up any more.

I don't think that's right and that's not what my testing shows (please find attached 0004-CV-POC.txt,
a .txt file to not break the CF bot), as:

- If it is awakened due to the CV broadcast, then we'll right after exit the loop (see above)
- If it is awakened due to the timeout or the socket event then we're still in the CV wait queue
(as nothing removed it from the CV wait queue).

> 
>>   (since IIUC they all rely on the same latch).
> 
> Relying on that fact seems like too much action-at-a-distance to me
> If
> we change the implementation of condition variables, then it would stop
> working.
> 

I'm not sure about this one. I mean it would depend what the implementation changes are.
Also the related TAP test (0005) would probably fail or start taking a long time due to
the corner case we are trying to solve here coming back (like it was detected in [1])

> Also, since they are using the same latch, that means we are still
> waking up too frequently, right? We haven't really solved the problem.
> 

I don't think so as the first CV broadcast will make us exit the loop.
So, ISTM that we'll wake up as we currently do, expect when there is a flush/replay
which is what we want, right?

>> That looks weird to use ConditionVariablePrepareToSleep() without
>> actually using ConditionVariableTimedSleep()
>> but it looks to me that it would achieve the same goal: having the
>> walsender being waked up
>> by the event on the socket, the timeout or the CV broadcast.
> 
> I don't think it actually works, because something needs to keep re-
> inserting it into the queue after it gets removed.

I think that's not needed as we'd exit the loop right after we are awakened by a CV broadcast.
> 
> To use condition variables properly, I think we'd need an API like
> ConditionVariableEventsSleep(), which takes a WaitEventSet and a
> timeout. I think this is what Andres was suggesting and seems like a
> good idea. I looked into it and I don't think it's too hard to
> implement -- we just need to WaitEventSetWait instead of WaitLatch.
> There are a few details to sort out, like how to enable callers to
> easily create the right WaitEventSet (it obviously needs to include
> MyLatch, for instance) and update it with the right socket events.
> 

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.

[1]: https://www.postgresql.org/message-id/47606911-cf44-5a62-21d5-366d3bc6e445%40enterprisedb.com

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com