Обсуждение: 9.4 logical replication - walsender keepalive replies

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9.4 logical replication - walsender keepalive replies

От
Steve Singer
Дата:
In 9.4 we've the below  block of code to walsender.c as

/* * We only send regular messages to the client for full decoded * transactions, but a synchronous replication and
walsendershutdown * possibly are waiting for a later location. So we send pings * containing the flush location every
nowand then.
 
*/
if (MyWalSnd->flush < sentPtr && !waiting_for_ping_response)
{    WalSndKeepalive(true);    waiting_for_ping_response = true;
}


I am finding that my logical replication reader is spending a tremendous 
amount of time sending feedback to the server because a keep alive reply 
was requested.  My flush pointer is smaller than sendPtr, which I see as 
the normal case (The client hasn't confirmed all the wal it has been 
sent).   My client queues the records it receives and only confirms when 
actually processes the record.

So the sequence looks something like

Server Sends LSN 0/1000
Server Sends LSN 0/2000
Server Sends LSN 0/3000
Client confirms LSN 0/2000


I don't see why all these keep alive replies are needed in this case 
(the timeout value is bumped way up, it's the above block that is 
triggering the reply request not something related to timeout)

Steve







Re: 9.4 logical replication - walsender keepalive replies

От
Andres Freund
Дата:
Hi Steve,

On 2014-06-30 11:40:50 -0400, Steve Singer wrote:
> In 9.4 we've the below  block of code to walsender.c as
> 
> /*
>  * We only send regular messages to the client for full decoded
>  * transactions, but a synchronous replication and walsender shutdown
>  * possibly are waiting for a later location. So we send pings
>  * containing the flush location every now and then.
> */
> if (MyWalSnd->flush < sentPtr && !waiting_for_ping_response)
> {
>     WalSndKeepalive(true);
>     waiting_for_ping_response = true;
> }
> 
> 
> I am finding that my logical replication reader is spending a tremendous
> amount of time sending feedback to the server because a keep alive reply was
> requested.  My flush pointer is smaller than sendPtr, which I see as the
> normal case (The client hasn't confirmed all the wal it has been sent).   My
> client queues the records it receives and only confirms when actually
> processes the record.
> 
> So the sequence looks something like
> 
> Server Sends LSN 0/1000
> Server Sends LSN 0/2000
> Server Sends LSN 0/3000
> Client confirms LSN 0/2000

> I don't see why all these keep alive replies are needed in this case (the
> timeout value is bumped way up, it's the above block that is triggering the
> reply request not something related to timeout)

Right. I thought about this for a while, and I think we should change
two things. For one, don't request replies here. It's simply not needed,
as this isn't dealing with timeouts. For another don't just check ->flush
< sentPtr but also && ->write < sentPtr. The reason we're sending these
feedback messages is to inform the 'logical standby' that there's been
WAL activity which it can't see because they don't correspond to
anything that's logically decoded (e.g. vacuum stuff).
Would that suit your needs?

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services



Re: 9.4 logical replication - walsender keepalive replies

От
Steve Singer
Дата:
On 07/06/2014 10:11 AM, Andres Freund wrote:
> Hi Steve,

> Right. I thought about this for a while, and I think we should change
> two things. For one, don't request replies here. It's simply not needed,
> as this isn't dealing with timeouts. For another don't just check ->flush
> < sentPtr but also && ->write < sentPtr. The reason we're sending these
> feedback messages is to inform the 'logical standby' that there's been
> WAL activity which it can't see because they don't correspond to
> anything that's logically decoded (e.g. vacuum stuff).
> Would that suit your needs?
>
> Greetings,

Yes I think that will work for me.
I tested with the attached patch that I think  does what you describe
and it seems okay.



> Andres Freund
>


Вложения

Re: 9.4 logical replication - walsender keepalive replies

От
Steve Singer
Дата:
On 07/14/2014 01:19 PM, Steve Singer wrote:
> On 07/06/2014 10:11 AM, Andres Freund wrote:
>> Hi Steve,
>
>> Right. I thought about this for a while, and I think we should change
>> two things. For one, don't request replies here. It's simply not needed,
>> as this isn't dealing with timeouts. For another don't just check 
>> ->flush
>> < sentPtr but also && ->write < sentPtr. The reason we're sending these
>> feedback messages is to inform the 'logical standby' that there's been
>> WAL activity which it can't see because they don't correspond to
>> anything that's logically decoded (e.g. vacuum stuff).
>> Would that suit your needs?
>>
>> Greetings,
>
> Yes I think that will work for me.
> I tested with the attached patch that I think  does what you describe 
> and it seems okay.
>
>


Any feedback on this?  Do we want that change for 9.4, or do we want 
something else?


>
>> Andres Freund
>>
>
>
>




Re: 9.4 logical replication - walsender keepalive replies

От
Andres Freund
Дата:
On 2014-08-11 17:22:27 -0400, Steve Singer wrote:
> On 07/14/2014 01:19 PM, Steve Singer wrote:
> >On 07/06/2014 10:11 AM, Andres Freund wrote:
> >>Hi Steve,
> >
> >>Right. I thought about this for a while, and I think we should change
> >>two things. For one, don't request replies here. It's simply not needed,
> >>as this isn't dealing with timeouts. For another don't just check
> >>->flush
> >>< sentPtr but also && ->write < sentPtr. The reason we're sending these
> >>feedback messages is to inform the 'logical standby' that there's been
> >>WAL activity which it can't see because they don't correspond to
> >>anything that's logically decoded (e.g. vacuum stuff).
> >>Would that suit your needs?
> >>
> >>Greetings,
> >
> >Yes I think that will work for me.
> >I tested with the attached patch that I think  does what you describe and
> >it seems okay.
> >
> >
> 
> 
> Any feedback on this?  Do we want that change for 9.4, or do we want
> something else?

I plan to test and apply it in the next few days. Digging myself from
under stuff from before my holiday right now...

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services



Re: 9.4 logical replication - walsender keepalive replies

От
Andres Freund
Дата:
On 2014-08-11 23:52:32 +0200, Andres Freund wrote:
> On 2014-08-11 17:22:27 -0400, Steve Singer wrote:
> > On 07/14/2014 01:19 PM, Steve Singer wrote:
> > >On 07/06/2014 10:11 AM, Andres Freund wrote:
> > >>Hi Steve,
> > >
> > >>Right. I thought about this for a while, and I think we should change
> > >>two things. For one, don't request replies here. It's simply not needed,
> > >>as this isn't dealing with timeouts. For another don't just check
> > >>->flush
> > >>< sentPtr but also && ->write < sentPtr. The reason we're sending these
> > >>feedback messages is to inform the 'logical standby' that there's been
> > >>WAL activity which it can't see because they don't correspond to
> > >>anything that's logically decoded (e.g. vacuum stuff).
> > >>Would that suit your needs?
> > >>
> > >>Greetings,
> > >
> > >Yes I think that will work for me.
> > >I tested with the attached patch that I think  does what you describe and
> > >it seems okay.
> > >
> > >
> > 
> > 
> > Any feedback on this?  Do we want that change for 9.4, or do we want
> > something else?
> 
> I plan to test and apply it in the next few days. Digging myself from
> under stuff from before my holiday right now...

Committed. Thanks and sorry for the delay.

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services