Обсуждение: tcp keep alive don't work when the backend is busy

According to the documentation
https://www.postgresql.org/docs/12/runtime-config-connection.html
A backend must check connection to the client by tcp_keepalive messages. (Config option tcp_keepalives_idle).

But this is don't work if the backend is busy.

Simple example:

psql localhost

set tcp_keepalives_idle=1;
do $$begin loop perform pg_sleep(1);end loop;end;$$;

In other terminal kill -9 the psql on the first terminal.

select * from pg_stat_activity where state='active';
And we will see that the backend is still active and busy.

The more realistic example. In the real code one of the loops, due to bug with asynchronous communication, come to the
infiniteloop. And occupy a backend and locks for a two week after the client was killed, before we detected this.  

=?utf-8?B?0J7Qu9C10LMg0KHQsNC80L7QudC70L7Qsg==?= <splarv@ya.ru> writes:
> According to the documentation
> https://www.postgresql.org/docs/12/runtime-config-connection.html
> A backend must check connection to the client by tcp_keepalive messages. (Config option tcp_keepalives_idle).

> But this is don't work if the backend is busy.

You're reading something into the documentation that isn't there.

The TCP keepalive mechanism is something that the OS does, independently
of backend processing.  The backend isn't going to notice loss of client
connection until it tries to read or write on the connection.

If it were free to improve this, we might do so.  But it would be
very much not free.

            regards, tom lane

On 10/12/2019 15:06, Tom Lane wrote:
> =?utf-8?B?0J7Qu9C10LMg0KHQsNC80L7QudC70L7Qsg==?= <splarv@ya.ru> writes:
>> According to the documentation
>> https://www.postgresql.org/docs/12/runtime-config-connection.html
>> A backend must check connection to the client by tcp_keepalive messages. (Config option tcp_keepalives_idle).
> 
>> But this is don't work if the backend is busy.
> 
> You're reading something into the documentation that isn't there.
> 
> The TCP keepalive mechanism is something that the OS does, independently
> of backend processing.  The backend isn't going to notice loss of client
> connection until it tries to read or write on the connection.
> 
> If it were free to improve this, we might do so.  But it would be
> very much not free.
> 
>             regards, tom lane
> 
> 


At what points does the backend bite the bullet to test the state of 
that file descriptor?

I'd expect select() and poll() to return immediately when keepalive 
probes timeout, so idling clients are covered (and that's the main use 
case); does any other code path go out if its way to ensure that there's 
still a client without actually needing to read()/write()/send()/recv()? 
(obviously at the cost you mentioned)








-- 
Regards

Fabio Ugo Venchiarutti
OSPCFC Network Engineering Dpt.
Ocado Technology

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On Wed, Dec 11, 2019 at 4:17 AM Fabio Ugo Venchiarutti
<f.venchiarutti@ocado.com> wrote:
> On 10/12/2019 15:06, Tom Lane wrote:
> > =?utf-8?B?0J7Qu9C10LMg0KHQsNC80L7QudC70L7Qsg==?= <splarv@ya.ru> writes:
> >> According to the documentation
> >> https://www.postgresql.org/docs/12/runtime-config-connection.html
> >> A backend must check connection to the client by tcp_keepalive messages. (Config option tcp_keepalives_idle).
> >
> >> But this is don't work if the backend is busy.
> >
> > You're reading something into the documentation that isn't there.
> >
> > The TCP keepalive mechanism is something that the OS does, independently
> > of backend processing.  The backend isn't going to notice loss of client
> > connection until it tries to read or write on the connection.
> >
> > If it were free to improve this, we might do so.  But it would be
> > very much not free.
>
> At what points does the backend bite the bullet to test the state of
> that file descriptor?
>
> I'd expect select() and poll() to return immediately when keepalive
> probes timeout, so idling clients are covered (and that's the main use
> case); does any other code path go out if its way to ensure that there's
> still a client without actually needing to read()/write()/send()/recv()?
> (obviously at the cost you mentioned)

It has been proposed that busy backends should (optionally)
periodically try to do a MSG_PEEK so they can learn about a client
that has gone away some time before they eventually try to write:

https://www.postgresql.org/message-id/flat/77def86b27e41f0efcba411460e929ae%40postgrespro.ru

More work is needed to move that forward, though.

> 10 дек. 2019 г., в 18:36, Justin <zzzzz.graf@gmail.com> написал(а):
>
> Hi Oner
>
> It appears that you looking for a way to detect and kill of idle connections or process that are running for a long
time Correct?? 

Nope, not idle. Only to stop an active connection if the client is already died.