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

On 10/12/2019 15:06, Tom Lane wrote:
> =?utf-8?B?0J7Qu9C10LMg0KHQsNC80L7QudC70L7Qsg==?= <splarv(at)ya(dot)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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