Re: Walsender timeouts and large transactions

On 02/08/17 19:35, Yura Sokolov wrote:
> The following review has been posted through the commitfest application:
> make installcheck-world: tested, passed
> Implements feature: not tested
> Spec compliant: not tested
> Documentation: not tested
>
> There is no check for (last_reply_timestamp <= 0 || wal_sender_timeout <= 0) as in other places
> (in WalSndKeepaliveIfNecessary for example).
>
> I don't think moving update of 'now' down to end of loop body is correct:
> there are calls to ProcessConfigFile with SyncRepInitConfig, ProcessRepliesIfAny that can
> last non-negligible time. It could lead to over sleeping due to larger computed sleeptime.
> Though I could be mistaken.
>
> I'm not sure about moving `if (!pg_is_send_pending())` in a body loop after WalSndKeepaliveIfNecessary.
> Is it necessary? But it looks harmless at least.
>

We also need to do actual timeout handing so that the timeout is not
deferred to the end of the transaction (Which is why I moved `if
(!pg_is_send_pending())` under WalSndCheckTimeOut() and
WalSndKeepaliveIfNecessary() calls).

> Could patch be reduced to check after first `if (!pg_is_sendpending())` ? like:
>
> if (!pq_is_send_pending())
> - return;
> + {
> + if (last_reply_timestamp <= 0 || wal_sender_timeout <= 0)
> + {
> + CHECK_FOR_INTERRUPTS();
> + return;
> + }
> + if (now <= TimestampTzPlusMilliseconds(last_reply_timestamp, wal_sender_timeout / 2))
> + return;
> + }
>
> If not, what problem prevents?

We should do CHECK_FOR_INTERRUPTS() independently of pq_is_send_pending
so that it's possible to stop walsender while it's processing large
transaction.

--
Petr Jelinek http://www.2ndQuadrant.com/
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