Re: Walsender timeouts and large transactions

On 9 August 2017 at 21:23, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>
wrote:

> 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.
>
>
Is there any chance of getting this bugfix into Pg 10?

We've just cut back branches, so it'd be a sensible time.

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