Re: logical decoding - GetOldestXmin

On 2012-12-13 18:29:00 +0100, Andres Freund wrote:
> On 2012-12-13 00:05:41 +0000, Peter Geoghegan wrote:
> > Initialisation means finding a free “logical slot” from shared memory,
> > then looping until the new magic xmin horizon for logical walsenders
> > (stored in their “slot”) is that of the weakest link (think local
> > global xmin).
> >
> > +      * FIXME: think about solving the race conditions in a nicer way.
> > +      */
> > + recompute_xmin:
> > +     walsnd->xmin = GetOldestXmin(true, true);
> > +     ComputeLogicalXmin();
> > +     if (walsnd->xmin != GetOldestXmin(true, true))
> > +         goto recompute_xmin;
> >
> > Apart from the race conditions that I'm not confident are addressed
> > here, I think that the above could easily get stuck indefinitely in
> > the event of contention.
>
> I don't like that part the slightest bit but I don't think its actually
> in danger of looping forever. In fact I think its so broken it won't
> ever loop ;). (ComputeLogicalXmin() will set the current global minimum
> which will then be returned by GetOldestXmin()).
>
> I would like to solve this properly without copying GetOldestXmin once
> more (so we can compute and set the logical xmin while holding
> ProcArrayLock), but I am not yet sure how a nice way to do that would
> look like.
>
> I guess GetOldestXminNoLock? That gets called while we already hold the
> procarray lock? Yuck.

Does anybody have an opinion on the attached patches? Especially 0001,
which contains the procarray changes?

It moves a computation of the sort of:

result -= vacuum_defer_cleanup_age;
if (!TransactionIdIsNormal(result))
   result = FirstNormalTransactionId;

inside ProcArrayLock. But I can't really imagine that to be relevant...


Another alternative to this would be to get a snapshot with
GetSnapshotData(), copy the xmin to the logical slot, then call
ProcArrayEndTransaction(). But that doesn't really seem to be nicer to
me.

Greetings,

Andres Freund

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