5. mai 2016 6:14 AM kirjutas kuupƤeval "Andres Freund" <and...@anarazel.de>:
> On 2016-05-05 06:08:39 +0300, Ants Aasma wrote:
> > On 5 May 2016 1:28 a.m., "Andres Freund" <and...@anarazel.de> wrote:
> > > On 2016-05-04 18:22:27 -0400, Robert Haas wrote:
> > > > How would the semantics change?
> > >
> > > Right now the time for computing the snapshot is relevant, if
> > > maintenance of xids is moved, it'll likely be tied to the time xids
> > > assigned. That seems perfectly alright, but it'll change behaviour.
> >
> > FWIW moving the maintenance to a clock tick process will not change user
> > visible semantics in any significant way. The change could easily be
> > in the next release.
> I'm not convinced of that - right now the timeout is computed as a
> offset to the time a snapshot with a certain xmin horizon is
> taken. Moving the computation to GetNewTransactionId() or a clock tick
> process will make it relative to the time an xid has been generated
> (minus a fuzz factor).  That'll behave differently in a number of cases,

Timeout is calculated in TransactionIdLimitedForOldSnapshots() as
GetSnapshotCurrentTimestamp() minus old_snapshot_timeout rounded down to
previous minute. If the highest seen xmin in that minute is useful in
raising cleanup xmin the threshold is bumped. Snapshots switch behavior
when their whenTaken is past this threshold. Xid generation time has no
effect on this timing, it's completely based on passage of time.

The needed invariant is, that no snapshot having whenTaken after timeout
timestamp can have xmin less than calculated bound. Moving the xmin
calculation and storage to clock tick actually makes the bound tighter. The
ordering between xmin calculation and clok update/read needs to be correct
to ensure the invariant.

Ants Aasma

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