Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations

Hi,

On 2022-02-25 15:28:17 -0800, Peter Geoghegan wrote:
> But why should we ever get to the FreezeMultiXactId() loop with the
> stuff from 0002 in place? The whole purpose of the loop is to handle
> cases where we have to remove *some* (not all) XIDs from before
> cutoff_xid that appear in a MultiXact, which requires careful checking
> of each XID (this is only possible when the MultiXactId is <
> cutoff_multi to begin with, which is OldestMxact in the patch, which
> is presumably very recent).
>
> It's not impossible that we'll get some number of "skewed MultiXacts"
> with the patch -- cases that really do necessitate allocating a new
> MultiXact, just to "freeze some XIDs from a MultiXact". That is, there
> will sometimes be some number of XIDs that are < OldestXmin, but
> nevertheless appear in some MultiXactIds >= OldestMxact. This seems
> likely to be rare with the patch, though, since VACUUM calculates its
> OldestXmin and OldestMxact (which are what cutoff_xid and cutoff_multi
> really are in the patch) at the same point in time. Which was the
> point I made in my email yesterday.

I don't see why it matters that OldestXmin and OldestMxact are computed at the
same time? It's a question of the workload, not vacuum algorithm.

OldestMxact inherently lags OldestXmin. OldestMxact can only advance after all
members are older than OldestXmin (not quite true, but that's the bound), and
they have always more than one member.

> How many of these "skewed MultiXacts" can we really expect?

I don't think they're skewed in any way. It's a fundamental aspect of
multixacts.

Greetings,

Andres Freund