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

On Fri, Feb 25, 2022 at 2:00 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > Hm. I guess I'll have to look at the code for it. It doesn't immediately
> > "feel" quite right.
>
> I kinda think it might be. Please let me know if you see a problem
> with what I've said.

Oh, wait. I have a better idea of what you meant now. The loop towards
the end of FreezeMultiXactId() will indeed "Determine whether to keep
this member or ignore it." when we need a new MultiXactId. The loop is
exact in the sense that it will only include those XIDs that are truly
needed -- those that are still running.

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.

How many of these "skewed MultiXacts" can we really expect? Seems like
there might be very few in practice. But I'm really not sure about
that.

--
Peter Geoghegan