Re: Removing more vacuumlazy.c special cases, relfrozenxid optimizations - Mailing list pgsql-hackers

On Sun, Feb 20, 2022 at 12:27 PM Peter Geoghegan <pg@bowt.ie> wrote:
> You've given me a lot of high quality feedback on all of this, which
> I'll work through soon. It's hard to get the balance right here, but
> it's made much easier by this kind of feedback.

Attached is v9. Lots of changes. Highlights:

* Much improved 0001 ("loosen coupling" dynamic relfrozenxid tracking
patch). Some of the improvements are due to recent feedback from
Robert.

* Much improved 0002 ("Make page-level characteristics drive freezing"
patch). Whole new approach to the implementation, though the same
algorithm as before.

* No more FSM patch -- that was totally separate work, that I
shouldn't have attached to this project.

* There are 2 new patches (these are now 0003 and 0004), both of which
are concerned with allowing non-aggressive VACUUM to consistently
advance relfrozenxid. I think that 0003 makes sense on general
principle, but I'm much less sure about 0004. These aren't too
important.

While working on the new approach to freezing taken by v9-0002, I had
some insight about the issues that Robert raised around 0001, too. I
wasn't expecting that to happen.

0002 makes page-level freezing a first class thing.
heap_prepare_freeze_tuple now has some (limited) knowledge of how this
works. heap_prepare_freeze_tuple's cutoff_xid argument is now always
the VACUUM caller's OldestXmin (not its FreezeLimit, as before). We
still have to pass FreezeLimit to heap_prepare_freeze_tuple, which
helps us to respect FreezeLimit as a backstop, and so now it's passed
via the new backstop_cutoff_xid argument instead. Whenever we opt to
"freeze a page", the new page-level algorithm *always* uses the most
recent possible XID and MXID values (OldestXmin and oldestMxact) to
decide what XIDs/XMIDs need to be replaced. That might sound like it'd
be too much, but it only applies to those pages that we actually
decide to freeze (since page-level characteristics drive everything
now). FreezeLimit is only one way of triggering that now (and one of
the least interesting and rarest).

0002 also adds an alternative set of relfrozenxid/relminmxid tracker
variables, to make the "don't freeze the page" path within
lazy_scan_prune simpler (if you don't want to freeze the page, then
use the set of tracker variables that go with that choice, which
heap_prepare_freeze_tuple knows about and helps with). With page-level
freezing, lazy_scan_prune wants to make a decision about the page as a
whole, at the last minute, after all heap_prepare_freeze_tuple calls
have already been made. So I think that heap_prepare_freeze_tuple
needs to know about that aspect of lazy_scan_prune's behavior.

When we *don't* want to freeze the page, we more or less need
everything related to freezing inside lazy_scan_prune to behave like
lazy_scan_noprune, which never freezes the page (that's mostly the
point of lazy_scan_noprune). And that's almost what we actually do --
heap_prepare_freeze_tuple now outsources maintenance of this
alternative set of "don't freeze the page" relfrozenxid/relminmxid
tracker variables to its sibling function, heap_tuple_needs_freeze.
That is the same function that lazy_scan_noprune itself actually
calls.

Now back to Robert's feedback on 0001, which had very complicated
comments in the last version. This approach seems to make the "being
versus becoming" or "going to freeze versus not going to freeze"
distinctions much clearer. This is less true if you assume that 0002
won't be committed but 0001 will be. Even if that happens with
Postgres 15, I have to imagine that adding something like 0002 must be
the real goal, long term. Without 0002, the value from 0001 is far
more limited. You need both together to get the virtuous cycle I've
described.

The approach with always using OldestXmin as cutoff_xid and
oldestMxact as our cutoff_multi makes a lot of sense to me, in part
because I think that it might well cut down on the tendency of VACUUM
to allocate new MultiXacts in order to be able to freeze old ones.
AFAICT the only reason that heap_prepare_freeze_tuple does that is
because it has no flexibility on FreezeLimit and MultiXactCutoff.
These are derived from vacuum_freeze_min_age and
vacuum_multixact_freeze_min_age, respectively, and so they're two
independent though fairly meaningless cutoffs. On the other hand,
OldestXmin and OldestMxact are not independent in the same way. We get
both of them at the same time and the same place, in
vacuum_set_xid_limits. OldestMxact really is very close to OldestXmin
-- only the units differ.

It seems that heap_prepare_freeze_tuple allocates new MXIDs (when
freezing old ones) in large part so it can NOT freeze XIDs that it
would have been useful (and much cheaper) to remove anyway. On HEAD,
FreezeMultiXactId() doesn't get passed down the VACUUM operation's
OldestXmin at all (it actually just gets FreezeLimit passed as its
cutoff_xid argument). It cannot possibly recognize any of this for
itself.

Does that theory about MultiXacts sound plausible? I'm not claiming
that the patch makes it impossible that FreezeMultiXactId() will have
to allocate a new MultiXact to freeze during VACUUM -- the
freeze-the-dead isolation tests already show that that's not true. I
just think that page-level freezing based on page characteristics with
oldestXmin and oldestMxact (not FreezeLimit and MultiXactCutoff)
cutoffs might make it a lot less likely in practice. oldestXmin and
oldestMxact map to the same wall clock time, more or less -- that
seems like it might be an important distinction, independent of
everything else.

Thanks
--
Peter Geoghegan