Re: New strategies for freezing, advancing relfrozenxid early

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From: Jeff Davis <pgsql@j-davis.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: John Naylor <john.naylor@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: New strategies for freezing, advancing relfrozenxid early
Date: Fri, 30 Dec 2022 12:43:04 -0800
Message-ID: <acb89d4554f072f9dd30f2ae5b8b60092f3eb7c9.camel@j-davis.com> (raw)
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On Mon, 2022-12-26 at 12:53 -0800, Peter Geoghegan wrote:
> * v12 merges together the code for the "freeze the page"
> lazy_scan_prune path with the block that actually calls
> heap_freeze_execute_prepared().
> 
> This should make it clear that pagefrz.freeze_required really does
> mean that freezing is required. Hopefully that addresses Jeff's
> recent
> concern. It's certainly an improvement, in any case.

Better, thank you.

> * On a related note, comments around the same point in
> lazy_scan_prune
> as well as comments above the HeapPageFreeze struct now explain a
> concept I decided to call "nominal freezing". This is the case where
> we "freeze a page" without having any freeze plans to execute.
> 
> "nominal freezing" is the new name for a concept I invented many
> months ago, which helps to resolve subtle problems with the way that
> heap_prepare_freeze_tuple is tasked with doing two different things
> for its lazy_scan_prune caller: 1. telling lazy_scan_prune how it
> would freeze each tuple (were it to freeze the page), and 2. helping
> lazy_scan_prune to determine if the page should become all-frozen in
> the VM. The latter is always conditioned on page-level freezing
> actually going ahead, since everything else in
> heap_prepare_freeze_tuple has to work that way.
> 
> We always freeze a page with zero freeze plans (or "nominally freeze"
> the page) in lazy_scan_prune (which is nothing new in itself). We
> thereby avoid breaking heap_prepare_freeze_tuple's working assumption
> that all it needs to focus on what the page will look like after
> freezing executes, while also avoiding senselessly throwing away the
> ability to set a page all-frozen in the VM in lazy_scan_prune when
> it'll cost us nothing extra. That is, by always freezing in the event
> of zero freeze plans, we won't senselessly miss out on setting a page
> all-frozen in cases where we don't actually have to execute any
> freeze
> plans to make that safe, while the "freeze the page path versus don't
> freeze the page path" dichotomy still works as a high level
> conceptual
> abstraction.

I always understood "freezing" to mean that a concrete action was
taken, and associated WAL generated.

"Nominal freezing" is happening when there are no freeze plans at all.
I get that it's to manage control flow so that the right thing happens
later. But I think it should be defined in terms of what state the page
is in so that we know that following a given path is valid. Defining
"nominal freezing" as a case where there are no freeze plans is just
confusing to me.


-- 
Jeff Davis
PostgreSQL Contributor Team - AWS

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