Re: New strategies for freezing, advancing relfrozenxid early

On Thu, Dec 15, 2022 at 6:50 AM Matthias van de Meent
<boekewurm+postgres@gmail.com> wrote:
> This first run of (auto)vacuum after the 8GB threshold seems
> to appear as a significant IO event (both in WAL and relation
> read/write traffic) with 50% of the table updated and WAL-logged. I
> think this should be limited to some degree, such as only freeze
> all_visible blocks up to 10% of the table's blocks in eager vacuum, so
> that the load is spread across a larger time frame and more VACUUM
> runs.

I agree that the burden of catch-up freezing is excessive here (in
fact I already wrote something to that effect on the wiki page). The
likely solution can be simple enough.

In v9 of the patch, we switch over to eager freezing when table size
crosses 4GB (since that is the value of the
vacuum_freeze_strategy_threshold GUC). The catch up freezing that you
draw attention to here occurs when table size exceeds 8GB, which is a
separate physical table size threshold that forces eager relfrozenxid
advancement. The second threshold is hard-coded to 2x the first one.

I think that this issue can be addressed by making the second
threshold 4x or even 8x vacuum_freeze_strategy_threshold, not just 2x.
That would mean that we'd have to freeze just as many pages whenever
we did the catch-up freezing -- so no change in the added *absolute*
cost of freezing. But, the *relative* cost would be much lower, simply
because catch-up freezing would take place when the table was much
larger. So it would be a lot less noticeable.

Note that we might never reach the second table size threshold before
we must advance relfrozenxid, in any case. The catch-up freezing might
actually take place because table age created pressure to advance
relfrozenxid. It's useful to have a purely physical/table-size
threshold like this, especially in bulk loading scenarios. But it's
not like table age doesn't have any influence at all, anymore. The
cost model weighs physical units/costs as well as table age, and in
general the most likely trigger for advancing relfrozenxid is usually
some combination of the two, not any one factor on its own [1].

[1] https://wiki.postgresql.org/wiki/Freezing/skipping_strategies_patch:_motivating_examples#Opportunistically_advancing_relfrozenxid_with_bursty.2C_real-world_workloads
-- 
Peter Geoghegan