On Tue, 2022-10-04 at 11:09 -0700, Peter Geoghegan wrote:
> So a simplistic threshold
> (combined with dynamic per-page decisions about freezing) should be
> enough to avoid most of the downside of eager freezing.
...
> I want to keep
> the cost as low as possible (often "negative cost" relative to
> Postgres 15), but overall I am consciously making a trade-off. There
> are downsides.
I am fine with that, but I'd like us all to understand what the
downsides are.
If I understand correctly:
1. Eager freezing (meaning to freeze at the same time as setting all-
visible) causes a modest amount of WAL traffic, hopefully before the
next checkpoint so we can avoid FPIs. Lazy freezing (meaning set all-
visible but don't freeze) defers the work, and it might never need to
be done; but if it does, it can cause spikes at unfortunate times and
is more likely to generate more FPIs.
2. You're trying to mitigate the downsides of eager freezing by:
a. when freezing a tuple, eagerly freeze other tuples on that page
b. optimize WAL freeze records
3. You're trying to capture the trade-off in #1 by using the table size
as a proxy. Deferred work is only really a problem for big tables, so
that's where you use eager freezing. But maybe we can just always use
eager freezing?:
a. You're mitigating the WAL work for freezing.
b. A lot of people run with checksums on, meaning that setting the
all-visible bit requires WAL work anyway, and often FPIs.
c. All-visible is conceptually similar to freezing, but less
important, and it feels more and more like the design concept of all-
visible isn't carrying its weight.
d. (tangent) I had an old patch[1] that actually removed
PD_ALL_VISIBLE (the page bit, not the VM bit), which was rejected, but
perhaps its time has come?
Regards,
Jeff Davis
[1]
https://www.postgresql.org/message-id/1353551097.11440.128.camel%40sussancws0025
