Re: POC: make mxidoff 64 bits

On Wed, 12 Nov 2025 at 16:00, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

>
> I added an
> inlined fast path to SlruReadSwitchPage and SlruWriteSwitchPage to
> eliminate the function call overhead of those in the common case that no
> page switch is needed. With that, the 100 million mxid test case I used
> went from 1.2 s to 0.9 s. We could optimize this further but I think
> this is good enough.
>
I agree with you.

- I added an SlruFileName() helper function to slru_io.c, and support
> for reading SLRUs with long_segment_names==true. It's not needed
> currently, but it seemed like a weird omission. AllocSlruRead() actually
> left 'long_segment_names' uninitialized which is error-prone. We
> could've just documented it, but it seems just as easy to support it.
>
Yeah, I didn't particularly like that place either. But then I decided it
was
overkill to do it for the sake of symmetry and would raise questions.
It turned out much better this way.

> I kept all the new test cases for now. We need to decide which ones are
> worth keeping, and polish and speed up the ones we decide to keep.
>
I think of two cases here.
A) Upgrade from "new cluster":
* created cluster with pre 32-bit overflow mxoff
* consume around of 2k of mxacts (1k before 32-bit overflow
and 1k after)
* run pg_upgrade
* check upgraded cluster is working
* check data invariant
B) Same as A), but for an "old cluster" with oldinstall env.

On Thu, 13 Nov 2025 at 19:04, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:

>
> Here's a new patch version that addresses the above issue. I resurrected
> MultiXactMemberFreezeThreshold(), using the same logic as before, just
> using pretty arbitrary thresholds of 1 and 2 billion offsets instead of
> the safe/danger thresholds derived from MaxMultiOffset. That gives
> roughly the same behavior wrt. calculating effective freeze age as before.
>
Yes, I think it's okay for now. This reflects the existing logic well.
I wonder what the alternative solution might be? Can we make a
"vacuum freeze" also do pg_multixact segments truncation?
In any case, this can be discussed later.

--
Best regards,
Maxim Orlov.