Re: Monitoring multixact members growth

On 8/19/22 12:52 AM, Vido Vlahinic wrote:
> My goal here is to predict where multixact members are growing the
> fastest so I can perform manual VACUUM FREEZE only on those tables
>
> (typically with multi-billion row count) when system is relatively
> idle as opposed to just sit and wait for wraparound protection to take
> over
>
> when autovacuum_multixact_freeze_max_age threshold is reached (slowing
> the whole system down). 
>

I think that you're probably approaching this wrong. Vacuum is something
that you generally want to run more aggressively, not less. But to be
fair, it's a very common misunderstanding that waiting to do vacuum
processing until later can be a good idea... even though in fact it
works in the opposite way - on systems with significant load (where it
matters) - sometimes a long-running report or query that needs old row
versions for its own processing might cause a lot of table and index
bloat and negatively impact real-time transactional performance. (For
really long-running stuff, it's sometimes better to use a snapshot of
the DB or maybe a standby system that's disconnected from the primary
for reporting and periodically replays logs to catch up. But obviously
you start simple and don't add this complexity to the architecture until
it's truly needed.)

Funny thing is that I've had to do exactly what you're asking about, as
part of troubleshooting problems - but the goal wasn't to run vacuum
later but to run a vacuum freeze IMMEDIATELY.  🙂  As one example,
pile-ups on LWLock multixact_offset.

Here's one pageinspect query that did the trick for me. In the first
line (WITH...) you change public.my_test to the table you want to
inspect. This only looks at a single table and it was for
troubleshooting the aforementioned wait event, so it's actually breaking
down mxid's by SLRU page numbers. If you're seeing a large number of
SLRU pages (lots of rows coming back) then that means you might want to
proactively run a manual vacuum freeze. (And then see if you can update
the app code to reduce mxid usage!)

I'm not answering your question, but thought it was a nice excuse to
share a related query and pontificate a bit... hopefully useful to someone!

-Jeremy

=====

pg-14.4 rw root(at)db1=# create extension pageinspect;
CREATE EXTENSION
Time: 7.561 ms

pg-14.4 rw root(at)db1=# with tab_name as (select 'public.my_test' t)
select min(now()) current_timestamp, count(*) number_mxid_on_page,
min(xmax) min_mxid, max(xmax) max_mxid,
  trunc((xmax)/(8192/4)) page_no
from (
  select ((attrs).t_infomask::bit(16) & x'1000'::bit(16))::int::boolean
is_multixact,
     (attrs).t_infomask::bit(16) infomask,
     (attrs).t_xmax::text::integer xmax
   from (
     select page,heap_page_item_attrs(get_raw_page((select t from
tab_name),page),
                                      (select t from
tab_name)::regclass) attrs
     from generate_series(0,(select relpages from pg_class where
oid=(select t from tab_name)::regclass)-1) page
   ) subq where (attrs).t_infomask is not null
 ) subq3 where is_multixact
group by trunc((xmax)/(8192/4));
 current_timestamp | number_mxid_on_page | min_mxid | max_mxid | page_no
-------------------+---------------------+----------+----------+---------
(0 rows)

Time: 2223.640 ms (00:02.224)

pg-14.4 rw root(at)db1=# begin;
BEGIN
Time: 0.466 ms

pg-14.4 rw root(at)db1=# select * from my_test where i<5 for update;
 i |         data
---+----------------------
 1 | XXXXXXXXXXXXXXXXXXXX
 2 | XXXXXXXXXXXXXXXXXXXX
 3 | XXXXXXXXXXXXXXXXXXXX
 4 | XXXXXXXXXXXXXXXXXXXX
(4 rows)

Time: 50.074 ms

pg-14.4 rw root(at)db1=# savepoint a;
SAVEPOINT
Time: 0.605 ms

pg-14.4 rw root(at)db1=# update my_test set i=i-10 where i<5;
UPDATE 4
Time: 49.481 ms

pg-14.4 rw root(at)db1=# with tab_name as (select 'public.my_test' t)
select min(now()) current_timestamp, count(*) number_mxid_on_page,
min(xmax) min_mxid, max(xmax) max_mxid,
  trunc((xmax)/(8192/4)) page_no
from (
  select ((attrs).t_infomask::bit(16) & x'1000'::bit(16))::int::boolean
is_multixact,
     (attrs).t_infomask::bit(16) infomask,
     (attrs).t_xmax::text::integer xmax
   from (
     select page,heap_page_item_attrs(get_raw_page((select t from
tab_name),page),
                                      (select t from
tab_name)::regclass) attrs
     from generate_series(0,(select relpages from pg_class where
oid=(select t from tab_name)::regclass)-1) page
   ) subq where (attrs).t_infomask is not null
 ) subq3 where is_multixact
group by trunc((xmax)/(8192/4));
       current_timestamp       | number_mxid_on_page | min_mxid |
max_mxid | page_no
-------------------------------+---------------------+----------+----------+---------
 2022-08-19 23:05:43.349723+00 |                   4 |        1 |       
1 |       0
(1 row)

Time: 2117.555 ms (00:02.118)

pg-14.4 rw root(at)db1=#

--
http://about.me/jeremy_schneider