Re: -1/0 virtualtransaction - Mailing list pgsql-general
| From | Vijaykumar Jain |
|---|---|
| Subject | Re: -1/0 virtualtransaction |
| Date | |
| Msg-id | CAM+6J95nkXHiGPw1=i7WPcAaz7ZqiL4OXdWhfeV8636y879AZQ@mail.gmail.com Whole thread Raw |
| In response to | Re: -1/0 virtualtransaction (Mike Beachy <mbeachy@gmail.com>) |
| List | pgsql-general |
Hi,
I am just trying to jump in, but ignore if not relevant.
when you said Eventually this results in an "out of shared memory" error
Can you rule out the below two scenarios (wrt /dev/shm too low in docker or query requesting for too many locks either due to parallellism/partition involved)
There have been multiple cases of out of shared memory i have read earlier for due to above.
also, is this repeatable (given you mention it happens and eventually lead to "out of shared memory")
I may be missing something, but i do not see a PID even though it has a lock granted on a page, was the process terminated explicitly or implicitly. ( and an orphan lingering ? )
ps auwwxx | grep postgres
I took the below from "src/test/regress/sql/tidscan.sql" to simulate SIReadLock with an orphan process (by killing the process), but it gets reaped fine for me :(
postgres=# \d tidscan
Table "public.tidscan"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
id | integer | | |
postgres=# INSERT INTO tidscan VALUES (1), (2), (3);
postgres=# BEGIN ISOLATION LEVEL SERIALIZABLE;
BEGIN
postgres=*# SELECT * FROM tidscan WHERE ctid = '(0,1)';
id
----
1
(1 row)
postgres=*# -- locktype should be 'tuple'
SELECT locktype, mode FROM pg_locks WHERE pid = pg_backend_pid() AND mode = 'SIReadLock';
locktype | mode
----------+------------
tuple | SIReadLock
(1 row)
postgres=*# -- locktype should be 'tuple'
SELECT pid, locktype, mode FROM pg_locks WHERE mode = 'SIReadLock';
pid | locktype | mode
------+----------+------------
2831 | tuple | SIReadLock
(1 row)
i thought one could attach a gdb or strace to the pid to figure out what it did before crashing.
As always, I have little knowledge on postgresql, feel free to ignore if nothing relevant.
Thanks,
Vijay
On Tue, 27 Apr 2021 at 19:55, Mike Beachy <mbeachy@gmail.com> wrote:
Hi Laurenz -On Tue, Apr 27, 2021 at 2:56 AM Laurenz Albe <laurenz.albe@cybertec.at> wrote:Not sure, but do you see prepared transactions in "pg_prepared_xacts"?No, the -1 in the virtualtransaction (https://www.postgresql.org/docs/11/view-pg-locks.html) for pg_prepared_xacts was another clue I saw! But, it seems more or less a dead end as I have nothing in pg_prepared_xacts.
Thanks for the idea, though.I still need to put more effort into Tom's idea about SIReadLock hanging out after the transaction, but some evidence pointing in this direction is that I've reduced the number of db connections and found that the '-1/0' locks will eventually go away! I interpret this as the db needing to find time when no overlapping read/write transactions are present. This doesn't seem completely correct, as I don't have any long lived transactions running while these locks are hanging out. Confusion still remains, for sure.
Mike
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