replication slot stats memory bug
replication slot stats memory bug
От:
Andres Freund <andres@anarazel.de>
Дата:
Hi,
in the course of https://postgr.es/m/3471359.1615937770%40sss.pgh.pa.us
I saw a leak in pgstat_read_statsfiles(), more precisely:
/* Allocate the space for replication slot statistics */
replSlotStats = palloc0(max_replication_slots * sizeof(PgStat_ReplSlotStats));
the issue is that the current memory context is not set by
pgstat_read_statsfiles().
In some cases CurrentMemoryContext is going to be a long-lived context,
accumulating those allocations over time. In other contexts it will be a
too short lived context, e.g. an ExprContext from the pg_stat_*
invocation in the query. A reproducer for the latter:
postgres[2252294][1]=# SELECT pg_create_logical_replication_slot('test', 'test_decoding');
┌────────────────────────────────────┐
│ pg_create_logical_replication_slot │
├────────────────────────────────────┤
│ (test,0/456C1878) │
└────────────────────────────────────┘
(1 row)
postgres[2252294][1]=# BEGIN ;
BEGIN
postgres[2252294][1]*=# SELECT * FROM pg_stat_replication_slots ;
┌───────────┬────────────┬─────────────┬─────────────┬─────────────┬──────────────┬──────────────┬─────────────┐
│ slot_name │ spill_txns │ spill_count │ spill_bytes │ stream_txns │ stream_count │ stream_bytes │ stats_reset │
├───────────┼────────────┼─────────────┼─────────────┼─────────────┼──────────────┼──────────────┼─────────────┤
│ test │ 0 │ 0 │ 0 │ 0 │ 0 │ 0 │ (null) │
└───────────┴────────────┴─────────────┴─────────────┴─────────────┴──────────────┴──────────────┴─────────────┘
(1 row)
postgres[2252294][1]*=# SELECT * FROM pg_stat_replication_slots ;
┌─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
│ >
├─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
│ \x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F\x7F>
└─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────>
(1 row)
I'll push the minimal fix of forcing the allocation to happen in
pgStatLocalContext and setting it to NULL in pgstat_clear_snapshot().
But it seems like we just shouldn't allocate it dynamically at all?
max_replication_slots doesn't change during postmaster lifetime, so it
seems like it should just be allocated once?
Greetings,
Andres Freund
Re: replication slot stats memory bug
От:
Andres Freund <andres@anarazel.de>
Дата:
Hi, On 2021-03-17 16:04:47 -0700, Andres Freund wrote: > I'll push the minimal fix of forcing the allocation to happen in > pgStatLocalContext and setting it to NULL in pgstat_clear_snapshot(). Done: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=5f79580ad69f6e696365bdc63bc265f45bd77211 Greetings, Andres Freund
Re: replication slot stats memory bug
От:
Andres Freund <andres@anarazel.de>
Дата:
Hi, On 2021-03-17 19:36:46 -0400, Tom Lane wrote: > > But it seems like we just shouldn't allocate it dynamically at all? > > max_replication_slots doesn't change during postmaster lifetime, so it > > seems like it should just be allocated once? > > Meh. I don't see a need to wire in such an assumption here. It does make it easier for the shared memory stats patch, because if there's a fixed number + location, the relevant stats reporting doesn't need to go through a hashtable with the associated locking. I guess that may have colored my perception that it's better to just have a statically sized memory allocation for this. Noteworthy that SLRU stats are done in a fixed size allocation as well... Greetings, Andres Freund
Re: replication slot stats memory bug
От:
Tom Lane <tgl@sss.pgh.pa.us>
Дата:
Andres Freund writes: > I saw a leak in pgstat_read_statsfiles(), more precisely: > /* Allocate the space for replication slot statistics */ > replSlotStats = palloc0(max_replication_slots * sizeof(PgStat_ReplSlotStats)); Yeah, I just found that myself. I think your fix is good. > But it seems like we just shouldn't allocate it dynamically at all? > max_replication_slots doesn't change during postmaster lifetime, so it > seems like it should just be allocated once? Meh. I don't see a need to wire in such an assumption here. regards, tom lane
Re: replication slot stats memory bug
От:
Amit Kapila <amit.kapila16@gmail.com>
Дата:
On Thu, Mar 18, 2021 at 4:55 AM Andres Freund wrote: > > Hi, > > On 2021-03-17 16:04:47 -0700, Andres Freund wrote: > > I'll push the minimal fix of forcing the allocation to happen in > > pgStatLocalContext and setting it to NULL in pgstat_clear_snapshot(). > > Done: https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=5f79580ad69f6e696365bdc63bc265f45bd77211 > Thank you! -- With Regards, Amit Kapila.
