RE: Memory issues with PostgreSQL 15

Hi all,
I started this discussion in May and was then dragged into other topics, so I could never follow up. Sorry for that!
Since then, the problem has resurfaced from time to time. Right now, we seem to have issues again, which gives me the opportunity to follow up on your various suggestions.

The current error messages are similar to what we have seen before:

<2024-07-25 12:27:38 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
<2024-07-25 12:27:38 CEST - mailprocessor> ERROR: could not resize shared memory segment "/PostgreSQL.1226901392" to 189280 bytes: No space left on device

As far as I understand, it does not make much sense to look into SysV shared memory (which is what ipcs does). Indeed, there is only the same small shared memory segment as we have seen back then:

# ipcs -m
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x04000194 45 postgres 600 56 20

Francisco and Tom both pointed at Posix shared memory instead; however, this also does not seem to be used a lot:

# df -h /dev/shm
Filesystem Size Used Avail Use% Mounted on
tmpfs 7.8G 6.6M 7.8G 1% /dev/shm

# ls -lhR /dev/shm
/dev/shm:
total 6.6M
-rw------- 1 postgres postgres 1.0M Jul 25 06:26 PostgreSQL.1095217316
-rw------- 1 postgres postgres 1.0M Jul 23 06:20 PostgreSQL.124772332
-rw------- 1 postgres postgres 1.0M Jul 23 06:18 PostgreSQL.1475196260
-rw------- 1 postgres postgres 1.0M Jul 23 06:18 PostgreSQL.1725210234
-rw------- 1 postgres postgres 193K Jul 23 06:15 PostgreSQL.2581015990
-rw------- 1 postgres postgres 193K Jul 23 06:15 PostgreSQL.2929101952
-rw------- 1 postgres postgres 193K Jul 23 06:17 PostgreSQL.3018875836
-rw------- 1 postgres postgres 65K Jul 23 06:15 PostgreSQL.3403523208
-rw------- 1 postgres postgres 193K Jul 23 06:15 PostgreSQL.3482890896
-rw------- 1 postgres postgres 193K Jul 23 06:18 PostgreSQL.3824279998
-rw------- 1 postgres postgres 193K Jul 23 06:18 PostgreSQL.3891977516
-rw------- 1 postgres postgres 193K Jul 23 06:15 PostgreSQL.3929720846
-rw------- 1 postgres postgres 1.0M Jul 23 06:34 PostgreSQL.3969232506
-rw------- 1 postgres postgres 193K Jul 23 06:18 PostgreSQL.4222425006

We also still see a lot of available memory:

# free -m
total used free shared buff/cache available
Mem: 15882 6966 191 2109 8725 6477
Swap: 1999 271 1728

Again, exactly the same situation as before.

Tom suggested that we hit some kernel limits, but I could not find any related kernel setting. The only limit I am aware of is the size of the /dev/shm filesystem itself. This could be changed, but the default value of 8 GB (which is half of the machine's memory) seems to be enough (given that it is not even used).

Is there anything else I can analyze? Sorry again for reviving this old thread.

Best,
Christian

PS: The database does not run in a Docker container.

-----Original Message-----
From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Sent: Wednesday, May 29, 2024 11:44 PM
To: Christian Schröder <christian(dot)schroeder(at)wsd(dot)com>
Cc: Francisco Olarte <folarte(at)peoplecall(dot)com>; pgsql-general(at)lists(dot)postgresql(dot)org; Eric Wong <eric(dot)wong(at)wsd(dot)com>
Subject: Re: Memory issues with PostgreSQL 15

[EXTERNAL]

=?utf-8?B?Q2hyaXN0aWFuIFNjaHLDtmRlcg==?= <christian(dot)schroeder(at)wsd(dot)com> writes:
> # ipcs -m

> ------ Shared Memory Segments --------
> key shmid owner perms bytes nattch status
> 0x04000194 35 postgres 600 56 19

> I am surprised to see this since I would have expected much more shared memory to be used by the database. Is there anything in the configuration that prevents the shared memory from being used?

SysV shared memory isn't that relevant to Postgres anymore. Most of what we allocate goes into POSIX-style shared memory segments, which are not shown by "ipcs". We do still create one small fixed-size data structure in SysV memory, which is what you're seeing here, for arcane reasons having to do with the lifespan of the shared memory segments being different in those two APIs.

>> <2024-05-21 11:34:46 CEST - mailprocessor> ERROR: could not resize
>> shared memory segment "/PostgreSQL.2448337832" to 182656 bytes: No
>> space left on device

This seems to indicate that you're hitting some kernel limit on the amount of POSIX shared memory. Not sure where to look for that.

regards, tom lane

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