Memory issues with PostgreSQL 15

Hi all,
We migrated from PostgreSQL 9.4 to PostgreSQL 15 a while ago. Since then, we have a lot of memory issues in our QA environment (which is a bit tense in resources). We did not have these problems before the migration, and we do not have them in our production environment, which has a lot more memory. So, it is not super critical for us, but I would still like to understand better how we can improve our configuration.

Our PostgreSQL version is "PostgreSQL 15.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44), 64-bit". The database server is a dedicated server with 15 GB RAM (and 4 cores, if this matters).
We used the following settings:
shared_buffers = 4GB
work_mem = 4MB

After a while, we saw the following error in the logs:

<2024-05-20 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory

However, according to "free", a lot of memory was available:

# free -m
total used free shared buff/cache available
Mem: 15882 4992 463 4195 10427 6365
Swap: 1999 271 1728

Our Grafana charts showed a slow increase in memory consumption until it plateaus at 4.66 GB.
We also found the following error:

<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

I thought this could all be related to our "shared_buffers" setting, so I increased it to 8 GB. This almost immediately (after a few minutes) gave me these errors:

<2024-05-27 11:45:59 CEST - > ERROR: out of memory
<2024-05-27 11:45:59 CEST - > DETAIL: Failed on request of size 201088574 in memory context "TopTransactionContext".
...
<2024-05-27 11:58:02 CEST - > ERROR: out of memory
<2024-05-27 11:58:02 CEST - > DETAIL: Failed while creating memory context "dynahash".
<2024-05-27 11:58:02 CEST - > LOG: background worker "parallel worker" (PID 21480) exited with exit code 1
...
<2024-05-27 12:01:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory
<2024-05-27 12:01:03 CEST - > LOG: could not fork autovacuum worker process: Cannot allocate memory
<2024-05-27 12:02:02 CEST - > LOG: could not fork new process for connection: Cannot allocate memory

Since this seemed worse than before, I changed the setting back to 4 GB. I noticed that "free" now reports even more available memory:

# free -m
total used free shared buff/cache available
Mem: 15882 621 320 2256 14940 12674
Swap: 1999 199 1800

So, does the "shared_buffers" setting have the opposite effect than I though? If I correctly remember similar discussions years ago, the database needs both "normal" and shared memory. By increasing the "shared_buffers" to 8 GB, I may have deprived it of "normal" memory. On the other hand, I would have expected the remaining 7 GB to still be enough.

At this point, I am out of ideas. I clearly seem to misunderstand how the database manages its memory. This may have changed between 9.4 and 15, so my prior knowledge may be useless. I definitely need some help. ☹

Thanks in advance,
Christian

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