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* Re: Kubernetes, cgroups v2 and OOM killer - how to avoid?
@ 2025-04-08 18:15 Joe Conway <[email protected]>
0 siblings, 1 reply; 3+ messages in thread
From: Joe Conway @ 2025-04-08 18:15 UTC (permalink / raw)
To: Ancoron Luciferis <[email protected]>; [email protected]
On 4/8/25 13:58, Ancoron Luciferis wrote:
> On 2025-04-07 15:21, Joe Conway wrote:
>> On 4/5/25 07:53, Ancoron Luciferis wrote:
>>> I've been investigating this topic every now and then but to this day
>>> have not come to a setup that consistently leads to a PostgreSQL backend
>>> process receiving an allocation error instead of being killed externally
>>> by the OOM killer.
>>>
>>> Why this is a problem for me? Because while applications are accessing
>>> their DBs (multiple services having their own DBs, some high-frequency),
>>> the whole server goes into recovery and kills all backends/connections.
>>>
>>> While my applications are written to tolerate that, it also means that
>>> at that time, esp. for the high-frequency apps, events are piling up,
>>> which then leads to a burst as soon as connectivity is restored. This in
>>> turn leads to peaks in resource usage in other places (event store,
>>> in-memory buffers from apps, ...), which sometimes leads to a series of
>>> OOM killer events being triggered, just because some analytics query
>>> went overboard.
>>>
>>> Ideally, I'd find a configuration that only terminates one backend but
>>> leaves the others working.
>>>
>>> I am wondering whether there is any way to receive a real ENOMEM inside
>>> a cgroup as soon as I try to allocate beyond its memory.max, instead of
>>> relying on the OOM killer.
>>>
>>> I know the recommendation is to have vm.overcommit_memory set to 2, but
>>> then that affects all workloads on the host, including critical infra
>>> like the kubelet, CNI, CSI, monitoring, ...
>>>
>>> I have already gone through and tested the obvious:
>>>
>>> https://www.postgresql.org/docs/current/kernel-resources.html#LINUX-
>>> MEMORY-OVERCOMMIT
>>
>> Importantly vm.overcommit_memory set to 2 only matters when memory is
>> constrained at the host level.
>>
>> As soon as you are running in a cgroup with a hard memory limit,
>> vm.overcommit_memory is irrelevant.
>>
>> You can have terabytes of free memory on the host, but if cgroup memory
>> usage exceeds memory.limit (cgv1) or memory.max (cgv2) the OOM killer
>> will pick the process in the cgroup with the highest oom_score and whack
>> it.
>>
>> Unfortunately there is no equivalent to vm.overcommit_memory within the
>> cgroup.
>>
>>> And yes, I know that Linux cgroups v2 memory.max is not an actual hard
>>> limit:
>>>
>>> https://www.kernel.org/doc/html/latest/admin-guide/cgroup-
>>> v2.html#memory-interface-files
>>
>> Read that again -- memory.max *is* a hard limit (same as memory.limit in
>> cgv1).
>>
>> "memory.max
>>
>> A read-write single value file which exists on non-root cgroups. The
>> default is “max”.
>>
>> Memory usage hard limit. This is the main mechanism to limit memory
>> usage of a cgroup. If a cgroup’s memory usage reaches this limit and
>> can’t be reduced, the OOM killer is invoked in the cgroup."
>
> Yes, I know it says "hard limit", but then any app still can go beyond
> (might just be on me here to assume any "hard limit" to imply an actual
> error when trying to go beyond). The OOM killer then will kick in
> eventually, but not in any way that any process inside the cgroup could
> prevent. So there is no signal that the app could react to saying "hey,
> you just went beyond what you're allowed, please adjust before I kill you".
No, that really is a hard limit and the OOM killer is *really* fast.
Once that is hit there is no time to intervene. The soft limit
(memory.high) is the one you want for that.
Or you can monitor PSI and try to anticipate problems, but that is
difficult at best. If you want to see how that is done, check out
senpai: https://github.com/facebookincubator/senpai/blob/main/README.md
>> If you want a soft limit use memory.high.
>>
>> "memory.high
>>
>> A read-write single value file which exists on non-root cgroups. The
>> default is “max”.
>>
>> Memory usage throttle limit. If a cgroup’s usage goes over the high
>> boundary, the processes of the cgroup are throttled and put under
>> heavy reclaim pressure.
>>
>> Going over the high limit never invokes the OOM killer and under
>> extreme conditions the limit may be breached. The high limit should
>> be used in scenarios where an external process monitors the limited
>> cgroup to alleviate heavy reclaim pressure.
>>
>> You want to be using memory.high rather than memory.max.
>
> Hm, so solely relying on reclaim? I think that'll just get the whole
> cgroup into ultra-slow mode and would not actually prevent too much
> memory allocation. While this may work out just fine for the PostgreSQL
> instance, it'll for sure have effects on the other workloads on the same
> node (which I have apparently, more PG instances).
>
> Apparently, I also don't see a way to even try this out in a Kubernetes
> environment, since there doesn't seem to be a way to set this field
> through some workload manifests field.
Yeah, that part I have no idea about. I quit looking at kubernetes
related things about 3 years ago. Although, this link seems to indicate
there is a way related to how it does QoS:
https://kubernetes.io/blog/2023/05/05/qos-memory-resources/#:~:text=memory.high%20formula
>> Also, I don't know what kubernetes recommends these days, but it used to
>> require you to disable swap. In more recent versions of kubernetes you
>> are able to run with swap enabled but I have no idea what the default is
>> -- make sure you run with swap enabled.
>
> Yes, this is what I wanna try out next.
Seriously -- this is *way* more than half the battle. If you do nothing
else, be sure to do this...
--
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
^ permalink raw reply [nested|flat] 3+ messages in thread
* Postgres_fdw- User Mapping with md5-hashed password
@ 2025-04-08 20:00 Dirschel, Steve <[email protected]>
parent: Joe Conway <[email protected]>
0 siblings, 1 reply; 3+ messages in thread
From: Dirschel, Steve @ 2025-04-08 20:00 UTC (permalink / raw)
To: [email protected] <[email protected]>
I know I can create user steve_test with password testpassword122 as md5 by doing:
select 'md5'||md5('testpassword122steve_test'); Returns --> md5eb7e220574bf85096ee99370ad67cbd3
CREATE USER steve_test WITH PASSWORD 'md5eb7e220574bf85096ee99370ad67cbd3';
And then I can login as steve_test with password testpassword122.
I'm trying to use similar logic when creating a user mapping:
CREATE USER MAPPING FOR postgres SERVER steve_snap0 OPTIONS (user 'steve_test', password 'md5eb7e220574bf85096ee99370ad67cbd3');
When I try and import a foreign schema I get an error:
ERROR: could not connect to server "steve_snap0"
If I create the user mapping with the password:
CREATE USER MAPPING FOR postgres SERVER steve_snap0 OPTIONS (user 'steve_test', password 'testpassword122');
It works fine.
Is it not possible to use the same logic for the user mapping password that can be used when creating a user?
Thanks in advance.
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^ permalink raw reply [nested|flat] 3+ messages in thread
* Re: Postgres_fdw- User Mapping with md5-hashed password
@ 2025-04-08 20:33 Adrian Klaver <[email protected]>
parent: Dirschel, Steve <[email protected]>
0 siblings, 0 replies; 3+ messages in thread
From: Adrian Klaver @ 2025-04-08 20:33 UTC (permalink / raw)
To: Dirschel, Steve <[email protected]>; [email protected] <[email protected]>
On 4/8/25 13:00, Dirschel, Steve wrote:
> I know I can create user steve_test with password testpassword122 as md5 by doing:
>
> select 'md5'||md5('testpassword122steve_test'); Returns --> md5eb7e220574bf85096ee99370ad67cbd3
>
> CREATE USER steve_test WITH PASSWORD 'md5eb7e220574bf85096ee99370ad67cbd3';
>
> And then I can login as steve_test with password testpassword122.
>
> I'm trying to use similar logic when creating a user mapping:
>
> CREATE USER MAPPING FOR postgres SERVER steve_snap0 OPTIONS (user 'steve_test', password 'md5eb7e220574bf85096ee99370ad67cbd3');
>
> When I try and import a foreign schema I get an error:
>
> ERROR: could not connect to server "steve_snap0"
>
> If I create the user mapping with the password:
>
> CREATE USER MAPPING FOR postgres SERVER steve_snap0 OPTIONS (user 'steve_test', password 'testpassword122');
>
> It works fine.
>
> Is it not possible to use the same logic for the user mapping password that can be used when creating a user?
A) Short version
No you can't.
b) Long version
From here:
CREATE ROLE
https://www.postgresql.org/docs/current/sql-createrole.html
"If the presented password string is already in MD5-encrypted or
SCRAM-encrypted format, then it is stored as-is regardless of
password_encryption (since the system cannot decrypt the specified
encrypted password string, to encrypt it in a different format). This
allows reloading of encrypted passwords during dump/restore."
Whereas from here:
https://www.postgresql.org/docs/current/postgres-fdw.html
" user mapping, defined with CREATE USER MAPPING, is needed as well to
identify the role that will be used on the remote server:
CREATE USER MAPPING FOR local_user
SERVER foreign_server
OPTIONS (user 'foreign_user', password 'password');
"
In the above you are just supplying values to the connection string not
actually creating a password as in the first case.
>
> Thanks in advance.
> This e-mail is for the sole use of the intended recipient and contains information that may be privileged and/or confidential. If you are not an intended recipient, please notify the sender by return e-mail and delete this e-mail and any attachments. Certain required legal entity disclosures can be accessed on our website: https://www.thomsonreuters.com/en/resources/disclosures.html
--
Adrian Klaver
[email protected]
^ permalink raw reply [nested|flat] 3+ messages in thread
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2025-04-08 18:15 Re: Kubernetes, cgroups v2 and OOM killer - how to avoid? Joe Conway <[email protected]>
2025-04-08 20:00 ` Postgres_fdw- User Mapping with md5-hashed password Dirschel, Steve <[email protected]>
2025-04-08 20:33 ` Re: Postgres_fdw- User Mapping with md5-hashed password Adrian Klaver <[email protected]>
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