Thread: Kubernetes, cgroups v2 and OOM killer - how to avoid?
Hi, 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 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 Any help is greatly appreciated! Cheers, Ancoron
On Sat, 2025-04-05 at 13:53 +0200, 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. You don't have to explain why that is a problem. It clearly is! > Ideally, I'd find a configuration that only terminates one backend but > leaves the others working. There isn't, but what you really want is: > 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 cannot answer your question, but I'd like to make two suggestions: 1. set the PostgreSQL configuration parameters "work_mem", "shared_buffers", "maintenance_work_mem" and "max_connections" low enough that you don't go out of memory. A crash is bad, but a query failing with an "out of memory" error isn't nice either. 2. If you want to run PostgreSQL seriously in Kubernetes, put all PostgreSQL pods on a dedicated host machine where you can disable memory overcommit. Yours, Laurenz Albe
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." 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. 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. The combination of some swap being available, and the throttling under heavy reclaim will likely mitigate your problems. -- Joe Conway PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
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". > > > 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. > > 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. > > The combination of some swap being available, and the throttling under > heavy reclaim will likely mitigate your problems. > Thank you for your insights, I have something to think about. Cheers, Ancoron
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
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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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
anyattachments. Certain required legal entity disclosures can be accessed on our website:
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--
Adrian Klaver
adrian.klaver@aklaver.com