Re: Safe vm.overcommit_ratio for Large Multi-Instance PostgreSQL Fleet

On 8/5/25 13:01, Priya V wrote:
> *Environment:*
> *PostgreSQL Versions:* Mix of 13.13 and 15.12 (upgrades in progress
> to be at 15.12 currently both are actively in use)

PostgreSQL 13 end of life after November 13, 2025

> *OS / Kernel:* RHEL 7 & RHEL 8 variants, kernels in the 4.14–4.18 range

RHEL 7 has been EOL for quite a while now. Note that you have to watch
out for collation issues/corrupted indexes after OS upgrades due to
collations changing with newer glibc versions.

> *Swap:* Currently none

bad idea

> *Workload:* Highly mixed — OLTP-style internal apps with
> unpredictable query patterns and connection counts
>
> *Goal:* Uniform, safe memory settings across the fleet to avoid
> kernel or database instability

> We’re considering:
> *|vm.overcommit_memory = 2|* for strict accounting

yes

> Increasing |vm.overcommit_ratio| from 50 → 80 or 90 to better
> reflect actual PostgreSQL usage (e.g., |work_mem| reservations that
> aren’t fully used)

work_mem does not reserve memory -- it is a maximum that might be used
in memory for a particular operation

> *Our questions for those running large PostgreSQL fleets:*
> 1.
> What |overcommit_ratio| do you find safe for PostgreSQL without
> causing kernel memory crunches?

Read this:
https://www.cybertec-postgresql.com/en/what-you-should-know-about-linux-memory-overcommit-in-postgresql/

> 2.
> Do you prefer |overcommit_memory = 1| or |= 2| for production stability?

Use overcommit_memory = 2 for production stability

> 3.
> How much swap (if any) do you keep in large-memory servers where
> PostgreSQL is the primary workload? Is having swap configured a good
> idea or not ?

You don't necessary need a large amount of swap, but you definitely
should not disable it.

Some background on that:
https://chrisdown.name/2018/01/02/in-defence-of-swap.html

> 4.
> Any real-world cases where kernel accounting was too strict or too
> loose for PostgreSQL?

In my experience the biggest issues are when postgres is running in a
memory constrained cgroup. If you want to constrain memory with cgroups,
use cgroup v2 (not 1) and use memory.high to constrain it, not memory.max.

> 5. What settings to go with if we are not planning on using swap ?

IMHO do not disable swap on Linux, at least not on production, ever.

> We’d like to avoid both extremes:
> Too low a ratio → PostgreSQL backends failing allocations even with
> free RAM

Have you actually seen this or are you theorizing?

> Too high a ratio → OOM killer terminating PostgreSQL under load spikes

If overcommit_memory = 2, overcommit_ratio is reasonable (less than 100,
maybe 80 or so as you suggested), and swap is not disabled, and you are
not running in a memory constrained cgroup, I would be very surprised if
you will ever get hit by the OOM killer. And if you do, things are so
bad the database was probably dying anyway.

HTH,

--
Joe Conway
PostgreSQL Contributors Team
Amazon Web Services: https://aws.amazon.com