docs: add memory tuning with GOMEMLIMIT (#27914)

Signed-off-by: Julian-Chu <[email protected]>

USERS.md

@@ -103,6 +103,7 @@ Currently, the following organizations are **officially** using Argo CD:
 1. [DaoCloud](https://daocloud.io/)
 1. [Datarisk](https://www.datarisk.io/)
 1. [Daydream](https://daydream.ing)
+1. [Delivery Hero](https://www.deliveryhero.com/)
 1. [Deloitte](https://www.deloitte.com/)
 1. [Dematic](https://www.dematic.com)
 1. [Deutsche Telekom AG](https://telekom.com)

docs/operator-manual/high_availability.md

@@ -632,6 +632,35 @@ $ kubectl port-forward svc/argocd-metrics 8082:8082
 $ go tool pprof http://localhost:8082/debug/pprof/heap
 ```
 
+## Mitigating OOMKilled Events from Memory Spikes
+
+To mitigate `OOMKilled` events caused by sudden memory spikes without over-provisioning resources, you can configure the `GOMEMLIMIT` environment variable on the relevant Argo CD containers (supported in Argo CD versions >= 2.7.0).
+
+Setting `GOMEMLIMIT` to **80%–90%** of your container's total memory limit forces the Go runtime to trigger garbage collection before the Kubernetes hard limit is reached. For more detail, see the [Go GC Guide](https://go.dev/doc/gc-guide#Memory_limit) and the [Environment variables](https://pkg.go.dev/runtime#hdr-Environment_Variables) reference.
+
+```yaml
+containers:
+  - name: argocd-application-controller
+    resources:
+      limits:
+        memory: "2Gi"
+    env:
+      - name: GOMEMLIMIT
+        value: "1800MiB"  # ~90% of the 2Gi memory limit above; GOMEMLIMIT uses MiB/GiB units
+```
+
+### Known Use Cases
+
+* Application controller cold-start memory spike
+* Expensive per-request memory spikes in argocd-server
+
+### Trade-Offs & Tuning
+
+> [!WARNING]
+> Setting `GOMEMLIMIT` too close to the application's actual working set can cause **GC thrashing**. The Go runtime will spend excessive CPU cycles continuously attempting to reclaim memory, significantly degrading application performance.
+
+If you observe sustained high CPU utilization alongside frequent GC activity or ongoing `OOMKilled` events, increase the container's total memory limit and recalculate `GOMEMLIMIT` proportionally to give the runtime more breathing room.
+
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