Why Redis in K8S restarts all the time? - kubernetes

Question

Redis pod restarts like crazy.
How can I find out the reason for this behavior?
I figured out, that the resources quota should be upgraded, but I have no clue what would be the best cpu/ram ratio. And why there are no crash events or logs?
Here are the pods:
> kubectl get pods
redis-master-5d9cfb54f8-8pbgq 1/1 Running 33 3d16h
Here are the logs:
> kubectl logs --follow redis-master-5d9cfb54f8-8pbgq
[1] 08 Sep 07:02:12.152 # Server started, Redis version 2.8.19
[1] 08 Sep 07:02:12.153 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
[1] 08 Sep 07:02:12.153 * The server is now ready to accept connections on port 6379
[1] 08 Sep 07:03:13.085 * 10000 changes in 60 seconds. Saving...
[1] 08 Sep 07:03:13.085 * Background saving started by pid 8
[8] 08 Sep 07:03:13.101 * DB saved on disk
[8] 08 Sep 07:03:13.101 * RDB: 0 MB of memory used by copy-on-write
[1] 08 Sep 07:03:13.185 * Background saving terminated with success
[1] 08 Sep 07:04:14.018 * 10000 changes in 60 seconds. Saving...
[1] 08 Sep 07:04:14.018 * Background saving started by pid 9
...
[93] 08 Sep 08:38:30.160 * DB saved on disk
[93] 08 Sep 08:38:30.164 * RDB: 2 MB of memory used by copy-on-write
[1] 08 Sep 08:38:30.259 * Background saving terminated with success
[1] 08 Sep 08:39:31.072 * 10000 changes in 60 seconds. Saving...
[1] 08 Sep 08:39:31.074 * Background saving started by pid 94
Here is previous logs of the same pod.
> kubectl logs --previous --follow redis-master-5d9cfb54f8-8pbgq
[1] 08 Sep 09:41:46.057 * Background saving terminated with success
[1] 08 Sep 09:42:47.073 * 10000 changes in 60 seconds. Saving...
[1] 08 Sep 09:42:47.076 * Background saving started by pid 140
[140] 08 Sep 09:43:14.398 * DB saved on disk
[140] 08 Sep 09:43:14.457 * RDB: 1 MB of memory used by copy-on-write
[1] 08 Sep 09:43:14.556 * Background saving terminated with success
[1] 08 Sep 09:44:15.073 * 10000 changes in 60 seconds. Saving...
[1] 08 Sep 09:44:15.077 * Background saving started by pid 141
[1 | signal handler] (1599558267) Received SIGTERM scheduling shutdown...
[1] 08 Sep 09:44:28.052 # User requested shutdown...
[1] 08 Sep 09:44:28.052 # There is a child saving an .rdb. Killing it!
[1] 08 Sep 09:44:28.052 * Saving the final RDB snapshot before exiting.
[1] 08 Sep 09:44:49.592 * DB saved on disk
[1] 08 Sep 09:44:49.592 # Redis is now ready to exit, bye bye...
Here is the description of the pod. As you can see the limit is 100Mi, but I can't see the threshold, after which the pod restarts.
> kubectl describe pod redis-master-5d9cfb54f8-8pbgq
Name: redis-master-5d9cfb54f8-8pbgq
Namespace: cryptoman
Priority: 0
Node: gke-my-cluster-default-pool-818613a8-smmc/10.172.0.28
Start Time: Fri, 04 Sep 2020 18:52:17 +0300
Labels: app=redis
pod-template-hash=5d9cfb54f8
role=master
tier=backend
Annotations: <none>
Status: Running
IP: 10.36.2.124
IPs: <none>
Controlled By: ReplicaSet/redis-master-5d9cfb54f8
Containers:
master:
Container ID: docker://3479276666a41df502f1f9eb9bb2ff9cfa592f08a33e656e44179042b6233c6f
Image: k8s.gcr.io/redis:e2e
Image ID: docker-pullable://k8s.gcr.io/redis#sha256:f066bcf26497fbc55b9bf0769cb13a35c0afa2aa42e737cc46b7fb04b23a2f25
Port: 6379/TCP
Host Port: 0/TCP
State: Running
Started: Wed, 09 Sep 2020 10:27:56 +0300
Last State: Terminated
Reason: OOMKilled
Exit Code: 0
Started: Wed, 09 Sep 2020 07:34:18 +0300
Finished: Wed, 09 Sep 2020 10:27:55 +0300
Ready: True
Restart Count: 42
Limits:
cpu: 100m
memory: 250Mi
Requests:
cpu: 100m
memory: 250Mi
Environment: <none>
Mounts:
/var/run/secrets/kubernetes.io/serviceaccount from default-token-5tds9 (ro)
Conditions:
Type Status
Initialized True
Ready True
ContainersReady True
PodScheduled True
Volumes:
default-token-5tds9:
Type: Secret (a volume populated by a Secret)
SecretName: default-token-5tds9
Optional: false
QoS Class: Guaranteed
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal SandboxChanged 52m (x42 over 4d13h) kubelet, gke-my-cluster-default-pool-818613a8-smmc Pod sandbox changed, it will be killed and re-created.
Normal Killing 52m (x42 over 4d13h) kubelet, gke-my-cluster-default-pool-818613a8-smmc Stopping container master
Normal Created 52m (x43 over 4d16h) kubelet, gke-my-cluster-default-pool-818613a8-smmc Created container master
Normal Started 52m (x43 over 4d16h) kubelet, gke-my-cluster-default-pool-818613a8-smmc Started container master
Normal Pulled 52m (x42 over 4d13h) kubelet, gke-my-cluster-default-pool-818613a8-smmc Container image "k8s.gcr.io/redis:e2e" already present on machine

Answer 1

This is the limit after which it restarts. CPU is just throttled, memory is OOM'ed.
Limits:
cpu: 100m
memory: 250Mi
Reason: OOMKilled
Remove requests & limits
Run the pod, make sure it doesn't restart
If you already have prometheus, run VPA Recommender to check how much resources it needs. Or just use any monitoring stack: GKE Prometheus, prometheus-operator, DataDog etc to check actual resource consumption and adjust limits accordingly.

Answer 2

Max's answer is very complete. But if you don't have Prometheus installed or don't want to, there is another way simple to check actual resource consumption installing the metrics server project in your cluster. After installing it you can check the CPU and memory usage with kubectl top node to check consumption on the node, and kubectl top pod to check consumption on pods. I use it and is very useful.
Or you can just increase the CPU and memory limits, but you will not be able to ensure how much resources the container will need. Basically will be a waste of resources.

Answer 3

The main problem is that you didn't limit the redis application. So redis just increases the memory, and when it reaches Pod limits.memory 250Mb, it is killed by an OOM, and restart it.
Then, if you remove the limits.memory, the redis will continue eating memory until the Node has not enough memory to run other processes and K8s kills it and marks it as "Evicted".
Therefore, configure the memory in redis application to limit the memory used by redis inside redis.conf file and depending on your needs set a LRU or LFU policy to remove some keys (https://redis.io/topics/lru-cache):
maxmemory 256mb
maxmemory-policy allkeys-lfu
And limit the memory of the Pod at around the double of redis maxmemory to give the rest of the processes and objects saved in redis some margin:
resources:
limits:
cpu: 100m
memory: 512Mi

Answer 4

Now the pods are getting evicted.
Can I find out the reason?
NAME READY STATUS RESTARTS AGE
redis-master-7d97765bbb-7kjwn 0/1 Evicted 0 38h
redis-master-7d97765bbb-kmc9g 1/1 Running 0 30m
redis-master-7d97765bbb-sf2ss 0/1 Evicted 0 30m