3.6 KiB
3.6 KiB
Pod startup latency SLI/SLO details
Definition
| Status | SLI | SLO |
|---|---|---|
| Official | Startup latency of stateless1 and schedulable2 pods, excluding time to pull images and run init containers, measured from pod creation timestamp to when all its containers are reported as started and observed via watch, measured as 99th percentile over last 5 minutes | In default Kubernetes installation, 99th percentile per cluster-day <= 5s |
[1]A stateless pod is defined as a pod that doesn't
mount volumes with sources other than secrets, config maps, downward API and
empty dir.
[2]By schedulable pod we mean a pod that can be scheduled in the cluster without causing any preemption.
User stories
- As a user of vanilla Kubernetes, I want some guarantee how quickly my pods will be started.
Other notes
- Only schedulable and stateless pods contribute to the SLI:
- If there is no space in the cluster to place the pod, there is not much we can do about it (it is task for Cluster Autoscaler which should have separate SLIs/SLOs).
- If placing a pod requires preempting other pods, that may heavily depend on the application (e.g. on their graceful termination period). We don't want that to contribute to this SLI.
- Mounting disks required by non-stateless pods may potentially also require non-negligible time, not fully dependent on Kubernetes.
- We are explicitly excluding image pulling from time the SLI. This is because it highly depends on locality of the image, image registry performance characteristic (e.g. throughput), image size itself, etc. Since we have no control over any of those (and all of those would significantly affect SLI) we decided to simply exclude it.
- We are also explicitly excluding time to run init containers, as, again, this is heavily application-dependent (and does't depend on Kubernetes itself).
- The answer to question "when pod should be considered as started" is also
not obvious. We decided for the semantic of "when all its containers are
reported as started and observed via watch", because:
- we require all containers to be started (not e.g. the first one) to ensure that the pod is started. We need to ensure that potential regressions like linearization of container startups within a pod will be catch by this SLI.
- note that we don't require all container to be running - if some of them finished before the last one was started that is also fine. It is just required that all of them has been started (at least once).
- we don't want to rely on "readiness checks", because they heavily depend on the application. If the application takes couple minutes to initialize before it starts responding to readiness checks, that shouldn't count towards Kubernetes performance.
- even if your application started, many control loops in Kubernetes will not fire before they will observe that. If Kubelet is not able to report the status due to some reason, other parts of the system will not have a way to learn about it - this is why reporting part is so important here.
- since watch is so centric to Kubernetes (and many control loops are triggered by specific watch events), observing the status of pod is also part of the SLI (as this is the moment when next control loops can potentially be fired).
TODOs
- We should try to provide guarantees for non-stateless pods (the threshold may be higher for them though).
- Revisit whether we want "watch pod status" part to be included in the SLI.
Test scenario
TODO: Descibe test scenario.