Code change:
We've noticed a strange behavior of the scheduler in GCE 5K node scalability tests. There were periods of time, lasting about 6s, when scheduler didn't schedule any new pods. It's ruining our 5s pod-startup-latency SLO. See the example below, pod density-latency-pod-2325-dgqrv created on 09:08:59, scheduler didn't schedule it until 09:09:06. An interesting pattern can be found in the around that time. Before 09:09:00 scheduler schedules ~5 pods / s. Then it goes silent for about 6s and at 09:09:06 it outputs about 30 "bound successfully" statements and continues scheduling pods with ~5 pods / s pace. We checked api-server logs for that silent period of time and it turned out that scheduler was doing something, it was constantly sending GET and PUT request for a metrics-server and heapster pod it was trying to schedule. ! We dug deeper and found that scheduler was actually trying to schedule these two pods for over 12h, below is the plot of number of calls per minute that scheduler issued to api-server for these two pods: ! As we can see there are long periods of time when scheduler issues over 1500 requests per minute. That seems strangely a lot given that the requests are only for the two pods. During the whole run scheduler issued over 400K GETs and over 200K PUTs for these two pods. ! We figured out that scheduler wasn't able to schedule these two pods because there was not enough resources in the cluster. While it's understandable that scheduler wasn't able to schedule the pods, I'd expect scheduler to not spam the system with so many retries, which, as we can see in this example, can potentially block / slow-down scheduling of other pods. I've increased the scheduler log verbosity level in , we should have more logs from the tomorrow's run. Will post them here once available.
/sig scheduling
Reiterating the mentions to trigger a notification: <detailsIn response to : /sig scheduling </details
/assign for triage
What is the version of your cluster? If you are using the HEAD, we have a back-off mechanism to the scheduler to delay rescheduling unschedulable pods. It starts with 1 second delay and doubles it every time the pod is not schedulable up to 10 seconds.
It's from head, so apparently something doesn't work...
Yes, we're using head. The test run on Jan 21 and the cluster was built from head on the same day (commit to be more precise). As you can see in the logs it doesn't look like scheduler is using any back-off mechanism. Definitely not the one you described, it issues requests to api server much more often than 1 per second per pod. !
An update on more logs - we won't have more logs. We tried increasing scheduler's log level to v4 and it resulted in the log growing by few orders of magnitude and it was clogged with mostly useless stuff, e.g. it logged "Node gce-scale-cluster-minion-group-XXX is a potential node for preemption." over 250 000 000 times. We reverted the log level change in If more scheduler logs are needed to debug this we probably should add some new, more useful, ones.
cc/
/assign I can look into this next Tuesday
Friendly ping, any updates?
Whoops. Thanks for the ping. Log level v4 should not be needed. Although the log line you called out should probably get pushed into V9... First off, that's some really great debugging. Thanks for the detailed report. If the scheduler is getting into tight busy loops, that could in theory starve the scheduling thread, causing the observed 6 second pause in scheduling. The bug is in  A pod only gets marked for backoff if . And can increment very quickly in a high churn cluster. This would result in a pod being retried in a tight loop. One (unrelated) observation that stood out to me was this: This is being printed by the package and shows a trace with no steps logged to it, which means that we are hitting one of these four error cases in . Most likely, the PVC for those heapster & metrics-server pods are in a bad/unexpected state.
I think fixes the root problem here. If we eliminate the API server spam, the main scheduling thread hopefully will not get blocked for 6 second periods. Are you able to test with this patch?
Thanks Jonathan! I've reserved a scale-testing slot for Thursday (Feb 21) to test the fix. Will update this issue once I have the results.
I reproduced the issue (non-schedulable heapster pod due to not enough resources) on a 5K node cluster. Then I run the load test, first on the version without the fix and the on the version with the fix. Below are the graphs of the number of scheduler requests (for the unschedulable pod) per minute. Without fix: ! With fix: ! As we can see on the graphs, the fix definitely helps, without it there were minutes were scheduler was calling api server up to 2K times per minute (2K QPM). With the fix it didn't exceed 100 QPM. But even with the fix the average QPM is ~60 (per single pod) which is 1 query per second per pod on average. is average 60 QPM (1 QPS) reasonable here or should we expect something less from the rate-limiting mechanism?
By default the scheduler rate limit is 20 qps for all scheduler operations. Default backoff delay for unschedulable pods is 1 second and goes exponentially higher every time a pod is retried, but does not exceed 10 seconds. This 10 second max backoff time also means that if a pod is not retried for 10 seconds, for example because there are many other pods in front of it in the queue, its backoff time is reset to 1 second. So the next time it is retried and is found unschedulable it will be backed off for 1 second. I am basically trying to say that 1 qps is not unreasonable. We also an optimization () that should lower qps for unschedulable pods further down. This last PR got merged only a few hours ago. With that PR merged, I would expect to see even lower qps as the scheduler will not update pod status every time it retries scheduling a pod.
Thanks Bobby! That makes sense. In that case I believe we can close this issue once merges.
/milestone v1.14 We're entering and this looks relevant to the 1.14 release. Please if I am incorrect
Thanks for fixing!

Related discussion:
text: return p.schedulingCycle } <del> // AddUnschedulableIfNotPresent does nothing if the pod is present in any // queue. If pod is unschedulable, it adds pod to unschedulable queue if // p.moveRequestCycle > podSchedulingCycle or to backoff queue if p.moveRequestCycle // <= podSchedulingCycle but pod is subject to backoff. In other cases, it adds pod to // active queue. </del> <ins> // AddUnschedulableIfNotPresent inserts a pod that cannot be scheduled into // the queue, unless it is already in the queue. Normally, PriorityQueue puts // unschedulable pods in `unschedulableQ`. But if there has been a recent move // request, then the pod is put in `podBackoffQ`. </ins> func (p *PriorityQueue) AddUnschedulableIfNotPresent(pod *v1.Pod, podSchedulingCycle int64) error { p.lock.Lock() defer p.lock.Unlock()