Azure: keep refreshes spread over time #3631
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When a `vmssVmsCacheJitter` is provided, API calls (after start) will be randomly spread over the provided time range, then happens at regular interval (for a given VMSS). This prevents API calls spikes. But we noticed that the various VMSS' refreshes will progressively converge and agglomerate over time (in particular after a few large throttling windows affected the autoscaler), which defeats the purpose. Re-randomizing the next refresh deadline every time (rather than just at autoscaler start) keeps the calls properly spread. Configuring `vmssVmsCacheJitter` and `vmssVmsCacheTTL` allows users to control the average and worst case refresh interval (and avg API call rate). And we can count on VMSS size change detection to kick early refreshes when needed. That's a small behaviour change, but possibly still a good time for that, as `vmssVmsCacheJitter` was introduced recently and wasn't part of any release yet.
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When a
vmssVmsCacheJitteris provided, API calls will be randomly spread (at startup time) over the provided time range, then happens at regular interval (for a given VMSS). This is meant to prevents API calls spikes.But we noticed that the various VMSS' refreshes will progressively converge and agglomerate over time (in particular after a few large throttling windows affected the autoscaler), which defeats the purpose.
Re-randomizing the next refresh deadline every time (rather than just at autoscaler start) keeps the calls properly spread.
Configuring
vmssVmsCacheJitterandvmssVmsCacheTTLallows users to control the average and worst case refresh interval (and avg API call rate). And we can count on VMSS size change detection to kick early refreshes when needed.That's a small behaviour change, but possibly still a good time for that, as
vmssVmsCacheJitterwas introduced recently and wasn't part of any release yet.This also slightly simplifies the code (no need to check if scaleSet.instancesRefreshJitter is > 0, if it is then lastRefresh will be time.Now().Add(0), so current time anyway).
An autoscaler instance running with
"vmssVmsCacheJitter": 3540a few days after start:Once patched: