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Varnish: the lower the Age value, the slower the request
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Description

Here's some network performance data pulled from Media Viewer users: P172 Plotted in a google doc: https://docs.google.com/spreadsheets/d/1KpsPgr0XyyQUXkERdYkowYPE1g0C_hmt0kV5cUKKhSQ/edit?usp=sharing

I've discovered this strange phenomenon whereby thumbnails recently added to varnish perform worse on average than thumbnails that have been in varnish for longer. The pattern is obvious once plotted. The performance is as much as 40% worse when the thumbnail has been recently added to varnish. The issue seems to go away once the thumbnail has been in varnish for a day.

We should find out why that's happening and if there's anything we can do about it.

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Gilles raised the priority of this task from to Needs Triage.
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Gilles added projects: acl*sre-team, Varnish.
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+Brandon, perhaps this rings a bell

faidon triaged this task as Medium priority.

The left axis is ms and the bottom is cache age? It doesn't greatly surprise me in the general case that there's a relationship there and that the performance isn't equal, but I'm not sure there's much to do about that. It would basically be a result of the "silo" design: as it fills the cache, one silo is being actively worked on, while past silos are basically read-only and start getting cached better at the FS layer. Probably a better thing to focus on, if warranted, is whether we can improve the average with other tuning.

The left axis is ms and the bottom is cache age?

Yep

It would basically be a result of the "silo" design

Is this known to be how Varnish works under the hood?

No movement in over a year, and is more an observation than a question.