page performance report regression detection

[Robert Collins]

I discussed this briefly with Stuart - it would be lovely to have the ppr show regressions.

Math wise - something like 'the time which bounds 99% of requests completing has increased by more than 20% vs the highest such time in the previous week for a given page id' - that provides a decent starting point for determining pages that need attention even if they are not yet oopsing. Note that the 99% time there is simply mean + 3*stddev.

Implementation wise, Stuart has some ideas.

This isn't particularly urgent, but it would be really really lovely to have.