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Here's a gist of our unicorn.rb config (https://gist.github.com/apangeajwrubel/4953849) Using env variables lets us adjust parameters without a code push (still requires a restart). We saw dramatic reduction in 'long tail' requests when we dropped the backlog to 25. We're experimenting now (thanks to @scottshea) with even lower values. At some point the routing mesh will give up retrying and throw an H21 (https://devcenter.heroku.com/articles/error-codes#h21-backen...). One data point that would be useful from heroku is how many retries we get.

To expand on this:

You have to remove the port declaration from the line for Unicorn in your Procfile, and then add a line like this to your unicorn.rb file to define the listener port along with adjusting the backlog size:

listen ENV['PORT'], :backlog => Integer(ENV['UNICORN_BACKLOG'] || 100)

We did this in unicorn.rb `:backlog => Integer(ENV['UNICORN_BACKLOG'] || 200)` and then set the UNICORN_BACKLOG variable by the Heroku command line `heroku config:set UNICORN_BACKLOG=25 -a <app_name>`. We have been as high as 1024 and as low as 10. We settled in at 25 for us.