Rap Genius cofounder here. Below is the full unedited text of https://help.heroku.com/tickets/37665, a Heroku support ticket I logged about 1 year ago. Sorry it is so long, but I think you'll find it interesting:

Tom@Rapgenius| about 1 year ago
I know this is a bit of a vague problem, but I've been getting a bunch of Error H12 (Request Timeout)s recently, and I'm not sure what to do about it.
It's not like I have some particularly slow actions; I'm getting this error for actions that under most circumstances work totally fine (i.e., return in less than 300ms). Also I don't have a deep request queue (I'm running 40 dynos which is more than enough). Maybe I'm doing some slow queries? Should I upgrade my DB?
Also, I do notice that most of my app's time (according to New Relic) is being spent in Ruby (http://cl.ly/29132F272W2D0K1l2I3P). Would upgrading Ruby to 1.9 noticeably help this performance? (I'm a bit nervous it'll create a ton of problems).

Phil@Heroku
Hello - I can look into this, but I'll need access to your New Relic account. Will you make sure 'phil@heroku.com' has access?
Also, from your screenshot I notice your DB times are ~ 100 ms. We recommend keeping those times closer to 50 ms. You might be able to speed things up with a database upgrade.
I'll look into New Relic once I have access and let you know what I find.

Tom@Rapgenius
Thanks, Phil!
How do I give you access to my New Relic account? I tried clicking "account settings" and got this: http://cl.ly/0V2J3i0826400I2s3b2c

Phil@Heroku
Tom, I have access now. I'm not sure what was blocking me earlier.
After looking at New Relic and the database server, I think a larger database will help. At the very least, it will be helpful to try the next level for a week and compare performance statistics in New Relic with the prior week.
Your app is using an Ika right now, and the next step up is the Zilla database. We've made the upgrade process very simple, and it's outlined here - http://devcenter.heroku.com/articles/fast-database-changeove...
Your database is ~ 5.4 GB in size (via the 'heroku pg:info' command) so an upgrade shouldn't take too long. You will be able to test the process by adding a Follower and timing it via the 'heroku pg:wait' command. This should give you a good idea of how long it will take to spin up the new database. Also, should the Zilla not help much, the downgrade process to an Ika will be the same. You only pay for the resources used.

The current database server appears to be a bit under-powered when it comes to Compute Units. The Zilla has more power and should provide some room to grow.
As for an upgrade to Ruby 1.9.2, I'm not sure how much that would help. It would be an involved upgrade that would take time to plan and deploy. The database upgrade should be a quicker solution.
Long-term you may want to consider moving to the Cedar stack and Ruby 1.9.2.

Tom@Rapgenius
Thanks! I'm upgrading now

Tom@Rapgenius
I'm still getting a ton of "Request Timeout" errors. E.g.:
2011-12-08 14:46:53.222 219 1 2011-12-08T14:46:53+00:00 d. heroku router - - Error H12 (Request timeout) -> GET rapgenius.com/Wale-ambition-lyrics dyno=web.17 queue= wait= service=30000ms status=503 bytes=0
one weird thing: there aren't any values listed for the "queue" and "wait" parameters. Could that indicate a problem?
Could an exception have been thrown earlier in the request before the timeout? Or does the timeout error just indicate that the request took too long? If it's the latter I'm not sure how to troubleshoot all these errors since the associated actions are fast the vast majority of the time

Tom@Rapgenius
Here's another interesting example:

2011-12-08 15:59:32.293 222 1 2011-12-08T15:59:32+00:00 d. heroku router - - Error H12 (Request timeout) -> GET rapgenius.com/static/templates_for_js dyno=web.17 queue= wait= service=30000ms status=503 bytes=0
This action is extremely simple – it doesn't access the DB or any external services. Here's the template:
Ballin!
<% unless current_user %>
<% form_for User.new, :html => { :id => '' } do |f| %>
Tired of entering your email address? Create a Rap Genius account and you'll never have to worry about it (or anything else) ever again:
<%= render :partial => "/users/form", :object => f %>
<%= f.submit "Create Account" %> <small>(Already have an account? <%= link_to 'Sign in', login_path, :class => :facebox %>)</small>
<% end %>
<% end %>

Besides a big request queue (which there isn't), how could this action possibly time out?

Phil@Heroku
Tom - sorry for not getting back to you sooner.

It's possible for H12s to occur even for simple actions if there is already queueing for the app. With a busy site like your's, even a few H12s can cause a cascade of H12s for successive requests.

It looks like New Relic has not reported any downtime over the past 24 hours. Can we let the site run through the weekend and see how things look Monday after 3 days of New Relic data with the new Zilla?

Tom@Rapgenius
> It's possible for H12s to occur even for simple actions if there is already queueing for the app.

I feel what you're saying, but I don't think my app's queuing. For one thing, New Relic shows 0 time spent in the queue during the period in which I'm getting all these timeouts. For another, I'm running 40 dynos and my average request time is <400ms. So:

400 ms * 3000 requests / minute * 1 min / 60000 ms = 20 simultaneous requests (i.e., 20 dynos)
so 40 dynos should definitely be more than enough.

Also, shouldn't Heroku be showing me the queue / wait stats at the time of the timeout? That would help prove whether my app was queuing at the time in question

It looks like New Relic has not reported any downtime over the past 24 hours.

New Relic isn't great at catching intermittent problems like this; you really feel it when you're using the site continuously for an hour or whatever. Also, users make many more HTTP requests than New Relic (since every page load kicks off several AJAX requests).

That said, there has been downtime in the past 24 hours (though less than in the previous 24): http://cl.ly/3D2b1Z170B0w1f1m113m

Tom@Rapgenius
Here's some additional data: At 5am this morning (EST), Rap Genius went down. I woke up at 11am (it's a Saturday!), did a logs --tail and observed that basically every request was timing out. I did heroku restart, and now every request started returning a backlog too deep error

Finally, I added another 10 dynos (bumping the total to 50, which is a log of dynos!), and this seems to have fixed the problem – perhaps because my app needs the additional capacity, or perhaps because merely changing the number of dynos reset something else. Either way, I'm sticking with 50 dynos for now out of fear even though I doubt my app needs that many (right?)

Either way, the 5 hours of unexplained downtime (there weren't any application-level exceptions or anything) that was fixable by tweaking my dyno count further supports my theory that something's going on with my app on Heroku's end.

Phil@Rapgenius
Tom -
I've been looking over your New Relic stats.

First - the good news - the upgrade to a Zilla seems to have helped. Database times are down a bit, which can only help. I checked the actual database server and it's not showing signs of over-work like the previous Ika was.
Second, I notice that downtimes reported by New Relic over the past two weeks are in the early morning hours - 3 to 6 AM PST. Do you have any scheduled tasks that run during these times?

Also, request queueing is nearly zero, so 50 dynos does seem like a lot.
What are your usage patterns like? The RPM graph in New Relic indicates the normal cyclical usage pattern, lower during the night, but what does Google Analytics tell you?

Finally, the Heroku platform has been having issues over the past week, but none of them correspond to the downtime you had Saturday morning.