I've been using New Relic for several months on DanceJam ("my" startup). DanceJam uses Engine Yard and one of the perks of hosting on Engine Yard is a free account with New Relic. Recently they announced a 90-Day Trial which has proved useful for userscripts.org.
In the trial you only get the stats for 2 mongrels, but if you have enough volume, it still gives great insight into your application. I've used it to spot and fix 2 of the largest time consumers for userscripts.org.
The first issue was obvious and known. The "script" model was storing the source in the table. Some scripts can be up to 100KB, which lead to slow load time any time you touch a script. Also not having a seperate model for source didn't allow for versioning .. no diffs :( After segmenting the source of a script from the script model, New Relic's "Active Record" section reported that Comment/find was the top database consumer (double the time of Script.find).
A quick check showed that "comments" was missing an index. After deploying I got some nice charts from New Relic. All charts show 30 minutes before the change, a spike while the index was occuring, then a nice speedup (most recent ~25 minutes).
Average response time was cut in half.
Response time for finding a comment dropped to ~20ms instead of hovering around 160ms
And in "Total Database Time" we see that fixing Comment/find, the overall stress on the database dropped by half.
New Relic starts at $65/month for up to 10 Mongrels. My only wish is that they had a cheaper plan for up to 5 mongrels.
Update
My munin charts for CPU load shows the dramatic difference a few tweaks make. It has been 2 days since the changes went live and the CPU & RAM usage has halved.
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