Rails 3 + Heroku: Find out dyno's memory usage

Question

Is there any way to find out how much memory a heroku web dyno is using? Let's say I wanna write a rake task that runs periodically and checks every dyno's memory usage. How could I do this? Thanks!

Answer 1

Since Heroku runs on an Amazon instance with Linux, you can use the proc filesystem to get runtime system data. The /proc/<pid>/smaps file contains memory info on the running process and all the libraries that it loads. For the resident process size, do this:

f = File.open("/proc/#{Process.pid}/smaps")
f.gets # Throw away first line
l = f.gets # Contains a string like "Size:               2148 kB\n"
l =~ /(\d+) kB/  # match the size in kB 
f.close
$1.to_i  # returns matched size as integer

Update: The proc file system has an even better source, /proc/<pid>/status. There's an entry VmRSS: for the total resident memory portion and VmHWM: for the highest resident memory used (high water mark). For more details of these and other fields, see The Linux Kernel docs for the proc file system.