Overview of Debugging and Profiling tools for Rails

1. Debugging and Profiling
Rails App
David Paluy
January 2013

3. Ruby is eating RAM

4. Agenda
● “Winter is coming!”
● Garbage Collector
● Debug Tools
● Profiling Tools

5. The Task: Send ~30,000 e-mails

6. Result Before

7. How Ruby Works?
Physical RAM
Process Heap
Ruby Heap Ruby Heap
Ruby Ruby Ruby Ruby Ruby Ruby
Object Object Object Object Object Object

8. New Object allocation
Free List
A
L
L
F
O
R
C
E
A
E
T
E
D

9. New Object allocation
Free List
A
L
L
O
C
A
T
E
D

10. New Object allocation
Free List is empty
A
L
L
O
C
A
T
E
D

11. New Object allocation
Free List is empty – Call GC
A
L
L
O
C
A
T
E
D

12. GC Process
● GC finds non-reachable objects and adds
them to Free List
● If Free List is still empty, another Heap
allocated

13. MRI GC
● “Conservative”: any bit pattern could be a
pointer (may produce false positive)
● “Stop the world”: no other Ruby code can
execute during GC
● “Mark & Sweep”:
mark all objects in use, than sweep away
unmarked objects

14. More Objects => Longer GC => Slow

15. In our case – Out of Memory!

16. How to Debug?
● gem "pry-debugger"
https://github.com/nixme/pry-debugger
● gem "debugger-pry"
https://github.com/pry/debugger-pry

17. Tools
● ObjectSpace.count_objects
● GC debug - Enable heap dump support
● gdb.rb (only Linux)
Note: memprof works only with Ruby 1.8

18. ObjectSpace.count_objects

19. Enable heap dump support to Ruby
Install custom patched version of ruby
Usage:

20. https://github.com/tmm1/gdb.rb
Attached to existing process and
examine the HEAP

21. Result After

22. Profiling Tools
● Ruby Benchmark
● ruby-prof
● perftools.rb (Google perftools for Ruby)

23. Benchmark
● gem 'benchmark_suite'
https://github.com/evanphx/benchmark_suite

24. ruby-prof
gem 'ruby-prof'
https://github.com/rdp/ruby-prof

25. ruby-prof Measurements
● process time (RubyProf::PROCESS_TIME)
● wall time (RubyProf::WALL_TIME)
● cpu time (RubyProf::CPU_TIME)
● object allocations (RubyProf::ALLOCATIONS)
● memory usage (RubyProf::MEMORY)
● garbage collections runs (RubyProf::GC_RUNS)
● garbage collection time (RubyProf::GC_TIME)

26. perftools.rb
https://github.com/tmm1/perftools.rb
gem 'rack-perftools_profiler', :require => 'rack/perftools_profiler'

27. rack-perftools_profiler usage

28. KCacheGrind

29. Summary
● More Objects => Longer GC => Slow
● Examine your HEAP
● Use Tools

30. Q&A
http://dpaluy.github.com
@dpaluy
dpaluy@gmail.com
http://www.linkedin.com/in/davidpaluy