Slides from meetup.com/madruby on tweaking Ruby GC settings. by Gleb Mazovetskiy (2010)

1. Ruby Performance
scaling your Ruby web app
Part 1: Garbage collection

2. MRI/REE Facts
• Mark & sweep GC
• freelists
• very expensive
• All refs are on the heap
• All code is on the heap

3. GC Settings
RUBY_HEAP_MIN_SLOTS 10k
RUBY_HEAP_SLOTS_GROWTH_FACTOR 1.8
Defaults
startup
+ 20k
+ 10k
+ 18k
...
10k
10k + 10k
10k + 10k + 18k
10k + 10k + 18k + 32.4k
...
RUBY_HEAP_SLOTS_INCREMENT 10k
10 000
20 000
38 000
70 400
128 720

4. A better strategy
• Stabilize on 1-2 heaps
RUBY_HEAP_MIN_SLOTS (480 000)
• Linear increment to keep it tight
RUBY_HEAP_SLOTS_GROWTH_FACTOR 1
RUBY_HEAP_SLOTS_INCREMENT (20 000)

5. Some tools
memprof, gdb, scrap
[68.63 MB] GET /stats/scrap
Number of objects : 815266 (627150 AST nodes, 76.93%)
Heap slot size : 20
GC cycles so far : 352
Number of heaps : 7
Total size of objects: 15923.16 KB
Total size of heaps : 18036.55 KB (2113.39 KB = 11.72%
unused)
Leading free slots : 18545 (362.21 KB = 2.01%)
Trailing free slots : 0 (0.00 KB = 0.00%)
Number of contiguous groups of 16 slots: 2685 (4.65%)
Number of terminal objects: 4697 (0.51%)

6. Thanks
Gleb Mazovetskiy. github.com/glebm