Kaminari | Closer to Code

Refactoring can be sometimes a pain in the ass. Especially when you're refactoring things that are really old. Today I was cleaning an almost 5 years old piece of code. It was a Rails 4.0 (migrated many times from 1.2) app that had helpers auto-loading turned on. I hated this, because it forced us to use names with prefixes for many methods.

Disabling it was really easy (in config/application.rb):

config.action_controller.include_all_helpers = false

Of course after that you need to require all the helpers that you use:

helper ApplicationHelper
helper FormHelper

Everything was fine except one small detail:

ActionView::Template::Error: undefined method `paginate' for #<#<Class:0x0000000929a3c0>:0x0000000aa2c5b0>
    app/views/admin/module/urls/index.html.haml:1:in `_app_views_admin_module_urls_index_html_haml__883223636925488268_77061720'
    app/controllers/application_controller.rb:57:in `respond_with'
    app/controllers/admin/module/urls_controller.rb:9:in `index'
    test/controllers/admin/module/urls_controller_test.rb:103:in `block in <class:UrlsControllerTest>'

Kaminari stopped working. It seems that it was loaded without issues, except the helper. To fix this, just include in you application controller following code:

helper KaminariHelper

Where is my memory?

Recently, when browsing large dataset from MongoDB using Padrino and Thin, Ruby started to have memory leaks. After each request it grew approximately 2-5 MB.

I've started debugging by putting following line in my action, to see memory usage increase per request:

puts 'RAM USAGE: ' + `pmap #{Process.pid} | tail -1`[10,40].strip

Results:

RAM USAGE: 796156K
RAM USAGE: 798284K
RAM USAGE: 798824K
RAM USAGE: 799088K
RAM USAGE: 799900K
RAM USAGE: 799900K
RAM USAGE: 812044K
RAM USAGE: 816152K
RAM USAGE: 816292K
RAM USAGE: 816836K
RAM USAGE: 818956K
RAM USAGE: 819088K
RAM USAGE: 830572K
RAM USAGE: 884604K
RAM USAGE: 887648K
RAM USAGE: 892800K
RAM USAGE: 897160K
RAM USAGE: 906960K

As you can see it grows rapidly. When looking at htop things get even worse:

88,4 MB
93,9 MB
97,5 MB
99,2 MB
109,4 MB
113,4 MB
122,7 MB
127,1 MB
...
1,2 GB!

It was definitely too much! Memory consumption reached it's limits and everything slowed down.

I knew that it had something to do with this line:

@analyses = Analysis.finished.page(params[:page] ||= 1).per(10)

Kaminari?

At the beginning I've suspected Kaminari and its pagination engine, however it is just a more complex layer covering some scopes. To check this I've removed Kaminari:

@analyses = Analysis.finished.skip(((params[:page] ||= 1)-1)*10).limit(10)

Unfortunately nothing good happened and memory consumption kept growing with same speed. Interesting is that, when I've turned off all MongoDB indexes:

db.collection1.dropIndexes()
db.collection2.dropIndexes()
...
db.collectionN.dropIndexes()

memory usage grew much slower than before. So WTF?

Identity map!

Finally I've discovered damn source of my problem. It was identity map in Mongoid. What is identity map?

The identity map in Mongoid is a current aid to assist with excessive database queries in relations, and is necessary for eager loading to work. (...) When a document is now loaded from the database, is is automatically added to the identity map by it's class and id. Subsequent request for that document by it's id will not hit the database, but rather pull the document back from the identity map itself. It's primary function in this capacity is to aid in cutting down queries for belongs_to relations when iterating over the parents.

Seems like identity map was never cleared (or it has a memory leak bug in it). Adding:

use Rack::Mongoid::Middleware::IdentityMap

didn't help at all so I've just turned identity map off:

mongo.identity_map_enabled = false

and everything went back to normal. Interesting thing is that identity map in ActiveRecord is by default turned off in Rails because it's known to cause similar problems.