Caching,
Memcached,
and Rails
Mike Perham
http://mikeperham.com
Wednesday, March 25, 2009

Me
Maintainer of memcache-client, shipped with Rails
Hardcore performance nerd
Wednesday, March 25, 2009

Performance
Let me Tell you a Story...
Wednesday, March 25, 2009

Performance Tuning
Remove unnecessary Functionality
Tune necessary Functionality
ONLY THEN, add caching last
ALWAYS track before/after timings
Wednesday, March 25, 2009

Caching
Cache the results of expensive operations
Why is this memoization dumb?
Wednesday, March 25, 2009

Why Dumb?
SCALABILITY!
FRESHNESS!
Wednesday, March 25, 2009

Rails
Built in support for memcached since Rails 2.1
config/environments/production.rb:
Wednesday, March 25, 2009

Request
Varnish Apache Rails DB
Memcached
Standard Toolset for Rails 2.3
Further == Slower
Wednesday, March 25, 2009

Caching Mechanisms
HTTP Proxy
FASTER
Page
Action
Fragment
SLOWER
Object
Wednesday, March 25, 2009

HTTP Proxy
HTTP/URL caching with Varnish
Expires, Etag, Cache-Control headers
5-10k req/sec!
Wednesday, March 25, 2009

Page Caching
ERB --> Static HTML
Uh oh, uses the filesystem!
Apache = 1-5k req/sec
Wednesday, March 25, 2009

Action Caching
Runs filters (e.g. authorization)
Can use any cache strategy
File, memory, memcached
Wednesday, March 25, 2009

Fragment Caching
Parts of the page are unique per user
Fragment cache the various UI boxes
Dynamically assemble HTML from the cache
Wednesday, March 25, 2009

Object Caching
Built-in AR support for “smart” keys
Rails.cache.fetch(post.cache_key) { expensive_calc(post) }
“#{table_name}/#{self.id}-#{self.updated_at.to_i}”
Support for complex dependencies
Wednesday, March 25, 2009

Expiration
“There are only two hard things in Computer
Science: naming and cache invalidation”
Explicit
expire_page :controller => ‘users’, :action => ‘index’
expire_action :controller => ‘users’, :action => ‘index’
Rails.cache.delete(‘key’)
Observers
Wednesday, March 25, 2009

Expiration (Cont’d)
Time-based
Rails.cache.fetch(‘key’, :expires_in => 1.hour) { ... }
expires_in(2.hours) # sets page’s HTTP max-age attribute
Versioning
Rails.cache.fetch(user.cache_key) { ... }
Wednesday, March 25, 2009

Memcached
Giant Hash of Fixed Size
Time-based expiration
LRU eviction
Operations: GET, SET, APPEND, INCR, CAS
Limited to 1MB per value, 250 chars per key
Wednesday, March 25, 2009

Facebook
100s of servers w/ 28TB of RAM
5 billion page views/day
400k req/sec from memcached
Wednesday, March 25, 2009

Performance
Wednesday, March 25, 2009

Mongrel + Nginx
Varnish
Nginx
WEB1
M1 M2
M1 M2
Memcached Memcached
APP1 APP2
Wednesday, March 25, 2009

Apache + Passenger
Varnish
HAProxy
WEB1
Memcached Memcached
Apache Apache
Rails Rails Rails Rails
APP1 APP2
Wednesday, March 25, 2009

Advanced
Spread your cache across a farm of memcached
Use the memcaches_page plugin
Use AJAX for user-specific widgets
Wednesday, March 25, 2009

Advice
Cache late in the development cycle, after tuning!
Memcached gives you time-based expiration,
versioning and multi-machine support.
Cache as early in the request as possible!
Varnish Apache Rails DB
Wednesday, March 25, 2009

THANK YOU!
HTTP://MIKEPERHAM.COM
MPERHAM@GMAIL.COM
QUESTIONS?
Wednesday, March 25, 2009