Common performance issues

rubyonrails-talk
1

Hi,

When writing a Rails app, what common tasks can likely result in performance issues?

The one I'm aware of is something like: Model.find(:all).each do |elem|   # do stuff end

Especially when there's lots of elements in the Model. What other ones are there?

Also, does Rails do caching of queries? If I'm in a view and I do: <% if user.admin? %> ...

and then later on in that same view, I have the same code again, does that execute two queries?

Joe

2

Another one: What's a faster way to write:

class User < AR   has_many :comments   def number_of_comments     comments.size   end end

(if I don't want to do caching). I assume that calling number_of_comments creates AR objects for each comment.

3

Joe-

You may find it useful to use “tail” to monitor the file development.log while you are developing - each SQL query that Rails executes will show up there. I believe (though you should double check this) that calling comments.size when comments has not been loaded will do a single “select count(*)…” query without instantiating a bunch of Comment objects.

-Steve Holder

4

Hi

the ruby Net::HTTP library allows calls like:

    Net::HTTP.get(url.path)

end similar looking post calls, Since the RESTful support uses DELETE and PUT, i was wondering how to make ruby calls like the above Net::HTTP of the kind DELETE and PUT?

Thanks in advance! Gustav

5

Steve Holder wrote:

Joe-

You may find it useful to use "tail" to monitor the file development.logwhile you are developing - each SQL query that Rails executes will show up there. I believe (though you should double check this) that calling comments.size when comments has not been loaded will do a single "select count(*)..." query without instantiating a bunch of Comment objects.

I thought that, when in development mode, Rails generates different SQL (i.e. more SQL queries) than when in production mode.

Is this not the case?

Joe

6

Definitely not.

Regarding your original question, have a look at the :counter_cache option for associations.

Michael

7

joevandyk@gmail.com wrote:

Hi,

When writing a Rails app, what common tasks can likely result in performance issues?

I found this article: A Look at Common Performance Problems in Rails

May be useful to others out there.

In my applications, the logs seem to indicate that the "rendering" part, and not the "database" part, is the bottleneck. Is ActiveRecord work included in the database figure?

Another one: I've noticed that, if I have a couple thousand <%= ... %> on a page, it slows down considerably. Would it be worth a try to build up strings of html using <% str += "more" %> and then <%= str %>?

Joe

8

if rendering is your bottleneck, you should check this out: http://www.railsexpress.de/blog/articles/2006/08/15/rails-template-optimizer-beta-test

ed

9

joevandyk wrote:

When writing a Rails app, what common tasks can likely result in performance issues?

I noticed in the Log folder that many SELECT statements had LIMIT 1 on the end.

We all know that's a no-no when you are _architecting_ a database. It's mostly harmless when _using_ it...

...however, I would have relied on assertions demanding that all select statements that only need 1 row return will only get 1 row. Without LIMIT 1. That would imply that the database is sufficiently normal, and the SELECT statements are sufficiently constrained.

So, in general, the ideal might be to transfer limit-checking to the tests, so the code can run faster without it, and with proper constraints.

(The spectre of SQL data normalization and query optimization also raises it's ugly head. We might presume that the slowest DB operation is still faster than the fastest Ruby page rendering cycle, no? :wink: