---
title: "Rails 6 reports object allocations while rendering"
description:
  "Rails 6 reports object allocations made while rendering view templates"
canonical_url: "https://www.bigbinary.com/blog/rails-6-reports-object-allocations-made-while-rendering-view-templates"
markdown_url: "https://www.bigbinary.com/blog/rails-6-reports-object-allocations-made-while-rendering-view-templates.md"
---

# Rails 6 reports object allocations while rendering

Rails 6 reports object allocations made while rendering view templates

- Author: Vishal Telangre
- Published: July 23, 2019
- Categories: Rails 6, Rails

Recently, Rails 6
[added `allocations` feature](https://github.com/rails/rails/pull/33449) to
`ActiveSupport::Notifications::Event`. Using this feature, an event subscriber
can see how many number of objects were allocated during the event's start time
and end time. We have written in detail about this feature
[here](https://blog.bigbinary.com/2019/04/24/rails-6-adds-cpu-time-idle-time-and-allocations-to-activesupport-notifications-event.html).

By taking the benefit of this feature, Rails 6 now reports the allocations made
while rendering a view template, a partial and a collection.

```plaintext
Started GET "/articles" for ::1 at 2019-04-15 17:24:09 +0530
Processing by ArticlesController#index as HTML
Rendering articles/index.html.erb within layouts/application
Rendered shared/\_ad_banner.html.erb (Duration: 0.1ms | Allocations: 6)
Article Load (1.3ms) SELECT "articles".\* FROM "articles"
↳ app/views/articles/index.html.erb:5
Rendered collection of articles/\_article.html.erb [100 times] (Duration: 6.1ms | Allocations: 805)
Rendered articles/index.html.erb within layouts/application (Duration: 17.6ms | Allocations: 3901)
Completed 200 OK in 86ms (Views: 83.6ms | ActiveRecord: 1.3ms | Allocations: 29347)
```

Notice the `Allocations: ` information in the above logs.

We can see that

- 6 objects were allocated while rendering `shared/_ad_banner.html.erb` view
  partial,
- 805 objects were allocated while rendering a collection of 100
  `articles/_article.html.erb` view partials,
- and 3901 objects were allocated while rendering `articles/index.html.erb` view
  template.

We can use this information to understand how much time was spent while
rendering a view template and how many objects were allocated in the process'
memory between the time when that view template had started rendering and the
time when that view template had finished rendering.

To learn more about this feature, please check
[rails/rails#34136](https://github.com/rails/rails/pull/34136).

Note that we can also collect this information by subscribing to
[Action View hooks](https://edgeguides.rubyonrails.org/active_support_instrumentation.html#action-view).

```ruby
ActiveSupport::Notifications.subscribe /^render\_.+.action_view\$/ do |event|
views_path = Rails.root.join("app/views/").to_s
template_identifier = event.payload[:identifier]
template_name = template_identifier.sub(views_path, "")
message = "[#{event.name}] #{template_name} (Allocations: #{event.allocations})"

ViewAllocationsLogger.log(message)
end
```

This should log something like this.

```plaintext
[render_partial.action_view] shared/\_ad_banner.html.erb (Allocations: 43)

[render_collection.action_view] articles/\_article.html.erb (Allocations: 842)

[render_template.action_view] articles/index.html.erb (Allocations: 4108)
```

## Links

- [Human page](https://www.bigbinary.com/blog/rails-6-reports-object-allocations-made-while-rendering-view-templates)