When a process uses more memory than expected, you need to know which method is responsible. ObjectSpace.memsize_of_all gives you a before/after delta.

The Pattern

require 'objspace'

GC.start
before = ObjectSpace.memsize_of_all
# ... run method here ...
after = ObjectSpace.memsize_of_all
puts "mem delta: #{((after - before) / 1024.0 / 1024.0).round(2)} MB"

GC.start forces a garbage collection cycle before measuring so prior allocations don’t skew the result.

A Real Example

Two approaches to build a list of 50,000 user records. Save as measure_memory.rb:

require 'objspace'

# old approach - full AR objects with map
GC.start
before = ObjectSpace.memsize_of_all
scope = User.where.not(confirmed_at: nil).order(:id).limit(50000).offset(0)
out1 = scope.map { |u| { email: u.email, fields: { first_name: u.first_name.to_s } } }
after = ObjectSpace.memsize_of_all
puts "map (old): #{out1.size} records, mem delta: #{((after - before) / 1024.0 / 1024.0).round(2)} MB"

# new approach - pluck
GC.start
before = ObjectSpace.memsize_of_all
scope = User.where.not(confirmed_at: nil).order(:id).limit(50000).offset(0)
out2 = scope.pluck(:email, :first_name).map { |email, first_name| { email: email, fields: { first_name: first_name.to_s } } }
after = ObjectSpace.memsize_of_all
puts "pluck (new): #{out2.size} records, mem delta: #{((after - before) / 1024.0 / 1024.0).round(2)} MB"

Then run:

rails runner measure_memory.rb

Result:

map (old):    50000 records, mem delta: 681.47 MB
pluck (new):  50000 records, mem delta:  30.13 MB

map instantiates full ActiveRecord objects — each one carries attribute hashes, dirty tracking, callbacks, and association caches. pluck returns raw arrays with only the columns you asked for.

681 MB versus 30 MB. The fix is 22x more efficient.

Why the Gap is so Large

map instantiates full ActiveRecord objects — each carries all columns, dirty tracking, callbacks, and association caches — then extracts just two fields. pluck issues a SELECT email, first_name and returns raw arrays with no AR overhead.

Use pluck when you only need a few columns and don’t need AR callbacks or associations. Use map (or find_each) when you need the full object.

References

• ObjectSpace
• ObjectSpace.memsize_of_all
• GC
• GC.start