`copy_data` memory bloat in v1.4

[davidtaylorhq]

We use Connection#copy_data to stream large volumes of data into a temporary table. We recently observed significant performance degradation and increased memory use for this system. Here's a minimal reproduction:

require "securerandom"
require "objspace"
require "bundler/inline"

PG_VERSION = "1.4.2"
gemfile do
  source 'https://rubygems.org'
  gem 'pg', PG_VERSION
end

puts "PG::version #{PG::VERSION}"

def memory_use
  3.times { GC.start }
  objspace_size_mb = ObjectSpace.memsize_of_all / 1024 / 1024
  rss_mb = `ps  -p #{Process.pid} -o rss`.split("\n")[1].to_i / 1024
  "objspace:#{objspace_size_mb}mb; rss:#{rss_mb}mb"
end

puts "Before: #{memory_use}"

start_at = Time.now()

connection = PG.connect(dbname: 'discourse_development')
table_name = "my_temp_table"
connection.exec("CREATE TEMP TABLE #{table_name}(url text UNIQUE)")
connection.copy_data("COPY #{table_name} FROM STDIN CSV") do
  1_000_000.times do |i|
    connection.put_copy_data("#{SecureRandom.hex(100)}\n")
  end
  puts "After loop, inside copy_data: #{memory_use}"
end

puts "After: #{memory_use}"
puts "Took #{Time.now - start_at}s"

With version 1.3.5, this script takes ~10s on my machine, and reports ~47mb RSS at the end. With version 1.4.0 (and 1.4.1, 1.4.2), it takes ~80s and reports ~182mb RSS at the end. The RSS appears to scale with the amount of data being copied.

[larskanis]

I can reproduce this issue. Not that dramatic as you measured, but also measurable. The root cause is #462. I made it when I noticed that put_copy_data is quite slow on Windows. Unfortunately it can result in a memory bloat when put_copy_data is called with more data than what actually can be transferred over the wire.

My proposal is to fix it like #474. Not ideal, but I think the only practical trade-off we can do.

CC @SamSaffron

[davidtaylorhq]

Thanks for the quick fix @larskanis - #474 does indeed fix the issue in my minimal repro script ❤️

it can result in a memory bloat when put_copy_data is called with more data than what actually can be transferred over the wire.

Aha I see! Indeed, adding a sleep 60 to the end of the repro script and then measuring again shows that the memory drops back to normal levels. 👍