Realm-cocoa: Read / write performance on models with a to-many relationship

[lcharlick]

I noticed one of my performance test cases failing since adding a to-many relationship to one of my models:

let tags = List<Tag>()

Even when not populated this seems to reduce read / write performance by ~200%. Is this expected behaviour?

Goals

What do you want to achieve?

Quickly read from a model that contains a to-many relationship

Expected Results

What did you expected to happen?

Results are returned in a similar time to a model without a relationship

Actual Results

What did happened instead?

Results are returned ~200% slower

Steps to Reproduce

Add a to-many relationship to a model:

let tags = List<Tag>()

Code Sample

Here's a minimal test case:

import XCTest
import RealmSwift

class A: Object {
  dynamic var id: Int = 0
//  let children = List<B>()
}

class B: Object {}

class RealmPerformanceTests: XCTestCase {
  var realm: Realm!

  override func setUp() {
    super.setUp()

    var config = Realm.Configuration()
    config.inMemoryIdentifier = self.name
    self.realm = try! Realm(configuration: config)
  }

  func writeObjects(count: Int) {
    self.realm.add((0...count).map { A(value: ["id": $0]) })
  }

  func testWritePerformance() {
    self.measure {
      try! self.realm.write {
        self.writeObjects(count: 100000)
      }
    }
  }

  func testReadPerformance() {
    try! self.realm.write {
      self.writeObjects(count: 100000)
    }

    self.measure {
      var objects = [A]()
      for object in self.realm.allObjects(ofType: A.self) {
        objects.append(object)
      }
    }
  }
}

Results:

testWritePerformance: 2.188 seconds
testReadPerformance: 0.263 seconds

Uncommenting the relationship line and re-running the tests gives:

testWritePerformance: 6.292 seconds
testReadPerformance: 0.863 seconds

Version of Realm and Tooling

ProductName:    Mac OS X
ProductVersion: 10.12
BuildVersion:   16A304a

/Applications/Xcode-beta6.app/Contents/Developer
Xcode 8.0
Build version 8S201h

/usr/local/bin/pod
1.1.0.beta.1
Realm (1.0.2)
RealmSwift (1.0.2)
Realm (from `https//github.com/realm/realm-cocoa.git`, branch `master`)
RealmSwift (from `https//github.com/realm/realm-cocoa.git`, branch `master`)

/bin/bash
GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin16)

carthage not found
(not in use here)

/usr/local/bin/git
git version 2.9.3

Thanks!

[tgoyne]

There is unfortunately a lot of unavoidable overhead. The most expensive part of reading an object from a Realm is just allocating the memory for the object, and adding a List property adds a few more allocations that can't be done lazily. That said, I think currently each List property requires three eager allocations, and two of them could probably be deferred until the property is actually used.