Question

I was attempting to use ObjectIDGenerator in C# to generate a unique ID during serialization, however, this class is not available in the XBox360 or Windows Phone 7 .NET frameworks (they use a compact version of .NET). I implemented a version using a dictionary of Object to Int64 and was able to get a fully working version up, however, the performance is unsatisfactory. I’m serializing on the order of tens of thousands of objects, and currently this is the greatest bottleneck in save/load performance. Using the actual .NET implementation on PC it takes about 0.3 seconds to serialize about 20,000 objects. Using my implementation, it takes about 6 seconds.

In profiling, I found that the heavy hitters were .TryGetValue and .Add on the dictionary (which makes sense since it’s both indexing and adding to the hash map). More importantly, the virtual equality operator was being called instead of simply comparing references, so I implemented an IEqualityComparer that only used ReferenceEquals (this resulted in a speed increase).

Does anyone have an insight into a better implementation of ObjectIDGenerator? Thanks for your help!

My Implementation: http://pastebin.com/H1skZwNK

[Edit]
Another note, the results of profiling says that the object comparison / ReferenceEquals is still the bottleneck, with a hit count of 43,000,000. I’m wondering if there’s a way to store data along side this object without having to look it up in a hash map…

Answer 1

Is it possible to use an Int32 Id property / handle for each object rather than Object? That may help things. It looks like you’re assigning an Id type number to each object anyway, only you’re then looking up based on the Object reference instead of the Id. Can you persist the object id (your Int64) within each object and make your dictionary into Dictionary<Int64, Object> instead?

You might also want to see if SortedDictionary<TKey, TValue> or SortedList<TKey, TValue> perform better or worse. But if your main bottleneck is in your IEqualityComparer, these might not help very much.

UPDATE

After looking at the ObjectIDGenerator class API, I can see why you can’t do what I advised at first; you’re creating the ids!

ObjectIDGenerator seems to be manually implementing its own hash table (it allocates an object[] and a parallel long[] and resizes them as objects are added). It also uses RuntimeHelpers.GetHashCode(Object) to calculate its hash rather than an IEqualityComparer which may be a big boost to your perf as its always calling Object.GetHashCode() and not doing the virtual call on the derived type (or the interface call in your case with IEqualityComparer).

You can see the source for yourself via the Microsoft Shared Source Initiative: