Are Objects Cheap?

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January 27, 2004

Are Objects Cheap?

I still don't have a handle on what objects cost. One .NET author, Jan Gray, calculated that the GC collects short-lived objects at a rate of 50 million objects per second. I am suspicious of that number; I think that rate can only be maintained with a small GC 0 heap, which the runtime sizes to fit into the cache. In another benchmark, another author calculated in high-allocation scenarios, that C# code often outperforms unmanaged C++; allocating an object in managed code is almost as fast as a stack allocation, involving in most cases a simple pointer increment.

I use SharpReader, an RSS reader that is written in managed code and consumes a large amount of memory--almost 200 MBs on a typical session. It's usually fairly responsive. Occasionally, after being active for several hours, the application becomes sluggish. I have wondered whether this was due to the garbage collector, or whether the cause is related to threading or network issues. The Performance Monitor (perfmon.exe) does not indicate any unusual GC activity.

Still, I am wary of using objects when a struct will do. The NLP group at Microsoft shares the same concern. When porting their natural language libraries from unmanaged code to managed code, they have deliberately distorted their API to favor structs and minimize the number of allocations under the view that avoiding the garbage collection will always be faster. For example, the TextFragment class encapsulates a stream of text that will undergo natural language analysis. The class has a Sentences property that returns a collection. When enumerating the collection through foreach, the collection reuses the same object and returns that same object after each call to IEnumerator.MoveNext() and IEnumerator.Current.

However, the Avalon team makes extremely liberal use of objects everywhere as if objects are free. Some developers even have the philosophy that structs have their own set of problems and should almost never be used. Then again, Avalon did rewrite the codebase to improve performance, but I doubt it had much to with the number of objects created.

I have started to rely more on reference types in place of value types, based on the Avalon example. My new thinking is that reference types are easier to work with. Later, I can look at the performance impact of using that reference type, and revert to a struct (or an interface) if need be.

To obtain the best of both worlds, I thought about writing a Perl script that takes a struct and produces a class or interface wrapper, which is more useful for referring to the value type in the heap. Of course, value types can be boxed too, but then the contexts of the object must be copied back to a value type for access to member functions.

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