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Using ThreadStatic to replace expensive locals -- good idea?

Asked 2011-02-01T16:11:54.153
15

Update: as I should have expected, the community's sound advice in response to this question was to "measure it and see." chibacity posted an answer with some really nice tests that did this for me; meanwhile, I wrote a test of my own; and the performance difference I saw was actually so huge that I felt compelled to write a blog post about it.

However, I should also acknowledge Hans's explanation that the ThreadStatic attribute is indeed not free and in fact relies on a CLR helper method to work its magic. This makes it far from obvious whether it would be an appropriate optimization to apply in any arbitrary case.

The good news for me is that, in my case, it seems to have made a big improvement.

I have a method which (among many other things) instantiates some medium-size arrays (~50 elements) for a few local variables.

After some profiling I've identified this method as something of a performance bottleneck. It isn't that the method takes an extremely long time to call; rather, it is simply called many times, very quickly (hundreds of thousands to millions of times in a session, which will be several hours). So even relatively small improvements to its performance should be worthwhile.

It occurred to me that maybe instead of allocating a new array on each call, I could use fields marked [ThreadStatic]; whenever the method is called, it will check if the field is initialized on the current thread, and if not, initialize it. From that point on all calls on the same th

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2 Answers

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[ThreadStatic] is no free lunch. Every access to the variable needs to go through a helper function in the CLR (JIT_GetThreadFieldAddr_Primitive/Objref) instead of being compiled inline by the jitter. It also isn't a true substitute for a local variable, recursion is going to byte. You really have to profile this yourself, guesstimating perf with that much CLR code in the loop isn't feasible.

answered 2011-02-01T17:22:09.133
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From results like this, ThreadStatic looks pretty fast. I'm not sure that anybody has a specific answer to if it's faster then reallocating a 50 element array though. That's the kind of thing you'll have to benchmark yourself. :)

I'm somewhat torn on if it's a "good idea" or not. So long as all the implementation details are kept inside the class it's not necessarily a bad idea (you really don't want the caller to have to worry about it), but unless benchmarks showed a performance gain from this method I would stick to simply allocating the array each time because it makes the code simpler and easier to read. As the more complicated of the two solutions, I'd need to see some benefit from the complexity before choosing this one.

answered 2011-02-01T17:00:34.457

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