Performance of Interlocked.Increment

Question

Is Interlocked.Increment(ref x) faster or slower than x++ for ints and longs on various platforms?

Answer 1

In our experience the InterlockedIncrement() et al on Windows are quite significant impacts. In one sample case we were able to eliminate the interlock and use ++/-- instead. This alone reduced run time from 140 seconds to 110 seconds. My analysis is that the interlock forces a memory roundtrip (otherwise how could other cores see it?). An L1 cache read/write is around 10 clock cycles, but a memory read/write more like 100.

In this sample case, I estimated the number of increment/decrement operations at about 1 billion. So on a 2Ghz CPU this is something like 5 seconds for the ++/--, and 50 seconds for the interlock. Spread the difference across several threads, and its close to 30 seconds.