I’m currently working on a ray-tracer in C# as a hobby project. I’m trying to achieve a decent rendering speed by implementing some tricks from a c++ implementation and have run into a spot of trouble.
The objects in the scenes which the ray-tracer renders are stored in a KdTree structure and the tree’s nodes are, in turn, stored in an array. The optimization I’m having problems with is while trying to fit as many tree nodes as possible into a cache line. One means of doing this is for nodes to contain a pointer to the left child node only. It is then implicit that the right child follows directly after the left one in the array.
The nodes are structs and during tree construction they are succesfully put into the array by a static memory manager class. When I begin to traverse the tree it, at first, seems to work just fine. Then at a point early in the rendering (about the same place each time), the left child pointer of the root node is suddenly pointing at a null pointer. I have come to the conclusion that the garbage collecter has moved the structs as the array lies on the heap.
I’ve tried several things to pin the addresses in memory but none of them seems to last for the entire application lifetime as I need. The ‘fixed’ keyword only seems to help during single method calls and declaring ‘fixed’ arrays can only be done on simple types which a node isn’t. Is there a good way to do this or am I just too far down the path of stuff C# wasn’t meant for.
Btw, changing to c++, while perhaps the better choice for a high performance program, is not an option.
Firstly, if you’re using C# normally, you can’t suddenly get a null reference due to the garbage collector moving stuff, because the garbage collector also updates all references, so you don’t need to worry about it moving stuff around.
You can pin things in memory but this may cause more problems than it solves. For one thing, it prevents the garbage collector from compacting memory properly, and may impact performance in that way.
One thing I would say from your post is that using structs may not help performance as you hope. C# fails to inline any method calls involving structs, and even though they’ve fixed this in their latest runtime beta, structs frequently don’t perform that well.
Personally, I would say C++ tricks like this don’t generally tend to carry over too well into C#. You may have to learn to let go a bit; there can be other more subtle ways to improve performance 😉