Question

Consider this in C#:

int[] a = new int[1024];

All integers will be set to 0. Traditionally, this is done by a highly optimized zero-fill operation (e.g. memset). However, as this array is allocated by the CLR I can think of an optimization which is pretty straight-forward; the CLR could maintain a zeroed memory area to skip this zero-filling at the time we request our memory.

Now, does CLR perform such an optimization? (I’m currently using .NET 4.0)

Answer 1

This operates at a much lower level. Windows keeps a low priority kernel thread alive whose only job is zero-ing the content of memory pages. Called the “zero page thread”. Those pages are kept in a pool, ready for use to any process that generates a page fault for reserved but not committed pages. Any code in Windows benefits from this, not just managed code. The intention is security, not zero-ing the RAM content of mapped memory pages would allow a program to spy on the memory of another process.

This won’t happen with your array, it is too small. It gets allocated in the gen #0 heap, a heap that will always have mapped pages. Large arrays however get allocated in the Large Object Heap, large is 85,000 bytes, 8000 bytes for an array of double. LOH allocations can take advantage of getting the pages pre-initialized to zero. Whether it actually does is hard to tell, the source code for that isn’t available anywhere. I’d say it is likely considering the amount of cpu cycles it saves.