Constant GC allocations when sending large messages to threads?

[cc]

Given the sample program at https://pastebin.com/u9sSNtj7
I'm experiencing GC allocations with every call to 
std.concurrency.send when sending larger messages (e.g. multiple 
ulongs).  These do not occur when sending uints in comparison, in 
the provided example.

For example, when the ManyAllocations version is set, I see 
results like:
allocations: 100  bytes: 3280

When commented out, I see:
allocations: 1  bytes: 80

Is there a way to mitigate this memory usage?

Using DMD32 D Compiler v2.089.1-dirty on Windows 10 x64
cmdline: rdmd.exe -m64