I have a PCIe device BAR mapped to a particular region in KVA. I use memcpy
to write into that region and measure the transfer rate. Recently, I was
able to increase the transfer rate by setting the memory attribute of the
BAR mapped region to VM_MEMATTR_WRITE_COMBINING. However, the transfer rate
fluctuates wildly and I cannot get a deterministic value. What could be the
cause for this?