the cudaHostAlloc() API call has, among others, the flags:
- cudaHostAllocMapped: Maps the allocation into the CUDA address space. The device pointer to the memory may be obtained by calling cudaHostGetDevicePointer().
- cudaHostAllocWriteCombined: Allocates the memory as write-combined (WC). WC memory can be transferred across the PCI Express bus more quickly on some system configurations, but cannot be read efficiently by most CPUs. WC memory is a good option for buffers that will be written by the CPU and read by the device via mapped pinned memory or host->device transfers.
I could quite understand when exactly I would prefer the "write-combined" option. I mean, it didn't say the transfer may be faster just in one direction, so why do they only recommend it for one direction? Also, which kind of systems benefit from this "write-combining"?
I read this white paper, Section 4.7, and still could not get it. Ok, so reading by the CPU is inefficient; but what if other benefits offset this inefficiency? Or - if they cannot, why can't they?
An elucidation would be appreciated.