A study of virtual memory MTU reassembly within the PowerPC architecture

Message transfer unit (MTU) reassembly schemes in modern operating systems cause I/O performance degradation when MTU sizes are larger than the architecture's page size. This can happen with emerging network technologies, such as Asynchronous Transfer Mode (ATM), where MTUs can be 64 KB or greater Traditional solutions either reassemble using memory copy or preallocate contiguous memory; these, however lack speed or consume excess resources, respectively. This paper presents an alternative scheme called Virtual Memory MTU Reassembly (VMMR) which reassembles non-contiguous pages through virtual memory remapping. VMMR allows hardware/software interfaces to efficiently DMA large MTUs in hardware pages and remap them to a contiguous address space. Studies done on a PowerPC 601 show that this method can outperform memcopy by one to two orders of magnitude (the maximum VMMR bandwidth is 14.7 Gbits/sec). High-performance multimedia applications, such as video on demand and video conferencing, can greatly benefit from such a performance boost.