Coupling U-Net with Virtual Memory Management
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           Anindya Basu, Matt Welsh and Thorsten von Eicken
                    Department of Computer Science
                          Cornell University

U-Net offers applications user-level access to the network such that the
network interface's DMA engine can transfer packet data directly to and from
application memory without compromising protection boundaries. Currently,
U-Net as well as similar approaches require the memory regions used for
communication to be pinned to physical memory in order to prevent the memory
manager from swapping out pages that are used in DMA transfers. This
approach is inflexible in that it requires data to be moved into and out of
this pinned "DMA window" for send and receive, and requires a considerable
amount of non-pageable memory per application using the network.

An new extension to the U-Net architecture (U-Net/MM) couples the operation
of the DMA engine with memory management to allow data to be transferred
directly to and from any region in an application's address space, thus
supporting true zero-copy messaging. The appropriate virtual-to-physical
mappings for DMA transfers are managed automatically to provide scalability
in the number of processes which can share the network interface.

U-Net/MM introduces a translation lookaside buffer (TLB) into the network
interface to translate virtual buffer addresses provided by the applications
to physical addresses used by the DMA engine. A new kernel module
communicates with the network interface to coordinate memory management
actions and, in particular, to provide page-translations to the network
interface when the TLB misses. The U-Net/MM architecture itself is
independent of any specific hardware platform — each of the components can
be implemented in hardware as well as software. 

The U-Net/MM architecture has been implemented using the FORE Systems PCA-200
PCI ATM network interface and Pentium workstations running the Linux
operating system. Preliminary results show performance comparable to that of
the original U-Net system with low overhead for TLB mappings and good
conservation of system memory under various job loads.

Additional information on the U-Net project can be obtained from
http://www.cs.cornell.edu/Info/Projects/U-Net/.