Experiences: Overcoming Data Transfer Bottlenecks across SUN-Transputer Interfaces

In many computationally-intensive tasks such as medical image reconstruction for which transputers can be used, the data communication rate between a display host such as a SUN-workstation and the transputer network becomes a constraint on system performance. Our goal is to maximise the SUN-transputer data exchange rate. Vy'e use several kinds of transputers connected to the VME-bus of the host SUN and this paper explores alternatives for designing their interfaces. We also use a VME-bus memory module memory-mapped to both the SUN and the transputers. Device drivers for the transputers connected by serial links must implement the required blocking semantics of the transputer links. There are two major designs for programmed i/o serial link interfaces-"polled" interfaces, and "interrupting" interfaces which work with a process scheduler to block the invoking process until the desired condition is true. W'e measured both kinds, expecting that the transputer hard link throughput would limit the performance. However, we found that the SUN's cpu cycles limit the performance of an Research funded by the Natural Science and Engineering Research Council ofCanada, under grants #8020 and #45395, and by the Sirnon Fraser University Center for Systems Science. @ Computing Systems, Vol. 5 ' No. 2 ' Spring 1992 159 unbuffered byte interface; a SUN4/110 is only capable of transferring 457,000 bytes/second. By reducing the software overhead in the SUN using a buffered FIFO block transfer we can increase throughput to 888,000 bytes/second. Now the bottleneck is the byte-wide VME-bus access mode used for the serial links. Throughput between the SUN and the transputers, using shared 32-bit wide VME-memory, is four times the throughput of the serial links. Hence, for optimum throughput, we use the transputer links for synchronisation of data which is exchanged in bulk using the shared memory. Further performance increases are only possible using different hardware interfaces which support the VME-bus block transfer mode.