Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!lll-winken!lll-lcc!ames!mailrus!uflorida!gatech!hubcap!wen-king From: wen-king@cit-vax.Caltech.Edu (King Su) Newsgroups: comp.parallel Subject: Re: Hmm! Keywords: HyperCube, Parallel Connection Schemes Message-ID: <3386@hubcap.UUCP> Date: 31 Oct 88 13:16:59 GMT Article-I.D.: hubcap.3386 Sender: fpst@hubcap.UUCP Lines: 30 Approved: parallel@hubcap.clemson.edu In article <3363@hubcap.UUCP> vrdxhq!ogccse.ogc.edu!pase@uunet.UU.NET (Douglas M. Pase) writes: >Another nice thing about hypercubes is that a large number of useful topologies slick feature is that a number of applications, such as FFTs, matrix ops, and are used with high efficiency (multihop messages need not occur). I do not understand how you equate the lack of multi-hop messages with high efficiency. When you map a 16x16 mesh onto an 8-D cube, you cannot possibly get more than 50% channel utilization because 4 of the 8 channels in each node are idle -- expansive hardwares that you paid for but can't use. Even if you can make use of all of the channels by mapping cube graph to cube machine, the node memory bus will not be able to spew out data fast enough to keep all of the channels busy all the time. Even in the original iPSC/1, the memory bus becomes saturated when more than 3 or 4 of the channel chips are doing DMA at the same time (in a 7-D iPSC, there are 14 channels, I+O). Today's hardware routers are about 20-30 times faster than iPSC/1's channel. Process graph mapping has been an important issue because each additional hop takes too long, not that the channels are getting saturated. Wormhole routing is one way to reduce that overhead. When each additional hop adds only 20-30 nanoseconds, process placement is much less important. Hot spots that are introduced by the communication pattern of the processes can generally be reduced by _RANDOMIZE_ the process placement, something that is un-thinkable when hop time is long. -- /*------------------------------------------------------------------------*\ | Wen-King Su wen-king@vlsi.caltech.edu Caltech Corp of Cosmic Engineers | \*------------------------------------------------------------------------*/