Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!RICHTER.MIT.EDU!krowitz
From: krowitz@RICHTER.MIT.EDU (David Krowitz)
Newsgroups: comp.sys.apollo
Subject: Re:  domain protocol over twisted pair
Message-ID: <8908012221.AA05657@richter.mit.edu>
Date: 1 Aug 89 22:21:52 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 40

A few comments ...

1) MIT's telecommunications office runs the campus-wide ethernet system
   which our Apollo ringnet is attached to via a DN660 gateway. The
   telecom office states that the maximum throughput of an ethernet
   segment is 3 MB/sec on a 10 Mhz thick ethernet cable. This is due
   to packet collisions eating up network bandwith. Thus, the maximum
   capacity of our building ethernet (a thick cable network) is 1/4 of
   the capacity of our 12 Mhz ringnet. (please excuse my typo ... ethernet
   throughput is 3 Mhz/sec). Apollo talking to each other over native
   ethernet connections will see a lot of collisions on a heavily loaded
   network. Apollos on native ringnets which are attached to each other
   via an ethernet backbone will also see a bottleneck.

2) Network throughput is directly proportional to the number of gateways
   seperating the nodes. Since the gateways are Apollo workstations and
   not dedicated hardwired boxes, the time lag to pass a packet from the
   ringnet interface to the ethernet interface is considerable. This is
   why diskless node service, /com/lcnode, /com/lusr, and other time
   dependent network services only work on your local network even if
   internet routing has been enabled.

3) Paging from diskless nodes and remote file access can eat up network
   bandwith in a hurry. Try running /com/dspst -a sometime while your
   node is copying a file to another node's disk. The number of paging
   requests per second can easily exceed 100/sec. If we assume a page
   size of 1024 bytes, 8 bits/byte, and 100 pages/sec we see that a
   single node can consume 0.8 Mhz of bandwith -- almost 25% of the
   usable bandwith of an ethernet! Now try copying two or three files
   at once (in seperate windows). You can easily get a single node to
   generate 200 or 300 paging requests per second!!


 -- David Krowitz

krowitz@richter.mit.edu   (18.83.0.109)
krowitz%richter@eddie.mit.edu
krowitz%richter@athena.mit.edu
krowitz%richter.mit.edu@mitvma.bitnet
(in order of decreasing preference)