Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!uunet!munnari.oz.au!labtam!graeme
From: graeme@labtam.labtam.oz (Graeme Gill)
Newsgroups: comp.arch
Subject: Re: processor for graphics terminal [was: PC/AT clones with RISC cpu]
Summary: PC Xterminals are rather slooowww .. here's why
Message-ID: <5530@labtam.labtam.oz>
Date: 5 Nov 90 01:11:11 GMT
References: <2081@aber-cs.UUCP> <0093F0E4.0B02A980@KING.ENG.UMD.EDU> <1990Nov3.235115.21250@zoo.toronto.edu>
Organization: Labtam Information Systems Pty. Ltd., Melbourne, Australia
Lines: 41

In article <1990Nov3.235115.21250@zoo.toronto.edu>, henry@zoo.toronto.edu (Henry Spencer) writes:
> In article <0093F295.10626840@KING.ENG.UMD.EDU> sysmgr@KING.ENG.UMD.EDU (Doug Mohney) writes:
> >People will want X-terms and people will want PC-compatability...
> 
> By this reasoning, nobody should be building anything except PC-compatible
> X terminals.  Strangely, many manufacturers manage to make money building
> machines that are neither.

	From our experience I can say that Xterminal performance is limited by
by three things:
	
	1) For small operations, network bandwidth is the ultimate limit.
The x11perf poly point performance multiplied by 4 is usually a good estimate
of the available network bandwidth in bytes per second.

	2) For large areas, memory write speed is the ultimate limit. To get
good speed you need to take advantage of 32 bit access, burst writes, 
interleaved memory and any special write modes Vrams support.

	3) Between 1) and 2), performance depends on cpu speed. Small operations
area also fairly sensitive to instruction cache size, function call 
overhead and the working register set size. The more registers the processor
has the better, since graphics code is notorious for having large numbers
of function parameters, and a large number of working variables.

	Given the above considerations, the current PC/AT clones do not
measure up well. Their network interfaces are generally based on 16 bit
controller cards which shuffle data via small static ram buffers, limiting
network performance to (typically) 50 - 200 Kbytes/sec. This compares to
600 - 800 Kbytes/sec for a purpose built design. The common video cards
are accessible only 8 or 16 bits at a time, over the slow PC bus, and
may involve bank switching and plane wise access to pixels. Neither 286
nor 386/486 processors support burst write to memory.
	All these problems are reflected in the performance quoted for PC
X-terminal emulation - generally 10 to 100 times slower than purpose
built Xterminals and workstations. Hence the market for Xterminals -
workstation performance graphics on your desktop at a lower price.

	Graeme Gill
	Labtam I.S.D. Pty Ltd
	graeme@labtam.oz.au