Mainframes vs PCs (was Re: discrete transistors)

From: William Donzelli <william_at_ans.net>
Date: Wed Oct 21 08:27:19 1998

> Ok folks, its true that mainframes typically have had better I/O bandwidth
> to their disk farms but that has changed. The PCI bus can and does sustain
> greater than 100 Mb/s over it. The "old" PCI bus spec had it at 33Mhz *
> 4bytes/write for 120Mbs and a properly designed PC can run the bus at 66Mhz
> these days giving an opportunity for 240Mbs. AGP does somewhat better.

240 Mbps is dead slow for todays big iron. Most S/390s today have
boatloads of fibrechannel on them - the old I/O channel architecture has
been replaced.

> The Crays and ConnectionMachines have, in the past, had the advantage of
> being vector processors where typical mainframes were often SIMD machines
> at best and simple pipelines at worst. Microprocessors caught up with the
> SIMD wave with multi-ALU pipelining, and with the Katmai and AMD-K7 they
> will get many of the vector features that made so-called "super computers"
> so fast.

Mainframes have never been designed for screaming CPU performance. In
fact, if you quote something in "Mips" to a hardcore mainframe guy, he is
going to look at you funny. Its all in the I/O.

The upcoming vector features in the new chips really is nothing for the
super people to get excited about. The features just are not that good -
they will get choked on large vectors, moving them between the registers
and memory. They may be good for games and MPEGs, but for high end
simulation, forget it.

By the way, most Connection Machines do not have vector units.

> If you build a "PC" (Pentium II class) with 256MB of SDRAM and dual PCI
> based fast/wide SCSI controllers running to a striped RAID array of "good"
> SCSI disks you can "beat" a lot of mainframes. Of course you best them with
> a $10,000 PC.

You have not seen a "real" disk array, have you? Thousands of disks,
capacity way up in the Tbytes. Two, or even twenty, fast/wide SCSI
controllers is not going to cut the mustard. In fact, most very high end
machines do not use any sort of SCSI, as it just does not perform well
enough.

William Donzelli
william_at_ans.net
Received on Wed Oct 21 1998 - 08:27:19 BST