Subject: Re: Setting up a T1 and email
To: Alex Barclay <>
From: Sheila //or// Bob (depends on who's writing) <>
List: port-vax
Date: 07/15/1998 18:59:56
Alex Barclay wrote:
> I fully agree. You need to remember that the serial ports on PeeCees are
> terrible from an efficiency point of view. The machine is getting swamped
> with interrupts. I have used a 486/33 as a fine IP router between two ethernets
> and that was 10MB. Ethernet cards place far less load on the CPU than a
> serial port. In the same way a decent T1 board will place a small load because
> it should be doing stuff via DMA.
> There was a paper in the early days of UNIX talking about the efficiency of
> devices on a VAX or PDP (I think it was Kernighan, Thompson or Ritchie
> who wrote it) Anyway they were saying that to run 50 terminals off a machine
> at 9600 baud at 1 interrupt per char would be 48000 interrupts/sec. This
> would bring the machine to its knees. An alternative would be to have
> the serial ports have some intelligence. The main processor only polls
> every 50Hz and gets every character since last time.
> I think that their paper directly parallels the Ethernet/T1 to serial case on
> PeeCees
Exactly Right and that was one of the reasons for the COMM-IOP from DEC
for the Unibus - it was a KMC-11 based Aux CPU that scanned DZ11s (with
interrupts turned off on the boards) and turned the unibus multimaster
capability into a means of supporting multiple serial lines with good
performance...I don't recall how it compared price wise to DH11 or DV
11, but it did allow you to run either sync or async interfaces with a
fairly low load on the CPU.

The KMV was after my time so I don't know much about it...

> A.
> --
> Alex Barclay                    E-mail:
> Vsys                            Tel:    +1 719 635 8066 x 17
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real address is shsrms at erols dot com
The Herbal Gypsy and the Tinker.