On Wed, 15 Jul 1998, 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

  Which is exactly why, in the VAX/PDP world, the DH11 series serial muxes were
designed.  On my 11/34a I had a DH11-AD, which was 16 serial ports with full
modem control that DMA'd incoming characters into buffers in memory, not
bothering the host CPU.  It was very, VERY nice...especially since it replaced
an interrupt-per-character DZ11. :)

               -Dave McGuire