[TUHS] Old mainframe I/O speed (was: core)

[Greg 'groggy' Lehey]

On Tuesday, 19 June 2018 at 23:41:41 -0600, Warner Losh wrote:
> On Tue, Jun 19, 2018 at 11:04 PM, Peter Jeremy <peter at rulingia.com> wrote:
>
>> On 2018-Jun-20 08:55:05 +1000, David Arnold <davida at pobox.com> wrote:
>>> Does the screen count as I/O?
>>
>> I was thinking about that as well.  1080p30 video is around 2MBps
>> as H.264 or about 140MBps as 6bpp raw.  The former is negligible,
>> the latter is still shy of the disparity in CPU power, especially
>> if you take into account the GPU power needed to do the decoding.
>>
>>> I???d suggest that it???s just that the balance is (intentionally) quite
>> different.  If you squint right, a GPU could look like a channelized I/O
>> controller.
>>
>> I agree.  Even back then, there was a difference between
>> commercial-oriented mainframes (the 1401 and 360/50 lineage - which
>> stressed lots of I/O) and the scientific mainframes (709x, 360/85 -
>> which stressed arithmetic capabilities).
>
> So what could an old mainframe do as far as I/O was concerned? Google
> didn't provide me a straight forward answer...

Looking at something like the IBM 370 series (mid-1970s), I/O was
performed by the channels, effectively separate processors with a very
limited instruction set.  Others, like the UNIVAC 1100 series, could
perform I/O directly or via separate processors.  This was similar on
the /360, but very different on the 1401.

In each case, from my recollection, main memory and the peripheral
were the bottleneck.  For the UNIVAC 1108 (1965, the one of which I
have the best recollection), memory was 36 bits every 750 ns, and you
could expect it to be interleaved at least 2 ways, so you could
transfer data across two channels to a FH 432 drum at in the order of
2.5 MW/s.  This could lead to underruns depending on what else was
going on in the system.  Other peripherals were slower, so this would
have been about the maximum.

Greg

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