[Info-vax] Why it is a good idea that OpenVMS isn't on x86-64 just yet

[Johnny Billquist]

On 2016-01-24 03:26, gcornelius at charter.net wrote:
> Johnny Billquist <bqt at softjar.se> wrote:
>> Norm Raphael wrote:
>>> TU58's used the DISK DRIVER, not the tape driver, so they blocked
>>> disk I/O when in use.  Slow doesn't really convey it.
>>
>> That was not necessary. We're talking about the ACP here, which would
>> only be involved if the TU58 was in Files-11 format. If it was in RT-11
>> format, you mounted it foreign and used EXCHANGE.
>> Second, I'm sure you could use a separate, unique ACP for the TU58, just
>> like in RSX, to keep it separated from all other system activity.
>
>   $ MOUNT/PROCESSOR=UNIQUE
>
> Of course, most of us considered that an advanced qualifier way back
> when and may not have taken the time to learn it.

Yeah. Most people don't even think about the ability, but it's there 
specifically for these kind of issues.

> Good point about $ EXCHANGE, though. I would not have thought of
> that.
>
>> Third, I'm sure that for plain operations reading/writing blocks, it did
>> not even block the ACP, unless VMS was more silly than RSX.
>
> Well VMS eventually went to the XQP, the Files-11 ACP as part of the
> executive.  It of course was/is multithreaded or would have become an
> extraordinary bottleneck in multidrive configurations.

In RSX, simple reads/writes never pass by the ACP, as the kernel do the 
translation between virtual and physical blocks in the I/O path and 
removes the need to pass by the ACP. The kernel knows the data 
structures that F11ACP have, and use them directly. Things like opening 
files, or extending them, however, needs to pass through the ACP, and 
thus one slow device can block a lot of other operations for extended 
times...

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol