[Info-vax] Volatile, was: Re: yet another sys$qiow question

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In article <mrnbn4$o0c$1 at dont-email.me>, David Froble <davef at tsoft-inc.com> writes:
>Bob Gezelter wrote:
>> On Wednesday, August 26, 2015 at 10:10:52 PM UTC-4, David Froble wrote:
>>> John Reagan wrote:
>>>> Looking at the code, the updating of the IOSB is done very carefully.  There
>>>> are EVAX_MB builtins used in the code (it is in Macro32) and they make very
>>>> sure that the first longword is always filled in last (with an MB between
>>>> storing into the 2nd longword and storing into the 1st longword).  I also
>>>> found code that worries about misaligned IOSBs to ensure atomic updating
>>>> (including letting an alignment fault occur just to get proper
>>>> synchronization).
>>>>
>>>> Looking in just the [SYS] facility (so that doesn't even count drivers, RMS,
>>>> RTLs, etc.), I found 389 uses of the MB instruction.  That EVAX_MB builtin is
>>>> mapped to the 'mf' instruction for Itanium.  There are equivalent
>>>> instructions for x86 although the stronger memory ordering rules might make
>>>> some of them unnecessary (but I doubt that the Macro compiler will be able to
>>>> figure that out without human assistance)
>>>
>>> Ok, a question.  Just curious.
>>>
>>> Do you have any feel for how much time might be saved by monitoring the IOSB 
>>> status instead of using one of the signaling methods, such as an AST or event flag?
>> 
>> David,
>> 
>> It depends upon what you mean by "monitoring the IOSB".
>> 
>> Spinning of any sort will be expensive. "Periodically checking" means that
>> there is some other event stream occurring, with its limits on latency.
>> 
>> ASTs are very low overhead.
>> 
>> - Bob Gezelter. http://www.rlgsc.com
>
>Well, yes Bob, that's how I see things also.  But we would not be discussing 
>polling the IOSB status unless someone came up with the (in my opinion very 
>poor) idea of doing so.  I'm trying to understand why anyone would come up with 
>the idea, unless as JF suggests they came from a MS-DOS background where there 
>were no ASTs or any signaling capabilities that I'm aware of.

Because, the programmer's language of choice equivalent of:

10$:	BLBC	IOSB,10$

is so efficient... NOT!  Why would anybody POLL the IOSB?  I thing that those
CPU cycles could be put to far better use.


>  As Steve 
>suggests, if I/O performance needs tweeking, there is IO_PERFORM.

And one STILL needs to check the status in the IOSA.

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