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

[VAXman- at SendSpamHere.ORG]

In article <mro851$8t4$3 at dont-email.me>, David Froble <davef at tsoft-inc.com> writes:
>VAXman- @SendSpamHere.ORG wrote:
>> In article <00AFC46D.EA88A745 at SendSpamHere.ORG>, VAXman-  @SendSpamHere.ORG writes:
>>> In article <mrnbn4$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$
>> 
>> Sorry, brainfart...
>> 
>> 10$:	TSTW	IOSB
>> 	BEQL	10$
>> 
>>> 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.
>>>
>>> -- 
>>> VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG
>>>
>>> I speak to machines with the voice of humanity.
>
>No problem Brian, I understood what you were trying to say.
>
>And I'm still asking, why would anyone do such?  Is there any valid reason?

I can't see any but I'll differ to others for an answer and then, point out
why it's still not a valid thing to do.

-- 
VAXman- A Bored Certified VMS Kernel Mode Hacker    VAXman(at)TMESIS(dot)ORG

I speak to machines with the voice of humanity.