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

[VAXman- at SendSpamHere.ORG]

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

I speak to machines with the voice of humanity.