[Info-vax] OpenVMS async I/O, fast vs. slow

[Arne Vajhøj]

On 11/7/2023 5:44 PM, Craig A. Berry wrote:
> On 11/6/23 6:18 PM, Arne Vajhøj wrote:
>> On 11/6/2023 6:31 AM, Johnny Billquist wrote:
> 
>>> Read ahead is something that the system can easily do both for normal 
>>> I/O and memory mapped I/O. It's just a question of speculative reads 
>>> assuming some pattern by the program. Most commonly that you are 
>>> reading files sequentially from start to finish.
>>
>> $QIO(w) and $IO_PERFORM(W) could.
>>
>> But at least for $QIO(W) then I would be very surprised if it did. It is
>> from before VIOC/XFC so originally it did not have anywhere to
>> store read ahead data. VMS engineering could have changed
>> behavior when VIOC/XFC was introduced. But I doubt it.
> 
> Are you saying that you think merely using $QIO bypasses XFC?  If so,
> how do you think SHOW MEM/CACHE can give you "Total QIOs"?  And note
> this from the performance management manual page 72:
> "I/O service can be optimized at the application level by using RMS 
> global buffers to share caches among processes. This method offers the 
> benefit of satisfying an I/O request without issuing a QIO; whereas 
> system memory software cache (that is, XFC) is checked to satisfy QIOs."

I am sure that $QIO(W) hits XFC. Else there would not be
much point in XFC.

The question is whether $QIO(W) get more data read into XFC
than asked for by the user. It could but I doubt it.

Arne