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

[Arne Vajhøj]

On 11/6/2023 6:31 AM, Johnny Billquist wrote:
> On 2023-11-04 22:44, Arne Vajhøj wrote:
>> On 11/4/2023 7:11 AM, Johnny Billquist wrote:
>>> So how would memory mapped I/O be any faster? You basically cannot be 
>>> any faster than one DMA transfer. In fact, with memory mapped I/O, 
>>> you might be also hitting the page fault handling, and a reading in 
>>> of a full page, which might be more than you needed, causing some 
>>> overhead as well.
>>
>> Fewer layers to go through. More freedom to read ahead.
> 
> 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.

$IO_PERFORM(W) maybe. But I think the spirit is still
"do exactly what the developer asked for and nothing more".

Arne