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

[Craig A. Berry]

On 11/4/23 1:42 PM, Arne Vajhøj wrote:
> On 11/4/2023 1:06 PM, Craig A. Berry wrote:
>>
>> On 11/4/23 6:11 AM, Johnny Billquist wrote:
>>
>>> I'm not sure I have ever understood why people think memory mapped 
>>> files would be faster than a QIO under VMS.
>>
>> I might've missed it but I haven't seen anyone say that. It's that
>> global sections are faster than mailboxes. The I/O API may be a
>> consideration but is secondary to the nature of the device.
> 
> I guess that I as usual will have to plead guilty.
> 
> I wrote:
> 
> # The normal assumption regarding speed of disk IO would be that:
> #
> # RMS record IO ($GET and $PUT) < RMS block IO ($READ and $WRITE) <
> # $QIO(W) < $IO_PERFORM(W) < memory mapped file
> #
> # (note that assumption and fact are spelled differently)
> 


Actually, I'm guilty of forgetting I read that message.  I had in mind
Jake's original problem of IPC, not disk I/O, when I responded to
Johnny's remark about memory mapped files. For disk I/O, yes, it's
almost certain that using virtual memory primitives to synchronize
integral pages between disk and memory will be faster than any other I/O
method; that's why pretty much every database product on every platform
does it.