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

[Dan Cross]

In article <ui64is$3go9i$1 at dont-email.me>,
Craig A. Berry <craigberry at nospam.mac.com> wrote:
>On 11/4/23 1:42 PM, Arne Vajhøj wrote:
>> On 11/4/2023 1:06 PM, Craig A. Berry wrote:
>>>
>>> On 11/4/23 6:11 AM, Johnny Billquist wrote:
>>>
>>>> I'm not sure I have ever understood why people think memory mapped 
>>>> files would be faster than a QIO under VMS.
>>>
>>> I might've missed it but I haven't seen anyone say that. It's that
>>> global sections are faster than mailboxes. The I/O API may be a
>>> consideration but is secondary to the nature of the device.
>> 
>> I guess that I as usual will have to plead guilty.
>> 
>> I wrote:
>> 
>> # The normal assumption regarding speed of disk IO would be that:
>> #
>> # RMS record IO ($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.

Everyone starts out thinking that, but most are wrong:
https://db.cs.cmu.edu/mmap-cidr2022/

	- Dan C.