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

[Johnny Billquist]

On 2023-11-04 22:41, Dan Cross wrote:
> 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/

*Thank you*. Finally someone who actually gives this some thought 
instead of just repeating what everyone else says.

   Johnny