[Info-vax] Large mailboxes

[Dave Froble]

On 11/26/2020 11:30 AM, Marc Van Dyck wrote:
> Unless I mis-read it, the OpenVMS documentation does not state any size
> limitation for permanent mailboxes. There is apparently just a
> limitation on the size of each message, but the number of messages can
> apparently be arbitrarily high. I have made a call to $CREMBX to
> create a mailbox for 1.000.000 messages of 200 bytes each, and VMS did
> not complain.
>
> I understand from a Digital Technical Journal article that mailbox space
> is not reserved at mailbox creation time, but allocated each time a
> new message is dropped in the mailbox. This I have been able to verify
> by loading a mailbox and see the values in $SHOW MEM/POOL decrease
> accordingly (loading chunks of 10.000 messages at a time...).
>
> The same documentation also says that it is possible to obtain the
> number of messages stored in a mailbox with a call to $GETJPI,
> specifying the item DVI$_DEVDEPEND. This call returns a longword
> of which only the two last bytes are significant, so the maximum
> number of outstanding messages can be 65535.

Which most likely seemed a huge number, back in 1978.

What can be reported, and what can exist, just might not be the same.

Run a simple test.  Store the messages, each one unique, then read them 
back insuring each message was stored and forwarded.  Might be interesting.

> I have been able to verify that too, with the same test as above,
> the number of outstanding messages growing steadily till 60.000 and
> then dropping back to 4000 or so after the next chunk of 10.000 messages
> were loaded.
>
> So my question is, why this limitation ? Is it just because when this
> interface was written, noone imagined that there could ever be a
> mailbox with more than 64k outstanding messages ? Or am I really going
> to break something other than this counter if I try loading more
> than 64k messages ?

What was the max memory on an 11/780?  Something like 8 MB?  Yes, it was 
a virtual memory machine.  But how many messages might the devs have 
anticipated, back then?

There just might be a word integer storing the # of messages, or, maybe 
VMS doesn't care, and it's some type of list.


-- 
David Froble                       Tel: 724-529-0450
Dave Froble Enterprises, Inc.      E-Mail: davef at tsoft-inc.com
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