[Info-vax] Large mailboxes

[Arne Vajhøj]

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.

You mean you tried maxmsg=200 and bufquo=200_000_000?

> 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.

SYS$QIO(W) IO$_SENSEMODE IOSB also only have 16 bit for
number messages in mailbox.

> 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 ?

I don't know.

But it seems likely that noone imagined it being a problem.

It uses non-paged pool. How big was available non-paged pool on
VAX systems?

My guess is that available non-paged pool divided by a
normal message size would fit into 16 bit.

Arne