[pups] Progress on 2.11BSD kernel

[Steven M. Schultz]

Hi -

> From: Johnny Billquist <bqt at update.uu.se>
> Are the UMRs allocated and set up statically?

	The clist and buffer cache UMRs are initialized at system boot time.

> (I haven't looked inside 2BSD for a while now, and can't remember much of
> the internals anymore.)

	And I haven't been around a UNIBUS system for a long time and the
	memory of having to deal with UMRs has faded.

> How many UMRs does the system use? 8 for buffer cache, you might expect
> one or two DH11s, that would require a few more, ethernet takes another
> few, but it seems there shouldn't be such a shortage.

	There are only 31 total - so a few allocated here, a few allocated
	there could result in not having enough.

> What did I miss?

	Swapping.  The kernel has to always be able to get UMRs to be
	able to swap out a process.   The memory is fuzzy but I think the
	kernel reserved 7 as to be able to swap 56KB at a time of a process
	but it could get by with less.   

	One UMR is 'reserved' by the hardware to cover the "I/O Page".

	The clist takes up 1 UMR for DMA terminal devices.

	Ethernet of course takes a UMR or two.  At one time a couple of the
	ethernet drivers (DEUNA) were a bit braindamaged and allocated more
	UMRs than they needed - I fixed that though.

	MSCP and TMSCP devices need UMRs to map their command and response
	rings.   The initial MSCP driver was definitely suboptimal in that
	regard - again, something I tended to (the problem showed up when
	more than 1 controller was present in a system).   Those UMRs are
	permanently allocated at boot time by the drivers when they initialize.

	Tape drivers need to have UMRs (although I don't know of too many
	folks with the old TE16 drives around on a UNIBUS system) available.

	It started adding up and on a system with several types of devices
	each of which wanted to reserve a couple UMRs the available pool for
	dynamic use (by tapes and swapping) was sometimes too small.

	Cheers,
	Steven Schultz