RMS global buffers for a VMS system's indexed files.

Global buffering can be used to save disk i/o to the system disk.


1) Brief description of global buffering.

Global buffers are system-wide as opposed to the normal per-process ones.
They can be very effective for indexed files being accessed by many processes.

When the first $CONNECT to a file occurs on a given VAX, if the FAB field
FAB$W_GBC (global buffer count) is non-zero, RMS creates a system page-file
global section to contain that number of buffers. For indexed files a buffer
holds one RMS bucket. 

When the last $DISCONNECT to a file on a given VAX occurs, the system
global section is deleted.

See the "Guide to VMS File Applications" and the "RMS Manual" for further
details.


2) Using global buffering.

To make a file a permanent candidate for global buffering one uses
SET FILE/GLOBAL_BUFFERS on the file.

To make a file's global buffers permanent on a running system, the
file must be $OPENed & $CONNECTed by a permanent process.

At Anglia College's Cambridge campus we use a permanent detached
system process to open the files (the process has many other functions too).
The files are opened for write-access with full sharing. This has the
additional effect of preventing anyone locking the files completely
(without sharing).

The following SYSGEN parameters are relevant (and need to be checked
when adding global buffering):

	GBLSECTIONS
	GBLPAGES
	GBLPAGFIL
	RMS_GBLBUFQUO


3) Example - our system indexed files & buffer sizes.

                                    Max
		    Approx   Record Bucket Global
File		    Records  Size   Size   Buffers	
SYSUAF		    3000     1412     6     40
RIGHTSLIST	    9000       64     3     80
VMSMAIL_PROFILE	    2500     2048     5     40
GROUPS		     800     2363     5     40   (database of user groups)
CONFIGURATION	       4       76     3      3   (system config. info)
COMMS_PROC_LINES     350      256     1     80   (Gandalf & terminal server ports)

(The bucket sizes were determined by the Optimize script of EDIT/FDL.
We use this automatically each month on SYSUAF, RIGHTSLIST & VMSMAIL_PROFILE.)


4) Example - a data sample.

(Sampled at 12-MAY-1989 13:10)

			VAX 11/780 ----------		VAX 8200 ------------
			Refcnt	  Hits Misses		Refcnt	  Hits Misses
SYSUAF			5	 76990	 5371		3	101392	 7446
RIGHTSLIST		5	278964	23087		4	464304	44890
VMSMAIL_PROFILE		2	 27833	  121		1	 31172	  144
GROUPS			1	 32611	 2680		1	 27374	 1939
CONFIGURATION		1	  2000	    2		1	  1905	    2
COMMS_PROC_LINES	3	166135	 7118		2	151031	 6802

Interactive login limit = 40 on both VAXes.
Typical number of concurrent users on each = 20-25.

A hit means that the required record was in the global section, a miss means
that it wasn't and the record (bucket) had to be read from disk. 

Refcnt shows the current number of $CONNECTions to the file.

As data buckets are fetched for indexed files rather than single records,
adjacent records are available in system memory after reading a record.
This is presumably one reason why the SYSUAF and RIGHTSLIST buffering is
so successful in our educational environment in which classes of users
(with adjacent UAF records) log-in at about the same time.

One has to be wary of scanning the whole of these files sequentially during
normal working hours as that would flush the active records out of the global
buffer cache. We do such housekeeping activity overnight (and no doubt the miss
counts are higher than they would otherwise be because of this periodic
scanning). 


5) Measuring the performance of global buffers.

[I haven't tried the VMS V5.0 MONITOR RMS facility yet.]

The sample data above was produced by the detached system process, which calls a
site-supplied Executive-mode system service to inspect RMS' statistics. 

This is an example Macro code fragment, to be run in Executive-mode,
which finds the RMS global buffer header (GBH) given an open FAB.

; Inspired by Bruce Ellis' Get_Hit program on the US VAX SIG 1987C tape.
	.library	/sys$library:lib.mlb/
	$impdef
	.link		/sys$system:rmsdef.stb/
	.link		/sys$system:sys.stb/
	(...)
; R5 contains the address of a FAB for an open file with global buffers
	movzwl	fab$w_ifi(r5),r6	; ifi
	moval	g^pio$gw_iioimpa,r7	; find image i/o impure area
	cmpw	r6,imp$w_num_ifabs(r7)	; valid ifi?
	bgtru	100$			; no

	movl	imp$l_ifabtbl(r7),r7	; find ifab table
	beql	100$			; none?
	movl	(r7)[r6],r7		; find ifab 
	beql	100$			; none?
	movl	ifb$l_gbh_ptr(r7),r7	; find gbh
	beql	100$			; none?
	(...)


The global buffer header contains the following fields (amongst others):

	gbh$l_miss	-- misses
	gbh$l_hit	-- hits
	gbh$l_usecnt	-- use count (reference count)