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+.\" Copyright (c) 1983, 1993
+.\"	The Regents of the University of California.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed by the University of
+.\"	California, Berkeley and its contributors.
+.\" 4. Neither the name of the University nor the names of its contributors
+.\"    may be used to endorse or promote products derived from this software
+.\"    without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+.\" SUCH DAMAGE.
+.\"
+.\"	@(#)d.t	8.1 (Berkeley) 6/8/93
+.\"
+.\".ds RH "Data Structure Sizing Rules
+.bp
+.LG
+.B
+.ce
+APPENDIX D. VAX KERNEL DATA STRUCTURE SIZING RULES
+.sp
+.R
+.NL
+.PP
+Certain system data structures are sized at compile time
+according to the maximum number of simultaneous users expected,
+while others are calculated at boot time based on the
+physical resources present, e.g. memory.  This appendix lists
+both sets of rules and also includes some hints on changing
+built-in limitations on certain data structures.
+.SH
+Compile time rules
+.PP
+The file \fI/sys/conf\|/param.c\fP contains the definitions of
+almost all data structures sized at compile time.  This file
+is copied into the directory of each configured system to allow
+configuration-dependent rules and values to be maintained.
+(Each copy normally depends on the copy in /sys/conf,
+and global modifications cause the file to be recopied unless
+the makefile is modified.)
+The rules implied by its contents are summarized below (here
+MAXUSERS refers to the value defined in the configuration file
+in the ``maxusers'' rule).
+Most limits are computed at compile time and stored in global variables
+for use by other modules; they may generally be patched in the system
+binary image before rebooting to test new values.
+.IP \fBnproc\fP
+.br
+The maximum number of processes which may be running at any time.
+It is referred to in other calculations as NPROC and is defined to be
+.DS
+20 + 8 * MAXUSERS
+.DE
+.IP \fBntext\fP
+.br
+The maximum number of active shared text segments.
+The constant is intended to allow for network servers and common commands
+that remain in the table.
+It is defined as
+.DS
+36 + MAXUSERS.
+.DE
+.IP \fBninode\fP
+.br
+The maximum number of files in the file system which may be
+active at any time.  This includes files in use by users, as 
+well as directory files being read or written by the system
+and files associated with bound sockets in the UNIX IPC domain.
+It is defined as
+.DS
+(NPROC + 16 + MAXUSERS) + 32
+.DE
+.IP \fBnfile\fP
+.br
+The number of ``file table'' structures.  One file
+table structure is used for each open, unshared, file descriptor.
+Multiple file descriptors may reference a single file table
+entry when they are created through a \fIdup\fP call, or as the
+result of a \fIfork\fP.  This is defined to be
+.DS
+16 * (NPROC + 16 + MAXUSERS) / 10 + 32
+.DE
+.IP \fBncallout\fP
+.br
+The number of ``callout'' structures.  One callout
+structure is used per internal system event handled with
+a timeout.  Timeouts are used for terminal delays,
+watchdog routines in device drivers, protocol timeout processing, etc.
+This is defined as
+.DS
+16 + NPROC
+.DE
+.IP \fBnclist\fP
+.br
+The number of ``c-list'' structures.  C-list structures are
+used in terminal I/O, and currently each holds 60 characters.
+Their number is defined as
+.DS
+60 + 12 * MAXUSERS
+.DE
+.IP \fBnmbclusters\fP
+.br
+The maximum number of pages which may be allocated by the network.  
+This is defined as 256 (a quarter megabyte of memory) in /sys/h/mbuf.h.
+In practice, the network rarely uses this much memory.  It starts off
+by allocating 8 kilobytes of memory, then requesting more as 
+required.  This value represents an upper bound.
+.IP \fBnquota\fP
+.br
+The number of ``quota'' structures allocated.  Quota structures
+are present only when disc quotas are configured in the system.  One
+quota structure is kept per user.  This is defined to be
+.DS
+(MAXUSERS * 9) / 7 + 3
+.DE
+.IP \fBndquot\fP
+.br
+The number of ``dquot'' structures allocated.  Dquot structures
+are present only when disc quotas are configured in the system.
+One dquot structure is required per user, per active file system quota.
+That is, when a user manipulates a file on a file system on which
+quotas are enabled, the information regarding the user's quotas on
+that file system must be in-core.  This information is cached, so
+that not all information must be present in-core all the time.
+This is defined as
+.DS
+NINODE + (MAXUSERS * NMOUNT) / 4
+.DE
+where NMOUNT is the maximum number of mountable file systems.
+.LP
+In addition to the above values, the system page tables (used to
+map virtual memory in the kernel's address space) are sized at
+compile time by the SYSPTSIZE definition in the file /sys/vax/vmparam.h.
+This is defined to be
+.DS
+20 + MAXUSERS
+.DE
+pages of page tables. 
+Its definition affects
+the size of many data structures allocated at boot time because
+it constrains the amount of virtual memory which may be addressed
+by the running system.  This is often the limiting factor
+in the size of the buffer cache, in which case a message is printed
+when the system configures at boot time.
+.SH
+Run-time calculations
+.PP
+The most important data structures sized at run-time are those used in
+the buffer cache.  Allocation is done by allocating physical memory
+(and system virtual memory) immediately after the system
+has been started up; look in the file /sys/vax/machdep.c.
+The amount of physical memory which may be allocated to the buffer
+cache is constrained by the size of the system page tables, among
+other things.  While the system may calculate
+a large amount of memory to be allocated to the buffer cache,
+if the system page
+table is too small to map this physical
+memory into the virtual address space
+of the system, only as much as can be mapped will be used.
+.PP
+The buffer cache is comprised of a number of ``buffer headers''
+and a pool of pages attached to these headers.  Buffer headers
+are divided into two categories: those used for swapping and
+paging, and those used for normal file I/O.  The system tries
+to allocate 10% of the first two megabytes and 5% of the remaining
+available physical memory for the buffer
+cache (where \fIavailable\fP does not count that space occupied by
+the system's text and data segments).  If this results in fewer
+than 16 pages of memory allocated, then 16 pages are allocated.
+This value is kept in the initialized variable \fIbufpages\fP
+so that it may be patched in the binary image (to allow tuning
+without recompiling the system),
+or the default may be overridden with a configuration-file option.
+For example, the option \fBoptions BUFPAGES="3200"\fP
+causes 3200 pages (3.2M bytes) to be used by the buffer cache.
+A sufficient number of file I/O buffer headers are then allocated
+to allow each to hold 2 pages each.
+Each buffer maps 8K bytes.
+If the number of buffer pages is larger than can be mapped
+by the buffer headers, the number of pages is reduced.
+The number of buffer headers allocated
+is stored in the global variable \fInbuf\fP,
+which may be patched before the system is booted.
+The system option \fBoptions NBUF="1000"\fP forces the allocation
+of 1000 buffer headers.
+Half as many swap I/O buffer headers as file I/O buffers
+are allocated,
+but no more than 256.
+.SH
+System size limitations
+.PP
+As distributed, the sum of the virtual sizes of the core-resident
+processes is limited to 256M bytes.  The size of the text
+segment of a single process is currently limited to 6M bytes.
+It may be increased to no greater than the data segment size limit
+(see below) by redefining MAXTSIZ.
+This may be done with a configuration file option,
+e.g. \fBoptions MAXTSIZ="(10*1024*1024)"\fP
+to set the limit to 10 million bytes.
+Other per-process limits discussed here may be changed with similar options
+with names given in parentheses.
+Soft, user-changeable limits are set to 512K bytes for stack (DFLSSIZ)
+and 6M bytes for the data segment (DFLDSIZ) by default;
+these may be increased up to the hard limit
+with the \fIsetrlimit\fP\|(2) system call.
+The data and stack segment size hard limits are set by a system configuration
+option to one of 17M, 33M or 64M bytes.
+One of these sizes is chosen based on the definition of MAXDSIZ;
+with no option, the limit is 17M bytes; with an option
+\fBoptions MAXDSIZ="(32*1024*1024)"\fP (or any value between 17M and 33M),
+the limit is increased to 33M bytes, and values larger than 33M
+result in a limit of 64M bytes.
+You must be careful in doing this that you have adequate paging space.
+As normally configured , the system has 16M or 32M bytes per paging area,
+depending on disk size.
+The best way to get more space is to provide multiple, thereby
+interleaved, paging areas.
+Increasing the virtual memory limits results in interleaving of
+swap space in larger sections (from 500K bytes to 1M or 2M bytes).
+.PP
+By default, the virtual memory system allocates enough memory
+for system page tables mapping user page tables
+to allow 256 megabytes of simultaneous active virtual memory.
+That is, the sum of the virtual memory sizes of all (completely- or partially-)
+resident processes can not exceed this limit.
+If the limit is exceeded, some process(es) must be swapped out.
+To increase the amount of resident virtual space possible,
+you can alter the constant USRPTSIZE (in
+/sys/vax/vmparam.h).
+Each page of system page tables allows 8 megabytes of user virtual memory.
+.PP
+Because the file system block numbers are stored in
+page table \fIpg_blkno\fP
+entries, the maximum size of a file system is limited to
+2^24 1024 byte blocks.  Thus no file system can be larger than 8 gigabytes.
+.PP
+The number of mountable file systems is set at 20 by the definition
+of NMOUNT in /sys/h/param.h.
+This should be sufficient; if not, the value can be increased up to 255.
+If you have many disks, it makes sense to make some of
+them single file systems, and the paging areas don't count in this total.
+.PP
+The limit to the number of files that a process may have open simultaneously
+is set to 64.
+This limit is set by the NOFILE definition in /sys/h/param.h.
+It may be increased arbitrarily, with the caveat that the user structure
+expands by 5 bytes for each file, and thus UPAGES (/sys/vax/machparam.h)
+must be increased accordingly.
+.PP
+The amount of physical memory is currently limited to 64 Mb
+by the size of the index fields in the core-map (/sys/h/cmap.h).
+The limit may be increased by following instructions in that file
+to enlarge those fields.