/share/doc/smm/02.config/d.t
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- .\".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.